Chap 7 Heat PDF
Chap 7 Heat PDF
Chap 7 Heat PDF
Notes
CHAPTER 7. HEAT
This chapter discusses sources of heat in industrial operations and associated equipment. A
description of each heat source, its general uses, operation, and common opportunities for energy
conservation are presented. There will be case studies referenced throughout the chapter that can be found in
Appendix E.
7.1 Boilers
A boiler is a device where energy extracted from some type of fuel is converted into heat that is
distributed to needed places to do useful work. In the process, the carrying media (water or steam) gives up
the heat and is cyclically reheated again and again. There are examples where the media (steam) is not
returned, such as locomotives, but in industrial processes covered in this manual it would constitute an
exception. For the most part, boilers take advantage of the phase changes that occur in some substances (for
example water). The phase change is associated with large amount of energy that can be harnessed to our
benefit.
There are four principal boiler categories: (1) natural draft, (2) forced draft, (3) hot water or steam,
and (4) fire tube or water tube. In a natural draft boiler, the combustion air is drawn in by natural convection
and there is no control of the air/fuel ratio. For forced draft boilers, a blower controls the quantities of
combustion air and the air/fuel mixture. Some boilers produce hot water, typically in the 160° to 190°F
range, while others produce steam. Steam boilers may be low pressure (approximately 15 psi), medium
pressure (15 to 150 psi), or high pressure (150 to 500 psi). Finally, boilers may be fire-tube or water-tube
boilers. In a fire-tube boiler, the hot gas flows through tubes immersed in water, whereas in a water-tube
boiler, the water flows through tubes heated by the hot combustion gases. There are also some very high
temperature and superheat boilers but these are seldom encountered in typical manufacturing operations. The
typical boiler used in small to medium sized industrial operations is a forced draft steam boiler at 120-150 psi
and approximately 150 hp. The following measures are also applicable to utility boilers. Other than the
major differences of not being natural draft boilers and producing steam at greater than 150 psi, utility boilers
are similar to boilers commonly used by industry.
This section includes energy conservation strategies for boiler systems. Combustion air blower
variable frequency drives, air/fuel ratio reset, turbulators, high-pressure condensate return systems, steam trap
repair, and steam leak repair are discussed in this section.
7.1.1 Boiler Operation and Efficiency
An ideal model of a boiler operation is based on the Carnot cycle. The Carnot cycle is defined as
two reversible isothermal and two reversible adiabatic processes. Heat is added to the cycle during the
isothermal process at high temperature (TH ), then follows an adiabatic process producing work as the
working fluid is expanded to a lower pressure. During the next isothermal stage, heat is rejected to the low
temperature reservoir at TL . During the last phase the working fluid is adiabatically compressed to finish the
cycle. The Carnot cycle is the most efficient cycle for the given low and high temperatures and its efficiency
is given by:
η = 1 − 1 T
T
h
The efficiency of a real boiler is always lower. A model Carnot cycle using the phase changing
medium, would be a boiler that operates at constant temperature while adding heat to the working medium,
then an expansion device (turbine) that operates adiabatically, a condenser that operates at constant
temperature while rejecting heat from the medium and a compressor or a pump that adiabatically brings the
medium to the starting point. The boilers are designed to operate at near constant pressure but in reality the
temperature and pressure vary. If the devices are operated near the saturation region, they will operate at
constant temperature as well as constant pressure. The quality of the medium is quite low at the end of
expansion and the fluid before compression is a mixture of liquid and vapor instead of just liquid.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 213
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The air fuel ratio should be adjusted to the recommended optimum values if possible; however, a
boiler with a wide operating range may require a control system to constantly adjust the air-fuel
ratio.
2. A high flue gas temperature often reflects the existence of deposits and fouling on the fire and/ or
water side(s) of the boiler. The resulting loss in boiler efficiency can be closely estimated on the
basis that a 1% efficiency loss occurs with every 40°F increase in stack temperature.
It is suggested that the stack gas temperature be recorded immediately after boiler servicing
(including tube cleaning) and that this value be used as the optimum reading. Stack gas
temperature readings should be taken on a regular basis and compared with the established
optimum reading at the same firing rate. A major variation in the stack gas temperature indicates a
drop in efficiency and the need for either air-fuel ratio adjustment or boiler tube cleaning. Exhibit
7.2 illustrates how the stack temperature rises with maladjusted air fuel ratios. In the absence of
any reference temperature, it is normally expected that the stack temperature be less than 100°F
above the saturated steam temperature at a high firing rate in a saturated steam boiler (this doesn’t
apply to boilers with economizers and air pre-heaters).
3. After an overhaul of the boiler, run the boiler and reexamine the tubes for cleanliness after thirty
days of operation. The accumulated amount of soot will establish the criterion as to the necessary
frequency of boiler tube cleaning.
4. Check the burner head and orifice once a week and clean if necessary.
5. Check all controls frequently and keep them clean and dry.
6. For water tube boilers burning coal or oil, blow the soot out once a day. The National Bureau of
Standards indicates that 8 days of operation can result in an efficiency reduction of as much as 8%,
caused solely by sooting of the boiler tubes.
7. Purity of water used for steam generation is extremely important. It is not usually possible to use
untreated waters found in nature as boiler feed water as there are many impurities. Water must be
treated to remove the impurities or convert them into some harmless form. Other means to remove
impurities and buildup from boilers is a systematic removal by blowdown. This way an excessive
accumulation of solids is prevented. Water treatment prevents the formation of scale and sludge
deposits on the internal surfaces of boilers. Scale formations severely retard the heat flow and
cause overheating of metal parts. The scale build-up and heat transfer relationship is demonstrated
in Exhibit 7.3.
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Notes
8. The frequency and amount of blowdown depend upon the amount and condition of the feed-water.
Check the operation of the blowdown system and make sure that excessive blowdown does not
occur. Normally, blowdown should be no more than 1% to 3% of steam output.
Exhibit 7.2: Boiler Efficiency (Natural Gas)
Economizers use heat from moderately low temperature combustion gases after the gases leave the
steam generating section (or in many cases also after going through a superheating segment) to preheat feed
water. Economizers are heating the feed water after it is received from the water feed pumps, so the water
arrives at a higher temperature into a steam generating area. A typical design uses steel tubes where the water
is fed at pressures higher than the pressure in the steam generation part. The feed rate has to correspond to
the steam output of the boiler. Exhibit 7.4 shows the effect of pre-heating of the feed water on the efficiency
of a boiler unit.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 215
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216 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
Although blowdowns are an absolute necessity for the operation of a boiler, it is important that one
realizes that, depending on the pressure, each blowdown decreases the efficiency of the boiler. Exhibit 7.5
illustrates the decrease in efficiency where the percent blowdown is calculated as follows:
&
M Blowdown
× 100
&
M Steam Produced
Note how sharply the efficiency loss increases with higher pressures.
Exhibit 7.5: Efficiency Loss Due to Blowdown
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 217
Heat: Boilers
Incomplete Combustion:
Notes
Soot + Aldehydes
Carbon Oxygen Water
+ = CO2
Hydrogen Nitrogen CO
Nitrogen
Perfect combustion (referred to as stoichiometric combustion) is the process of burning the fuel
without an excess of combustion air. This process should develop the “ULTIMATE CO2"" (see Exhibit 7.6).
Exhibit 7.6: Ultimate CO2 Values
Fuel CO2 %
Natural Gas (can vary) 11.7-12.1%
Propane 13.7%
No.2 Oil 15.2%
No.4 Oil 16.0%
While these values can be sometimes achieved, Exhibit 7.7: Boiler Combustion Mixtures shows
more realistic desired values.
Exhibit 7.7: Boiler Combustion Mixtures
Carbon, in burning to carbon monoxide, gives off only about one third of the available heat. A one
eighth inch coating of soot on the heat exchanger increases fuel consumption by over 8% as a rule of thumb.
Incomplete combustion that results in the formation of CO is dangerous because it is odorless, colorless,
tasteless, and contrary to popular belief, it is non-irritating. The gas is also lighter than air and consequently,
if it is escaping from a plugged or leaking boiler fireside, can rise to occupied areas. CO can only be
detected with special test or monitoring equipment.
Causes of Incomplete Combustion
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Notes
• Fuel is not vaporized - possible reasons
Worn nozzle
Clogged nozzle
Pump pressure is incorrect
Pump, lines, filter or tank lines are clogged
Cold fuel
• Water in fuel - possible causes
Supplier doesn’t supply quality fuel
Tank is located outside
Cover the fill opening and vent to protect from rain
3. Insufficient or inconsistent heat
• The ignition system is used to provide the proper temperature (called kindling point) for the
light off of the vaporized fuel under design conditions. When design conditions are not met,
light off will not occur.
• An established flame is usually sufficient to maintain the kindling point. However, anytime the
combustion temperature falls below the kindling point, the combustion triangle is broken and
combustion stops. A safety device will shut the fuel off within 3 seconds of flame failure.
Calculating Combustion Efficiency
The calculation of combustion efficiency is based upon three factors.
1. Chemistry of the fuel
2. Net temperature of the stack gases
3. The percentage of oxygen or carbon dioxide by volume in the stack gases
Eyeballing the flame for color, shape and stability is not enough for maximizing efficiency.
Commercial analyzers are available to accurately gauge combustion efficiency. The simplest units measure
only O2 or CO2 . Exhibit 7.8 lists efficiencies for common heat generation devices.
Exhibit 7.8: Combustion Efficiencies
There are no standard performance efficiency levels that commercial boiler manufacturers must
adhere to. Efficiency is reported in different terms:
• Thermal Efficiency – A measure of effectiveness of the heat exchanger that does not account for
radiation and convection losses.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 219
Heat: Boilers
Notes • Fuel to Steam Efficiency - A measure of the overall efficiency of the boiler accounting for radiation
and convection losses.
• Boiler Efficiency – Refers to either thermal efficiency or Fuel to Steam Efficiency.
Installation of controllers such as a temperature setback device can result in savings of up to 18% of
annual heating costs. A controller can sense the inside or outside temperature, or both. Controllers manage
the boiler cycling and/or control valves based upon the ratio of the two temperatures and the rate of change of
each. Burner controls maximize the burner’s efficiency. One way this can be done is by using two-stage
(high-low) burners. Another possibility is the utilization of higher voltage electronic ignition that improves
light off and consequently reduces associated soot accumulation. Employment of interrupted ignition reduces
the run time of ignition components by approximately 98% during heating season increasing ignition
component life.
7.1.2 Typical Performance Improvements
Some performance improvements are easily achieved and many of which are proper maintenance or
operation procedures. This section covers a few of the more common ones.
7.1.2.1 Adjustment of Fuel and Air Ratio
For each fuel type, there is an optimum value for the air/fuel ratio. The air/fuel ratio is the ratio of
combustion air to fuel supplied to the burner. For natural gas boilers, this is 10% excess air, which
corresponds to 2.2% oxygen in the flue gas. For coal-fired boilers, the values are 20% excess air and 4%
oxygen. Because it is difficult to reach and maintain these values in most boilers, it is recommended that the
boiler air/fuel ratio be adjusted to give a reading of 3% oxygen in the flue gas (about 15% excess air) for
gas-fired boilers and 4.5% (25% excess air) for coal-fired boilers. For natural gas boilers, the efficiency is a
function of excess/deficient air and stack temperature. The curves for oil and coal-fired boilers are similar.
Because the efficiency decreases rapidly with deficient air, it is better to have a slight amount of excess air.
Also, the efficiency decreases as the stack gas temperature increases. As a rule of thumb, the stack
temperature should be 50° to 100°F above the temperature of the heated fluid for maximum boiler efficiency
and to prevent condensation from occurring in the stack gases. It is not uncommon that as loads on the
boiler change and as the boiler ages, the air/fuel ratio will need readjusting. It is recommended that the
air/fuel ratio be checked as often as monthly. Combustion analyzers are available for less than $1,000, and it
is often recommended that these be purchased. Case studies illustrating this opportunity can be found in
Appendix E.
Exhibit 7.9 illustrates the average cost savings from implementation of this opportunity.
Exhibit 7.9: Air/Fuel Ratio Reset: Costs and Benefits
1. Tabulated data were taken from the Industrial Assessment Center (IAC) database. All values are
averages based on the database data. The implementation rate for this measure was 70%.
2. One example from the IAC database to further clarify the costs is as follows: Adjusting the air/fuel
ratio on a 6.3 MMBtu/hr boiler at a concrete plant resulted in energy and cost savings of 1,814
MMBtu/yr and $4,760/yr. The implementation cost was $1,500, which was the cost for flue gas
analysis equipment and labor.
3. The energy cost savings are based on proposed dollar savings as reported to IAC from the center,
which are usually almost identical to actual savings reported from the facility.
220 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
7.1.2.2 Elimination of Steam Leaks
Significant savings can be realized by locating and repairing leaks in live steam lines and in
condensate return lines. Leaks in the steam lines allow steam to be wasted, resulting in higher steam
production requirements from the boiler to meet the system needs. Condensate return lines that are leaky
return less condensate to the boiler, increasing the quantity of required make-up water. Because make-up
water is cooler than condensate return water, more energy would be required to heat the boiler feed water.
Water treatment would also increase as the make-up water quantity increased. Leaks most often occur at the
fittings in the steam and condensate pipe systems. Savings for this measure depend on the boiler efficiency,
the annual hours during which the leaks occur, the boiler operating pressure, and the enthalpies of the steam
and boiler feed water where enthalpy is a measure of the energy content the steam and feed water.
Exhibit 7.10 lists average cost savings and energy conservation from implementation of this
opportunity.
Exhibit 7.10: Steam Leak Repair: Costs and Benefits
1. Tabulated data were taken from the Industrial Assessment Center (IAC) database. All values are
averages based on the database data.
2. The implementation rate for this measure was 81%.One example from the IAC database to further
clarify the costs is as follows: Repairing steam leaks on a 600 hp boiler system at a rendering plant
resulted in energy and cost savings of 986 MMBtu/yr and $4,535/yr. The implementation cost was
$350.
3. The energy cost savings are based on proposed dollar savings as reported to IAC from the center,
which are usually almost identical to actual savings reported from the facility.
7.1.2.3 Variable Frequency Drives for Combustion Air Blowers
The load on a boiler typically varies with time, and, consequently, the boiler firing-rate varies
between low and high fire. The amount of combustion air required changes accordingly. Common practice
has been to control a damper or vary the positions of the inlet vanes in order to control the airflow; that is,
when inlet air is required the damper is essentially closed and opened as more air is required. This is an
inefficient method of airflow control because air is drawn against a partially closed damper whenever the
maximum amount of combustion air is not required. It is much more efficient to vary the speed of the blower
by installing a variable-frequency drive on a blower motor (note that it is sometimes expensive to install a
variable-frequency drive if inlet vanes exist). Because the power required to move the air is approximately
proportional to the cube of the airflow rate, decreasing the flow rate by a factor of two will result in a
reduction of power by a factor of eight. This measure is particularly significant on boilers of 3.3 MMBtu/h or
greater.
Combustion air blower variable-frequency drives are available from boiler manufacturers for new
boiler installation. They also may be retrofitted to an existing boiler with few changes to the boiler. Exhibit
7.11 presents average cost savings and energy conservation from implementation of this opportunity.
Exhibit 7.11: (ASD) - Variable-Frequency Drives: Costs and Benefits
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 221
Heat: Boilers
1. Tabulated data were taken from the Industrial Assessment Center (IAC) database. All values are
Notes
averages based on the database data. The implementation rate for this measure was 33%.
2. One example from the IAC database to further clarify the costs is as follows: Installing variable speed
drives and corresponding controls on two 250 hp combustion air fans at a food processing plant resulted
in energy and cost savings of 488,445 MMBtu/yr and $28,000/yr. The implementation cost was
$80,000.
3. The energy cost savings are based on proposed dollar savings as reported to IAC from the center, which
are usually almost identical to actual savings reported from the facility.
7.1.2.4 Maintenance of Steam Traps
A steam trap holds steam in the steam coil until the steam gives up its latent heat and condenses. In
a flash tank system without a steam trap (or a malfunctioning trap), the steam in the process heating coil
would have a shorter residence time and not completely condense. The uncondensed high-quality steam
would be then lost out of the steam discharge pipe on the flash tank. Steam trap operation can be easily
checked by comparing the temperature on each side of the trap. If the trap is working properly, there will be
a large temperature difference between the two sides of the trap. A clear sign that a trap is not working is the
presence of steam downstream of the trap. Non-working steam traps allow steam to be wasted, resulting in
higher steam production requirement from the boiler to meet the system needs. It is not uncommon that,
over time, steam traps wear and no longer function properly. Exhibit 7.12 lists average cost savings and
energy conservation from implementation of this opportunity.
Exhibit 7.12: Steam Trap Repair: Costs and Benefits
1. Tabulated data were taken from the Industrial Assessment Center (IAC) database. All values are
averages based on the database data. The implementation rate for this measure was 79%.
2. One example from the IAC database to further clarify the costs is as follows: Repairing one steam trap
resulted in energy and cost savings of 105 MMBtu/yr and $483/yr on a 600 hp boiler at a rendering
plant. The implementation cost was $220.
3. The energy cost savings are based on proposed dollar savings as reported to IAC from the center, which
are usually almost identical to actual savings reported from the facility.
7.1.2.5 High Pressure Condensate Return Systems
As steam looses it’s heat content is condenses into hot water called condensate. A sudden reduction
in the pressure of a pressurized condensate will cause the condensate to change phase into steam, more
commonly called flashing. Flash tanks are often designed into a pressurized return system to allow flashing
and to remove non-condensable gases from the steam. The resulting low-pressure steam in the flash tank
can often be used as a heat source.
A more efficient alternative is to return the pressurized condensate directly to the boiler through a
high-pressure condensate return system. Heat losses due to flashing are significant, especially for high-
pressure steam systems. Steam lost due to flashing must be replaced by water from the city mains (at
approximately 55°F). This causes the feed water mixture to the boiler to be significantly below its boiling
point, resulting in higher fuel consumption by the boiler. Water treatment costs are also greater with
increased flash losses. In a retrofit application, a closed, high-pressure condensate return system would
prevent the flashing that occurs in the existing system by returning the condensate to the boiler at a higher
pressure and temperature, thereby reducing boiler energy requirements and water treatment costs.
Non-condensable gases (such as air and those formed from the decomposition of carbonates in the
boiler feed water treatment chemicals) can be removed from a closed condensate return system through the
222 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
use of variable orifice discharge modules (VODMs). VODMs are similar to steam traps in that they return
condensate but also can remove non-condensable gases. In a system that does not contain VODMS, these
gases can remain in the steam coil of the equipment being heated and can form pockets of gas that have the
effect of insulating the heat transfer surfaces, thus reducing heat transfer and decreasing boiler efficiency.
Exhibit 7.13 lists average cost savings from installation of a condensate return system.
Exhibit 7.13: Condensate Return Systems: Costs and Benefits
1
Options Installed Energy Cost Savings Simple
Costs ($)2 Savings ($/yr)3 Payback (yr)
(MMBtu/yr)
High Pressure 6,931 9,688 12,738 0.5
Condensate Return
1. Tabulated data were taken from the Industrial Assessment Center (IAC) database. All values are
averages based on the database data. The implementation rate for this measure was 59%.
2. One example from the IAC database to further clarify the costs is as follows: Installing of high-pressure
condensate return system equipment at food processing plant resulted in energy and cost savings of 4,727
MMBtu/yr and $14,100/yr. The implementation cost was $37,000.
3. The energy cost savings are based on proposed dollar savings as reported to IAC from the center, which
are usually almost identical to actual savings reported from the facility.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 223
Heat: Heat Recovery Systems
Notes
Exhibit 7.14: Fuel Savings Realized by Preheating Combustion Air
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Notes
7.2.2.2 Heat Pipes
The heat pipe thermal recovery unit is a counterflow air-to-air heat exchanger. Hot air is passed
through one side of the heat exchanger and cold air is passed through the other side in the opposite direction.
Heat pipes are usually applied to process equipment in which discharge temperatures are between 150 and
850 °F. There are three general classes of application for heat pipes:
1. Recycling heat from a process back into a process (process-to-process)
2. Recycling heat from a process for comfort and make-up air heating (process-to-comfort)
3. Conditioning make-up air to a building (comfort-to-comfort)
Heat pipes recover between 60 to 80% of the sensible heat between the two air streams. A wide
range of sizes is available, capable of handling 500 to 20,000 cubic feet of air per minute. The main
advantages of the heat pipe are:
• No cross-contamination
• Operates without external power
• Operates without moving parts
• Occupies a minimum of space
7.2.2.3 Shell and Tube Heat Exchangers
Shell and tube heat exchangers are liquid-to-liquid heat transfer devices. Their primary application
is to preheat domestic water for toilets and showers or to provide heated water for space heating or process
purposes.
The shell and tube heat exchanger is usually applied to a furnace process cooling water system, and is
capable of producing hot water approaching 5 to 10°F of the water temperature off the furnace. To
determine the heat transfer capacity of the heat exchanger the following conditions of the operation must be
known:
1. The amount of water to be heated in gallons per hour
2. The amount of hot process water available in gallons per hour
3. Inlet water temperature and final water temperature desired
4. Inlet process water temperature
7.2.2.4 Regenerative Unit (Heat Wheel)
The heat wheel is a rotary air-to-air energy exchanger which is installed between the exhaust and
supply air duct work in a make-up or air heating system. It recovers 70 to 90% of the total heat from the
exhaust air stream. Glass fiber ceramic heat recovery wheels can be utilized for preheating combustion air
with exhaust flue gas as high as 2,000°F. Heat wheels consist of a rotating wheel, drive mechanism,
partitions, frames, air seals and purge section. Regeneration is continuous as the wheel rotates through the
hot section picking up energy that is then stored and transferred to the cooler air in the supply section.
7.2.2.5 Recuperators
Recuperators are air-to-air heat exchangers built to provide efficient transfer of heat from hot
exhaust gases to cooler air stream. Recuperators are generally used in the following processes:
• Preheating combustion air
• Preheating material that has to be heated in the process
• Recovery heat from hot gas to supplement or replace the primary heat source in process or comfort
heating applications
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 225
Heat: Heat Recovery Systems
Notes
There are many different types of recuperator designs available today. The recuperator described
below is primarily used for combustion air preheating.
It consists of three basic cylinders, the hot gases flow up through the inner cylinder, cold
combustion air enters at the bottom of the outer cylinder, flows upward and down through the middle
cylinder, exiting from the bottom of the middle cylinder. Heat energy from exhaust gases is transferred
through the inner cylinder wall to the combustion air by a combination of conduction and radiation heat
transfer. The net effect is preheated air temperature as high as 1,000°F with inlet exhaust gases entering at
Destratification fans are used to destratify air in buildings. Stratification is a result of an increasing
air temperature gradient between the floor and the ceiling in an enclosed area, usually due to stagnant air.
When there is insufficient air movement, the hot air will rise to the ceiling, resulting in warmer temperatures
in the upper portion of the area and cooler air temperatures at the working level near the floor. An example
of stratification is shown in Exhibit 7.15(a). If stratification is present, the heating requirements of the
facility are increased because the heating system is continually working to maintain the thermostat setpoint
temperature. The thermostat setpoint operates according to the temperature at the working level. Much
effort is required to make up for the heat the working level loses due to this physical occurrence. The
destratification process initiates the movement of the air, creating a more uniform temperature distribution
within the enclosed space. The air temperature at the floor level becomes nearly equal to the air temperature
at the ceiling thus reducing the amount of energy needed to heat the facility. The amount of heat lost to
ventilation and infiltration is also reduced due to the overall reduction in heat being generated.
7.3.1.1 Ceiling Fans
The basic function in destratification is to pull the air from the ceiling level down to the floor level
and allow it to mix with the cooler air and increase the temperature at the working level. This benefits the
226 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
comfort of the workers and also reduces the energy use of the facility. This process can be accomplished by
two different means. The first and most common device used is the ceiling fan. The fan draws the air from
above the fan and forces it downward by the power of the specific motor and blade combination. The
resulting motion is an air plume, with the warm air moving downward and outward and essentially creating
a mixture like the one shown in Exhibit 7.15(b). The total air volume and coverage is dependent on the
motor size, height of the fan and the specifications of the fan blade (design, size, rpm). Ceiling fans are also
applicable in cooling conditions. It creates motion in the air and this can assist with evaporative cooling of
the skin surface.
The total number of fans needed in a facility can be determined by the following equation.
Total Plant Area
= Number Fans Needed
Fan Coverage Area
The fan coverage area depends on the type and size of fan used and this information can usually be obtained
from the fan manufacturer. Placement of the fans is also important. The simplest method of determining
placement is to calculate the distance between each fan. This can be accomplished by using the following
equation.
Corner fans should be placed half this distance from each wall and consecutive fans should be
placed this distance apart to obtain maximum coverage. Obstacles such as stacked merchandise or office
partitions should be taken into consideration when choosing and placing fans.
7.3.1.2 Ducting
Another option for destratifying the air is to install a hanging device that uses a fan to pull the warm
air from the ceiling, sends it downward through a duct/tube and redistributes the air at the floor level as
shown in Exhibit 7.15(c). This device has advantages and disadvantages. It aids in the destratification
process and creates a more uniform temperature distribution without creating disturbing drafts. It is also
simple to install and can easily be relocated throughout the building. On the other hand, these devices may
be a bit cumbersome and unsightly. They extend from the ceiling down to the floor and create additional
obstacles for the workers and may not be appropriate for some areas of the plant. These devices also do not
possess the cooling applications of the ceiling fans.
Exhibit 7.15: Stratification and Destratification of Air
a) Stratification air pattern, (b) Destratification air pattern using a ceiling fan,
(c) Destratification air pattern using ducting
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 227
Heat: Heating Systems
Notes
savings these alternative sources can produce should be evaluated in relation to the cost to install them. For
example, consider the replacement of a 500,000-Btu-per hour electric heater with a 500,00-Btu-per-hour
natural gas heater.
Annual Cost of Electric Heater
= 500,000 Btu/hr x $14.65/106 Btu x 80% Eff. - 6,000 hrs/yr = $35,200
Annual Cost of Natural Gas Heater
= 500,000 Btu/hr x $3.00/106 Btu x 50% Eff. x 6,000 hrs/yr = $4,500
The energy cost saving is = $35,200 - $4,500 = $30,700/yr
7.3.2.1 Radiant Heaters
Radiant heaters are used for heating spaces by converting electric or gas energy to heat. It is
important to think thoroughly about the whole picture before recommending radiant heaters because
considered in isolation they probably would not be economically viable.
When dealing with the use of energy for the purpose of heating sometimes it is better to deal
directly with the source of the problem. Convection heaters are inefficient heating devices in that energy is
wasted in heating the space and using that heated air to convectively warm the people and/or objects within
that space. Radiant heaters take a different approach. Radiant heaters operate similar to the sun. Radiant
energy is transferred at the speed of light as electromagnetic waves. The heaters emit infrared radiation that
is absorbed by the people/objects that it strikes, which elevates the temperature of the body, but does not
heat the air through which it travels.
7.3.2.2 Types of Radiant Systems
Radiant heating systems can be gas-fired or electric. The type of radiant heating system used is
determined by the sources available. For example, electric radiant heating systems may be installed in an
area of the building where gas is unavailable even though natural gas is more cost effective than electricity.
The efficiencies for both electric and gas systems are approximately the same but natural gas infrared
systems have a longer lifetime. A radiant heating system is often a relatively easy retrofit measure but may
also be integrated into new construction. Radiant heaters come in different sizes, styles and shapes
according to their application. Exhibit 7.16 shows a typical example of a radiant heater.
In relation to equipment performance, radiant sources can be categorized into three groups. A low
temperature system has source temperatures up to 300°F and would typically be used as a floor or ceiling
heater. A low-intensity system has sources up to 1200°F. A medium-intensity system has temperatures up
to 1800°F and would typically include a porous matrix unit. High-intensity systems have source
temperatures up to 5000°F and usually consist of an electrical reflector lamp and high temperature resistors.
Low-temperature heating systems are usually used in residential and perimeter heating applications such as
schools, offices, and airports. These systems are often incorporated directly into the building structure.
Low-, medium-, and high-intensity systems have more industrial and commercial uses and are usually
assembled units that are installed into existing structures.
7.3.2.3 Applications
Use of radiant systems is ideal for comfort heating. Since the infrared radiation elevates body
temperature without heating the air through which it travels, the same degree of comfort provided by the
convection heaters can be maintained at lower indoor air temperatures with radiant heaters. This measure
also eliminates the problem of stratification. It is beneficial to use these heaters in spaces where the ceilings
are high and stratification is prominent. It is also very practical for areas that are frequently exposed to the
outside air such as loading dock areas. Radiant spot heating helps workers to maintain a comfortable
working temperature even though the space air may be cold. Radiant heat, unlike convection, does not
require a medium to travel through and thus has a much higher heat transfer rate. An advantage of this is its
short response time. The person or object will feel the effects of the system shortly after it is engaged. The
rate of energy transfer is dependent upon many different factors including temperature, emissivity,
reflectivity, absorptivity and transmissivity. Emissivity is a radiative property that indicates how efficiently
the surface emits compared to an ideal radiator and its value ranges between 0 and 1. Reflectivity,
228 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
absorptivity, and transmissivity are the fractions of incidental radiation reflected, absorbed, and transmitted,
respectively.
Exhibit 7.16: Infrared Radiant Heater
Radiant systems can also replace conventional heating methods in process heating. Since radiation
does not need to travel through a medium, more heating work can be accomplished in less space. The
response time when compared with convection heaters can prove to be an advantage in these industrial
applications. The shutdown time for an infrared burner varies from one to 30 seconds. Gas or electric
radiant heaters may be used for different heating applications. Applications include cooking, broiling,
melting and curing metals, curing and drying rubber and plastics, and preshrinking and finishing of textiles.
Providing the correct combustion controls will increase combustion efficiency measurably.
Complete combustion of natural gas yields carbon dioxide and water vapor. If gas is burned with out the
correct amount of air, an analysis of the products of combustion will show it contains about 11-12% CO2
and 20-22% water vapor. The remainder is nitrogen, which was present in the air and passed through the
combustion reaction essentially unchanged.
If the same sample of natural gas is burned with less than the correct amount of air (“rich” or
“reducing fire”), flue gas analysis will show the presence of hydrogen and carbon monoxide, products of
incomplete combustion. Both of these gases have fuel value, so exhausting them from furnaces is a waste of
fuel (see Exhibit 7.17).
If more than the required amount of air is used (lean or oxidizing flame), all the gas will be burnt
but the products of combustion will contain excess oxygen. This excess oxygen is an added burden on the
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 229
Heat: Furnaces and Burners
Notes
combustion system - it is heated and then thrown away thereby wasting fuel. The following steps should be
taken to upgrade burner and combustion controls to prevent these situations:
1. Use sealed burners. Make all combustion air go through the burner - open cage type burners are
very inefficient.
2. Use power burners. Inspirator or atmosphere burners have very poor mixing efficiency at low
inputs, especially for low pressure natural gas.
3. Install a fuel/air ratio control system.
Exhibit 7.17: Percent Excess Air From CO2 Reading
230 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
These systems feature fixed orifices in both gas and air streams, and these orifices are sized to pass
proportional amounts of gas and air at equal pressure drops, pressure drop signals are fed to a ratio controller
which compares them. One of the outstanding features of this system is that the air/fuel ratio can be
adjusted by turning a dial. Since a burner can be thrown off correct gas ratios by changes in ambient air
temperature and humidity, this ratio adjustment feature permits the operator to set the burner back to peak
operating efficiency with very little effort.
On multiple burner furnaces, the combustion products of all burners mix together before they reach
the flue gas sampling point. Furnaces should have manifolded flue gas outlets to obtain a common sampling
point for flue gas analysis. If, for example, some of the burners are unintentionally set lean, and others rich,
the excess air from the lean burners could consume the excess fuel from the rich burners, producing flue gas
with optimum CO2 and practically no free oxygen or combustibles. Samples of these gases could be
misleading and show correct air/gas ratio, when in fact they are not. Also, if a burner is set rich and the
excess comb ustibles in the flue gases find air in the stack and burn there, flue gas analysis will again suggest
that the burner is properly adjusted.
To overcome the problem of misleading flue gas analysis in multi-burner furnaces, metering
orifices should be installed on the gas lines to each burner. If pressure drops across all orifices are identical,
gas flow to each burner will be the same.
7.4.4 Furnace Pressure Controls
Furnace Pressure Controls afford additional energy savings, particularly on topflued furnaces. If a
furnace operates under negative pressure, cold air is drawn into it through badly fitted doors and cracks.
This cold air has to be heated, adding to the burden on the combustion system and wasting fuel. If the
furnace operates at high positive pressure, flames will sting out through doors, site ports and other openings,
damaging refractories and buckling shells. Ideally a neutral furnace pressure overcomes both these
problems. Automatic furnace pressure controls maintain a predetermined pressure at hearth level by
opening or closing dampers in response to furnace pressure fluctuations.
In summation, good air/fuel ratio control equipment and automatic furnace pressure controls are
two useful weapons for combating energy waste in heating operations. Properly applied, they also offer the
side benefits of improved product quality and shortest possible heating cycles.
7.4.5 Furnace Efficiency
Conventional refractory linings in heating furnaces can have poor insulating abilities and high heat
storage characteristics. Basic methods available for reducing the heat storage effect and radiation losses in
melt and heat treat furnaces are:
1. Replace standard refractory linings with vacuum-formed refractory fiber insulation material.
2. Install fiber liner between standard refractory lining and shell wall.
3. Install ceramic fiber linings over present refractory liner.
Refractory fiber materials offer exceptional low thermal conductivity and heat storage. These two
factors combine to offer very substantial energy savings in crucible, reverberatory and heat-treat furnaces.
With bulk densities of 12-22 lbs/cu ft, refractory fiber linings weigh 8% as much as equivalent volumes of
conventional brick or castables. In addition, refractory fibers are resistive to damage from ext reme and rapid
changes in temperature. These fiber materials are simple and fast to install. The density of fiber refractory
is low, therefore much less heat is required to bring the lining to operating temperature. This results in rapid
heating on the start-up. Conversely, cooling is also rapid, since there is less heat stored in the lining.
The basic design criteria for fiber lined crucible furnaces are the same as used for furnaces lined
with dense refractories. Two rules should be followed.
1. The midpoint of the burner should be at the same level as the bottom of the crucible, and the burner
should fire tangentially into the space between the crucible and lining.
2. The space between the outside of the crucible, and the furnace lining near the top should be about
10% of the crucible diameter.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 231
Heat: Furnaces and Burners
Notes
Crucible furnaces can be constructed using a combination of fiber with dense refractory or almost
entirely out of fiber. Increasing the proportion of fiber will increase the energy savings and maximize the
other benefits previously listed.
Fiber materials are available in varying thicknesses, suitable for a complete monolithic installation,
and composition to handle 2400°F, 2600°F, and 2800°F. The higher temperature compositions contain high
aluminum fiber, which lowers the amount of shrinkage at elevated operating temperatures.
7.4.6 Furnace Covers
Installation of furnace covers is necessary to reduce preheating of combustion air. Thermal shock
and spalling have caused problems in the fabrication and use of furnace covers. Materials available today,
such as refractory fiber, have eliminated these problems.
In addition to technological advantages of fiber insulation, industry has also developed the
capability of vacuum forming these materials over a variety of metallic support structures. Fiber insulation
can be formed over either expanded metal or angle iron frames, or both, with V-type anchors attached. The
anchors are made from high temperature alloys, holding the fiber to the metallic support structures to
provide an integral, fully secured assembly. No part of the anchor system is exposed to excessive
temperatures. This eliminates attachment problems for ladle pre-heaters, crucible furnace covers, and
induction furnace covers. Installation of furnace covers improves the thermal efficiency of the process by
approximately 50%.
7.5 Cogeneration
Cogeneration is the simultaneous production of electric power and use of thermal energy from a
common fuel source. Interest in cogeneration stems from its inherent thermodynamic efficiency. Fossil
fuel-fired central stations convert only about one-third of their energy input to electricity and reject two-
thirds in the form of thermal discharges to the atmosphere. Industrial plants with cogeneration facilities can
use the rejected heat in their plant process and thereby achieve a thermal efficiency as high as 80 percent.
7.5.1 The Economics of Cogeneration
In-plant generation of electricity alone is not usually economical; a variable use must be made of
the by-product waste heat. For this reason the demand for both types of energy must then be in balance,
typically 100 kW versus 600,000 Btuh, for a gas turbine installation.
In most potential applications of industrial cogeneration, more electric power would be produced in
meeting the plant’s thermal requirement than could be used internally. However, the enactment of PURPA
(Public Utility Regulatory and Policies Act of 1978) greatly expanded the application for cogeneration by
granting qualified cogenerators the right to:
• Interconnect with a utility’s grid
• Contract for backup power with the utility at nondiscriminatory rates
• Sell the power to the utility at the utility’s avoided cost.
There are several reasons for considering cogeneration besides energy savings.
• Energy independence
• Replacement of aging equipment
• Expansion of facilities
• Environmental considerations
• PURPA franchise to sell electricity
• Power factor improvement
However, plant conditions must fit certain requirements for a successful cogeneration application.
232 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
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Notes
Some factors are:
• The nature of the process must be suitable for cogeneration. Certain processes lend themselves
more readily to cogeneration, such as refining, petrochemical, and pulp and paper industries, which
have accounted for many of the larger cogeneration installations to date.
• The rate differential between electricity and fossil fuels should be relatively high on an equivalent
Btu basis.
• Plant operation of 6,000 hours per year is usually the minimum needed to justify installation and
continuous operation thus improving reliability by minimizing dependence on the starting system.
• A source of waste fuel in suitable quantity provides an attractive incentive for cogeneration.
Although plant conditions may appear favorable for cogeneration, the long-term situation should
also be considered before proceeding with a project. Factors that should be considered for long-term
evaluation of cogeneration are:
1. The long-range cost of fuel for gas- and oil-fired units must be considered
2. Excess coal-fired generating facilities and abundant coal supplies can result in increased
competition from utilities and lower avoided costs.
3. Utilities may press for repeal of PURPA or at least the ability to discount the avoided cost purchase
rate.
4. Long-term continuity of operations. Facilities that are expecting significant changes in operation or
ownership should determine the viability of the initially large investment.
5. Reliability requirements of the cogeneration facility will be important. Maintenance and reliability
of equipment is very important as the cost of penalty for additional utility charges for any outage
can be significant where demand charges are high.
Aside from long-term effects, other alternatives to cogeneration may negate some of its benefits.
These alternatives include renegotiation of electrical rates, load management, technology improvements,
process changes, and energy conservation.
1. Renegotiating rates may enable an industrial plant to duplicate the potential economic benefits of
cogeneration without the risk of building and operating a power plant.
2. Load management techniques may be able to modify peak demands.
3. Major technological improvements or process changes can occur and significantly alter the present
energy requirements.
4. Where available capital is limited, energy conservation may be able to reduce electrical
consumption significantly by using projects with more attractive returns.
7.5.2 Cogeneration Cycles
There are many possible types of cogeneration cycles but most can be considered variations of the
two basic cycles gas turbine and steam/turbine as shown in Exhibit 7.18.
In the case of the gas turbine cogeneration cycle, air is compressed and injected into the combustor
along with the fuel, generally natural gas. The combustion gases at high temperature and pressures expand
rapidly in the turbine, doing work in the process. The turbine drives an electrical generator and air
compressor. The exhaust gas from the turbine, which is still at a high temperature, is then used to generate
steam in a waste heat boiler.
The cost of a gas turbine with heat recovery equipment ranges between $600 to $1,000 per kW,
depending on the specific design conditions. Gas turbine systems costs are reduced by over 50 percent with
larger units.
There are several advantages of the gas turbine system in comparison with the steam/turbine
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 233
Heat: Cogeneration
Notes
system.
• Lower capital cost (normally 50 to 70 percent of steam/turbine cost)
• Lower operating and maintenance cost.
• Higher power-to-heat ratio that is generally more desirable in industrial applications.
A reciprocating engine, generally a diesel, can be used in lieu of the turbine to supply the motive power.
Since the exhaust from the engine is at a much lower temperature, only low pressure steam (maximum of 50
psig) or hot water can be generated without supplemental heating.
Exhibit 7.18: Cogeneration Cycles
234 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
Heat: Cogeneration
Notes
Exhibit 7.19: Gas-Turbine Cycle
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 235
Heat: Cogeneration
Notes
= [(30 MMBtu/hr) / (80% boiler eff.)] x (80,000 hrs) x ($3,00/MMBtu/hr) = $900,000/yr
Annual Saving = $688,000 + $900,000 - $1,056,000 = $532,000/yr
Investment = $1,500/kW x $1,720 kW = $2,580,000/yr
Payback = ($2580,000) / ($532,000) = 4.8 years
Exhibit 7.20: Steam-Turbine Cycle
Oil and gas-fired engine cogeneration systems are most suitable for smaller installations (under 1
MW). Packaged units are available from a few kilowatts to over a megawatt. The systems include a prime
mover, generator switchgear, heat recovery, and controls. Equipment costs range from $500 to $1,000/kW.
Installation costs for plumbing, electrical, and other facilities typically add 50 to 150 percent to the
equipment cost. Total turnkey costs range from $700 to $2,000/kW.
Experience with the smaller size units (under 100 kW) has been relatively short. In the
steam/turbine system, fuel is burned in a boiler to generate steam. The steam is passed through a topping
turbine that drives the electric generator. The exhaust steam is then used for process heating.
The greatest advantage of these systems is their ability to use practically any kind of fuel including
lower-cost solid or waste fuels, either alone or in combination. The capital cost of steam turbine systems is
higher, typically 50 to 100 percent greater than a gas turbine system using natural gas or oil.
7.5.2.2 Estimate of Savings
A high-spot estimate of savings should be made as early in the investigation as possible to confirm
that cogeneration is merited; a detailed energy-load analysis should be made. This involves preparing a
profile on the plant’s steam and electric usage, taking into account daily, weekly, monthly, and seasonal
variations. Using actual loads instead of average loads is important to determine whether periods of low-
load factor are a problem. System performance will be best where output is steady instead of fluctuating
with load.
With this data, plant personnel can select the most advantageous cogeneration cycle, taking into
account various possible operating conditions and equipment options. A computer model analysis is very
useful for this purpose. Equipment vendors can be utilized if outside assistance is needed to make the
computer analysis.
The options that can be considered are as follows:
• Combined cycle - permits the use of a flexible instead of fixed ratio of electrical to thermal energy
to adjust for variations in the steam demand
• Steam pressure - the higher the pressure the more efficient the turbine steam rate. When high-
pressure steam or gas must be reduced in pressure through a pressure-reducing valve, a simpler
system known as “induction generation” can be used to generate electricity.
• Steam injection - adds to turbine efficiency
• Extraction turbine - provides process steam for use at different pressures
236 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
Heat: Cogeneration
Notes
• Water treatment method - high-pressure steam turbines require more sophisticated boiler feed water
treatment
• Dual burners - burners capable of burning more than one fuel add flexibility to use lowest cost fuel
• Degree of automation - fully automatic systems increase price significantly
• Duct burner in exhaust stream - increases output and permits generation of higher pressure steam
• Steam condenser - permits additional electrical generation from steam turbine at some loss in
efficiency
• Generator type - power factor is improved with higher cost synchronous generator
• Parallel or independent operation will affect switchgear selection.
After the operating conditions and cogeneration facilities have been fully defined, the savings and
investment estimates should be revised to complete the initial evaluation of the cogeneration facility.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 237
Heat: Thermoenergy Storage Systems
Notes
7.6.2 Electric Load Analysis
A detailed electrical load analysis is necessary to determine the impact thermal storage will have on
the existing peak demand because of this interrelationship with other loads. Use of average loads will not be
satisfactory for this purpose.
The operating cost per ton for a thermal storage system is also higher than for a conventional
system. The refrigeration machine must operate at a lower temperature, which requires more energy per ton.
There is also some inherent loss in storage. One system reported that power consumption increased by 17
percent when the system was producing ice.
Exhibit 7.21 shows that incremental investment for thermal storage results in an attractive payback.
However, it should be emphasized that the example attributes maximize demand saving over the full year of
operation and for the full capacity of the unit. A well-documented analysis of all energy flows and costs is
needed for a more in-depth evaluation. A number of questions will also have to be answered as part of the
evaluation, such as:
• Should the thermal storage be for heating storage, cooling storage, or both?
• Should the system handle 100 percent of the cooling load or only the portion needed for load
leveling?
• Should the storage system be water or ice?
• Should the storage system be for a daily or weekly cycle?
Generally, systems have been for daily cycles and load levelers only.
Exhibit 7.21: Thermal Storage High Spot Evaluation
238 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency
Heat: Thermoenergy Storage Systems
Notes
REFERENCES
1. “Testing & Measurement” Bulletin 4011 Bacharch Inc.
2. “Boiler Efficiency” Bulletin CB 7767 Cleaver Brooks Co.
3. “Heat-Timer Model HWR” File No. 30-E-1 Heat Timer Co.
4. “Digi-Span” File No. 980M Heat Timer Co.
6. “Auto. Vent Damper” Form No. 60-2523 Honeywell Inc.
7. “Chronotherm 3” Form No. 68-0056-1 Honeywell Inc.
8. “Perfect Climate” Products Form 70-2317/8-92 Honeywell
9. “Flame Safeguard Manual” No. 708107 Honeywell
10. “Principles of Steam Heating” Dan Holohan
11. Maine Oil & Solid Fuel Board Rules
12. NFPA Code #31 Installation of Oil Burning Equipment
13. “Boiler Efficiency Improvement” Dyer/Maples
14. “Application Data for Burners” Form No. 30-60004A Iron Fireman Div. Dunham Bush Co.
15. Grainger, Inc., Air Circulators, Dayton Fans, p. 2318.
16. McMaster-Carr Supply Company, Net Prices Catalog.
17. Rutgers University, Industrial Assessment Report No. RU-00146, ECO No. 03, pp. 28-30.
18. Chase Industries, Bulletin No. 8102, 1981.
19. Colorado State University, Allied Signal Energy Conservation Training Program, 1994.
20. Buckley, Norman A., “Application of Radiant Heating Saves Energy,” ASHRAE Journal, V. 31, No. 9,
September 1991, p. 18.
21. Incropera, Frank P. and DeWitt, David P., Fundamentals of Heat and Mass Transfer, 3rd Ed., John
Wiley & Sons, 1990.
22. University of Tennessee, Industrial Assessment Report No. TN-0535, ECO No. 01, pp. 32-33.
23. Georgia Institute of Technology, Industrial Assessment Report No. GT-0541. ECO No. 02, pp. 8-11.
24. Marks’ Standard Handbook for Mechanical Engineers, McGraw-Hill Book Company, 1987.
25. Dyer, D. P., G. Maples, etc., Boiler Efficiency Improvement, Boiler Efficiency Institute, Auburn, AL,
1981, pp.4-31.
26. Witte, L.C., P. S. Schmidt, D.R. Brown, Industrial Energy Management and Utilization, Hemisphere
Publishing Corp., Washington, D.C., 1988, pp. 530-532.
27. Kennedy, W.J., W.C. Turner, Energy Management, Prentice-Hall, Englewood Cliffs, NJ, 1984.
Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency 239
Heat: Thermoenergy Storage Systems
Notes
240 Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency