1.

20 1993 Fundamentals Handbook (SI)

Column 13. The effect of subcooling in the subcooler on sys- Path 2-4. Cold, concentrated solution (2) absorbs low-pressure
tem performance is primarily on the irreversibilityof the subcooler refrigerant (8 and 9) in the absorber in equilibrium with the evapo-
and on the main expansion valve. Increased heat transfer in the rator pressure.
subcooler results in a corresponding increase in its irreversibility. Path 4-5. Solution (4) is pumped to the generator via the heat
This is offset by the decrease in irreversibility of the expansion exchanger, where it is heated (5) by the solution leaving the
valve. Increasing the amount of subcooling decreases the total generator.
irreversibility of the system and changes irreversibility in other Path 5-1. Hot, dilute solution (5) enters the generator, where heat
components. is added to distill refrigerant (6). Hot, concentrated solution leaves
the generator (I).
Recommendations Path 6-7. Hot, high-pressure refrigerant vapor (6) condenses (7).
These analyses illustrate a method of systems analysis; iden-
tifying sources of irreversibilities in a system provides a basis GENERATOR CONDENSER
for selecting design parameters. However,optimum system design
considers the cost of system components G\ndthen compares the
cost of improving the performance of a given component with the
cost of improving all other components in the system.
An extension of this work, the mathematical modeling of the
system components, allows study of a given system for a range of HEAT
steady-state operating conditions. An additional extension, the HEAT
OUT
consideration of thermal properties of the system components, EXCHANGER
allows calculation of transient operating conditions.

ABSORPTION
REFRIGERATION CYCLES RESTRICTOR

Absorption refrigeration cycles are heat-operated cycles RESTRICTOR

in which a secondary fluid (the absorbent) absorbs the pri-

mary fluid (gaseous refrigerant) that has been vaporized in the VAPOR
evaporator.
In the basic absorption cycle, low-pressure refrigerant vapor is CD SPILLOVER
converted to a liquid phase (solution) while still at low pressure.
Conversion is made possible by the vapor being absorbed by a' ABSORBER CD
EVAPORATOR
secondary fluid-the absorbent. Absorption proceeds because of
the mixing tendency of miscible substances, and because of an HEAT OUT LOW
TEMPERATURE
affinity between absorbent and refrigerant molecules. Thermal IN
energy released during the absorption process must be released to
a sink. This energy arises from the heat of condensation, sensi- Fig. 20a Lithium Bromide-Water Single-Stage
ble heats, and heat of dilution. Absorption Refrigeration Cycle
The refrigerant-absorbent solution is pressurized in the solu-
tion pump and conveyed via a heat exchanger to the generator
where refrigerant and absorbent are separated, i.e., regenerated,
by a distillation process. A simple still is adequate for the sepa-
ration when the pure absorbent material is nonvolatile, as in the CONSTANT UB, OONCENTRAnON LINES

water-lithium bromide system. However, fractional distillation

equipment is required when the pure absorbent material is vola-
tile, as in the ammonia-water system. Refrigerant that is not essen-
tially free of absorbent hampers vaporization in the evaporator
(Threlkeld 1970). The regenerated absorbent normally contains
a substantial amount of refrigerant. If the absorbent material
tends to become solid, as in the water-lithium bromide system,
enough refrigerant must be present to keep the pure absorbent
~ CONDENSER
material in a dissolved state. Certain considerations, particularly a. PRESSURE

avoiding excessivelyhigh temperatures in the generator, make it g

>-
desirable to leavea moderate amount of refrigerant in the regener- Z
<l
ated absorbent. [5 EVAPORATCR
Figure 20a diagrams a basic absorption cyclethat uses a lithium '" PRESSURE

bromide and water solution as an absorbent and water as a ~

w
0::
refrigerant. Figure 20b shows state points of the pressure- LINE
temperature diagram for this pair in the basic absorption cycle.
The flow path for these schematics is as follows: EVAPORATOR
TEMPERATURE
ABSORBER
TEMPERATURE
GENERATOR
TEMPERATURE
Path 1-2. Hot, concentrated solution (I), in equilibrium with the TEMPERATURE
condenser pressure, leaves the generator; this solution is cooled
in the heat exchanger by the incoming solution and throttled to Fig. 20b Pressure-Temperature State Points for Lithium
the absorber (2). Bromide-Water Single-Stage Absorption Refrigeration Cycle
Thermodynamics and Refrigeration Cycles 1.21

Path 7-8. Hot, liquid refrigerant is expanded into the evaporator, ature to which it might be subjected. If a solid forms, it presumably
whereit is evaporated at low pressure and temperature with heat would stop flow and cause equipment shutdown.
from the cooled space. Cold, low-pressure refrigerant vapor (8) is Volatility ratio. The refrigerant should be much more volatile
absorbed by the solution in the absorber (3). than the absorbent so the two can be easily separated. Otherwise,
cost and heat requirements can prohibit separation.
ABSORPTION CYCLES IN PRACTICE Affinity. The absorbent should have a strong affinity for the
refrigerant under conditions in which absorption takes place
Inefficienciesin the basic absorption cycleare caused by sensible (Buffington 1949). This affinity (1) causes a negative deviation
heat losses, heats of solution, and vaporization characteristics of from Raoult's law and results in an activity coefficient of less than
the absorbing fluid. Conveying hot absorbent from generator into unity for the refrigerant; (2) reduces the amount of absorbent to
absorberwastesconsiderablethermal energy.A liquid-to-liquid heat be circulated and, consequently, the waste of thermal energy from
exchanger transfers energy from this stream to the refrigerant- sensible heat effects; and (3) reduces the size of the liquid heat
absorbent solution being pumped back to the generator, saving a exchanger that transfers heat from absorbent to pressurized
major portion of the energy. Use of this liquid heat exchanger is refrigerant-absorbent solution in practical cycles.Calculations by
shown in the flow diagram for a water-lithium bromide cycle Jacob et al. (1969)indicate that strong affinity has disadvantages.
(Figure 20a) and for an ammonia-water cycle (Figure 21). This affinity is associated with a high heat of dilution; conse-
Modifications for the basic cycle do not bring the coefficient quently, extra heat is required in the generator to separate refriger-
of performance over a threshold of unity, e.g., heat required to ant from absorbent.
generate a kilogram of refrigerant is not less than the heat taken Pressure. Operating pressures, largely established by physical
up when this kilogram evaporates in the evaporator. Performance properties of the refrigerant, should be moderate. High pressures
can be improved by using the double-effect evaporation principle necessitate use of heavy-walled equipment, and significant elec-
and a double-effect generator (Whitlow and Swearingen 1958). trical power may be required to pump the fluids from low side to
With the water-lithium bromide pair, two generators can be used: high side. Low pressures (vacuum) necessitate use of large volume
one,at high temperature and pressure, heated by an external source equipment and special means of reducing pressure drop in
of thermal energy; a second, at lower pressure and temperature, refrigerant vapor flow.
heated by condensation of the vapor from the first generator. Stability. Almost absolute chemical stability is required, because
Condensate from both generators moves to the evaporator. fluids are subjected to severe conditions over many years of serv-
ice. Instability could cause the undesirable formation of gases,
CHARACTERISTICS OF solids, or corrosive substances.
REFRIGERANT-ABSORBENT PAIR Corrosion. Since the fluids or substances created by instability
can corrode materials used in constructing equipment, corrosion
The materials that make up the refrigerant-absorbent pair inhibitors should be used. •
should meet the following requirements to be suitable for absorp- Safety. Fluids must be nontoxic and nonflammable if they are
tion refrigeration: in an occupied dwelling. Industrial process refrigeration is less crit-
Absence of solid phase. The refrigerant-absorbent pair should
ical in this respect.
Transport properties. Viscosity, surface tension, thermal diffu-
not form a solid phase over the range of composition and temper-
sivity, and mass diffusivity are important characteristics of the
REFLUX COIL refrigerant and absorbent pair. For example, low viscosity of the
fluid promotes heat and mass transfer and reduces pumping
problems.
CONDENSER
Latent heat. The refrigerant's latent heat should be high so the
circulation rate of the refrigerant and absorbent can be kept at a
HEAT OUT
minimum.
No known refrigerant-absorbent pair meets all requirements
TOWER listed. Ammonia-water and water-lithium bromide are two pairs
in extensive commercial use. The ammonia-water pair meets most
requirements, but its volatility ratio is too low, and it requires
high operating pressures. Furthermore, ammonia is a Safety

bJCD
CD _HEAT
GENERATOR

IN
Code Group 2 fluid (ASHRAE Standard 15-1989),restricting its
indoor use.
Advantages of the water-lithium bromide pair include high
safety, high volatility ratio, high affinity, high stability, and high
EVAPORATOR
latent heat. However, this pair tends to form solids. Since the
refrigerant turns to ice at O°C, the pair cannot be used for low-
10
temperature refrigeration. Lithium bromide crystallizes at moder-
11 ate concentrations, especially when it is air cooled, limiting the
ASSORa'R pair to applications where the absorber is water cooled. However,
HEAT
OUT
PERIODIC SPILLOVER using a combination of salts as the absorbent can reduce this crys-
tallizing tendency enough to permit air cooling (Macriss 1968,Weil
HEAT IN
1968, Rush 1968).Other disadvantages of the water-lithium bro-
mide pair include the low operating pressures it requires and the
lithium bromide solution's high viscosity. Proper equipment
design can overcome these disadvantages.
Other important refrigerant-absorbent pairs include the
following:
Fig. 21 Ammonia-Water Single-Stage • Ammonia-salts (Blytas and Daniels 1962,Robertson et al. 1966)
Absorption Refrigeration Cycle • Methylamine-salts (Robertson et al. 1966, Macriss et al. 1969)
1.22 1993 Fundamentals Handbook (SI)

• Alcohols-salts (Aker et al. 1965) RA X - Rc (X - I) = 1 (68)

• Ammonia-organic solvents (Robertson et al. 1966)
• Sulfur dioxide-organic solvents (Albright et al. 1963) where
• Halogenated hydrocarbons-organic solvents (Albright et al.
1962, Albright et al. 1960, Albright and Thieme 1961, Hesselberth RA = refrigerant mass fraction in solution leaving absorber
and Albright 1966)
Rc = refrigerant mass fraction in solution leaving generator
X = mass of solution flow from absorber/unit mass flow rate of
refrigerant
Several pairs of refrigerant-absorbents appear suitable for a
X - I = mass of solution flow from generator/unit mass flow rate of
relatively simple cycle and may not create as much of a crystalli- refrigerant
zation problem as the water-lithium bromide pair. However, sta- I = unit mass flow of refrigerant
bility and corrosion information on most of them is limited. Also,
the refrigerants, except for fluororefrigerants in the last type, are In this equation, the refrigerant in the dilute solution (from the
somewhat hazardous. Adding corrosion inhibitors, crystallization absorber) minus the refrigerant in the concentrated solution from
retardants, or heat transfer enhancers may correct these problems. the generator equals the unit mass flow of refrigerant being con-
sidered.
This equation can be rewritten:
THERMODYNAMIC ANALYSIS
First law analysis is useful for investigating new refrigerant- (1 - WFSA) X - (1 - WFSc) (X - 1) = I (69)
solvent pairs, improving cycles, finding effects of operating con-
ditions, troubleshooting test equipment, and related studies. The where
method is illustrated in the following data on the water-lithium WFS A = mass fraction of lithium bromide in solution from absorber
bromide and the ammonia-water machines. For convenience in WFSc = mass fraction of lithium bromide in solution from generator
making first law analyses and other calculations, see appropriate
Therefore, if the concentrations of lithium bromide are
charts in Chapter 17.
Second law analysis is useful for quantifying the inefficiencies, WFSA = 0.595 and WFSc = 0.646, Equation (69) becomes:
or thermodynamic irreversibilities, of each process in a cycle. In
such an analysis, the concept of availability-a measure of the (l - 0.595) X - (l - 0.646) (X - 1) = 1
work that could be produced from a flow stream if it were carried 00405 X - 0.354 (X - 1) = 1
in an ideal reversible manner to an equilibrium condition with the and X = 12.67
environment-is used. For instance, the availability of a hot fluid
stream is work that could be produced by cooling and expanding
Flow rates can be developed for a hypothetical system with
that fluid to atmospheric temperature and pressure while con-
certain assumptions or prior knowledge of typical system condi-
verting the resulting heat flow to work by an ideal Carnot cycle and
tions as in the following example.
the fluid expansion to work by an ideal expansion turbine. The loss .
of availability by irreversibilities in a process can never be re- Example 5. A large lithium bromide machine operating according to the
covered. The principal irreversibilities leading to this loss are heat flow diagrams of Figures 20a and 20b (including identifying numbers) has
transfer through a large temperature difference, friction, and the following conditions:
unrestrained expansion. Availability is a state function or property. I. Refrigeration load, 1000 kW
If kinetic and potential energy are negligible, availability is cal- 2. Evaporator temperature (point 8), 5 °C
culated from: 3. Absorber equilibrium temperature (point 3), 42°C
4. Actual solution temperature (point 4), 38°C
5. Solution temperature (point 5), 82.4 °C
(66) 6. Solution temperature (point I), 103.8°C
7. Solution temperature (point 2), 55°C
where 8. Refrigerant vapor temperature (point 6), 96.1°C
9. Refrigerant temperature (point 7), 45°C
B = availability per unit mass
h enthalpy per unit mass 10. Refrigerant spillover rate (point 9), 2.5% of (point 8)
II. Concentrations of solution per above
ho = enthalpy at ambient conditions
12. Chilled water temperatures, 12.7°C
To = ambient absolute temperature
S = entropy per unit mass 13. Cooling water temperature entering, 30°C
14. Assume no inerts present
So = entropy at ambient conditions
15. Cooling tower water flow rate, 0.0646 m3/s
Second law analysis of absorption cycle machines generally shows
The lithium bromide charts in Chapter 17and the steam tables in Kee-
that the generator produces the largest loss of availability. nan et al. (1969) may be used to find fluid flow rates, heat loads for the
various components, and the COP for the system.
WATER-LITHIUM BROMIDE MACHINE
Enthalpy kJ/kg
The determination of essential fluid flow rates is based on the 2515.0
Of (8), hv =
following relationships (Figure 20): Of (7), hi = 188.7
Difference, ilH = 2326.3
(67)
lO00kW
1.025 2326.3 = 0.4406 kg/s refrigerant
where (in consistent units)
10.35 x 0.4406 = 4.5603 kg/s dilute solution
RE = mass flow of refrigerant from evaporator - 0.4406 kg/s refrigerant
QE = heat load at evaporator
hv = enthalpy of refrigerant vapor from evaporator 4.1197 kg/s concentrated solution
hi = enthalpy of refrigerant liquid from condenser Approximate absolute pressure at generator = 9.60 kPa
Thermodynamics and Refrigeration Cycles 1.23

lithium bromide-water solutions and the COP are reduced for the
Enthalpy kJ/kg
same delivered chilled water temperature and available cooling
Of (9) h, 20.97
tower water temperature. At lower cooling tower water tempera-
Of (2) h, 174.52
tures, the concentrations and COP can be maintained, even
Of (4) h, 106.15
though the available heat source temperature for the generator is
Of (5) h, 193.48
reduced, within the practical limits for a given design.
Of (1) h, 261.85
Of (6) hv 2685.30
AMMONIA-WATER CYCLE
Material and heat balances at each component follow:
Absorber
Figure 21 diagrams an ammonia-water single-stage refrigera-
Heat in kJ/kg tion cycle. This resembles a lithium bromide-water refrigeration
(2) 4.118784 x 174.52 719.0
1081.1
cycle except, in large systems, the relative volatilities of ammonia
(8) (.4406/1.025) x 2515
.2 and water require a fractional distillation, accomplished with a
(9) (.4406/1.025) x 0.025 x 20.97
packed tower, a bubble tower, or a tower with sieve trays. Reflux
Subtotal = 1800.3
purifies the ammonia vapors coming off the top of the tower and
Heat out assures the least possible water content of the refrigerant. The
(4) 4.5603 x 106.15 484.1 concentrated solution of ammonia-in-water (aqua), after being
Absorber load (difference) 1316.2 heated by the heat exchanger, is fed at some intermediate point
Generator generally near the lower portion of the tower (or rectifier/ana-
Heat out kJ/s lyzer). The same required evaporator duty should be assumed
(1) 4.1197 x 261.85 1078.8 when analyzing the system and comparing it with a lithium
(6) .4406 x 268.3 1183.1 bromide-water cycle.
Subtotal = 2261.9 In smaller systems, the degree of ammonia vapor purity off the
Heat in top of the tower (or rectifier/analyzer) is generally less than that
(5) 4.5603 x 193.5 882.4 in the following example. As a result, tower and condenser pres-
Generator load (difference) 1379.5 sure is less, but water contamination of the refrigerant must be
Condenser constantly bled by liquid spillover from the evaporator to the
Heat in kJ/s absorber. (In larger systems, a high purity of ammonia vapor off
(6) .4406 x 2685.3 1183.1 the tower is maintained, and spillover from the absorber can be
Heat out a periodic occurrence.) To bleed refrigerant, the valve in the vapor
(7) .4406 x 188.7 83.1 line off the evaporator is throttled to establish a slight pressure
1100.0 differential between the evaporator and the absorber. Then, the
Condenser load (difference)
valve in the liquid spillover line is cracked open until the ammo-
Evaporator nia in the evaporator is purified sufficiently (detected by check-
1000 kW x I = 1000 kJ/s ing the pressure in the evaporator versus the temperature of the
Heat Balance liquid in the evaporator).
Heat in kJ/s In large systems, a vertical liquid leg under the evaporator
Evaporator load 1000.0 provides a relatively inactive area and accumulates ammonia rich
Generator load 1379.52 in water. The spillover line is tapped into this liquid leg. At an
Subtotal = 2379.5 evaporator pressure of 517 kPa, a 10070by mass water content
Heat out increases the refrigerant temperature from 5.1to 7.7°C for a 2.6°C
Absorber load 1316.2 penalty (Jennings and Shannon 1938).For pure ammonia offthe
Condenser load 1100.0 top of the tower (attained by keeping the superheat near 3.9°C),
2416.2 the operating pressure and temperatures for the tower, generator,
Balance within 1.52070 and condenser are relatively high, established by the coolant tem-
Evaporator Load perature available at the condenser.
COP = ------ = 0.725 In lithium bromide-water systems, the cooling tower water is fed
Generator Load
to the absorber and then to the condenser. In ammonia-water
With 2070heat loss to ambient, COP = 0.710 systems, the cooling t0wer water is fed first to the condenser and
The rate of steam flow or hot water flow to the generator can be deter-
then to the absorber. In both cases, coolant can be in parallel to
mined by the enthalpies of steam and hot water available with assumed improve efficiencies; however,this requires high coolant flow rates
condensate and hot water temperatures off the generator. For saturated and excessively large cooling towers.
steamat 170kPa and 2°C, subcooling of steam condensate in the gener- In the ammonia-water cycle, the reflux can be created by a
ator, the steam rate is: separate condenser or by the main condenser. Reflux can flow by
gravity or pump to the tower.Ammonia-water systems do not have
1379.5 x 1.036
----- = 0.643 m3 Is of steam at 170 kPa the potential crystallization of solution problem of lithium
(2698 - 474) bromide-water systems, and the controls can be simpler. Also, the
corrosion characteristics of ammonia-water solutions are less
and for hot water at 115°C with a 5.6 °C water range, the flow rate of hot
water is: severe,although inhibitors are generally used for both types of sys-
tems. Whereas lithium bromide-water systems use combinations
1379.5 x 0.001056 of steel, copper, and copper-nickel materials for shells and heat
------ = 0.06937 m3 Is of hot water
(482.9 - 461.9) transfer surface, no copper-bearing materials can be used in
ammonia-water systems.
The actual temperatures of the generator's heat source vary with For the ammonia-water cycle, Equation (67) determines the
the design heat transfer surface available and the application. For refrigerant flow rate, and Equation (70)develops the solution flow
lowertemperatures of steam or hot water, the concentrations of rate per unit refrigerant rate:
1.24 1993 Fundamentals Handbook (Sf)

WFSA (X) - WFSG (X - 1) (70) The enthalpy of (2) hi is determined by a mass-enthalpy flow rate
balance, as follows:
where
For 6070split (43070 WA):
WFS A = mass fraction of ammonia in solution from absorber
WFSG = mass fraction of ammonia in solution from generator
X = mass of solution from absorber per unit mass of refrigerant
166.2 - 9.5 [112.4 - (- 65.9)]18.5 = - 33.1 kJ/kg
flow
X - I = mass of solution from generator per unit mass of refriger-
ant flow For 30070split (19070WA):
For large systems, a reasonable pressure drop between the
evaporator and absorber is 10 kPa. 509.1 - 2.7 [112.4 - (-65.9)]11.7 = 2t5.9 kJ/kg
Example 6. A large ammonia-water absorption plant operating accord-
ing to the flow diagram of Figure 21 has the following conditions:
These enthalpy values correspond to the following temperatures for
1. Refrigeration load, 1758.4 kW solution entering the absorbers: for 6070 split-47.8 DC; for 30070
2. Evaporator temperature (point lO), 5.0°C split-83.1 DC.
3. Evaporator pressure (point lO), 517 kPa For a given strong ammonia (SA) concentration, tower pressure and feed
4. Absorber pressure (point 11), 507 kPa temperature, and purity of ammonia produced in the main condenser at
5. Strong aqua solution temperature (point 3), 41°C a given tower pressure, there is a minimum reflux rate to the top of the tower
6. Condenser temperature (point 8), 38°C that requires an infinite number of trays or infinite tower height. For
7. Condenser and tower pressure (point 7), 1460 kPa
ammonia-water mixtures using the enthalpy-concentration diagram of IGT
8. Concentration split (WFSG - WFS A)' 6070by mass (1964) and the procedure of Brown and Associates (1956), this minimum
9. Cooling tower water temperature, 29.4 °C reflux ratio for 49070SA at the feed condition of 78.8 °C for the tower pres-
Chapter 17 shows the enthalpy-concentration diagram for ammonia- sure of 1460 kPa is 0.0658 kg/kg of 99.95070 ammonia refrigerant feed to
water mixture and the ammonia (R-717) properties table. Assume a 3070 the evaporator. The practical reflux rate for a reasonable tower height is
increase in the theoretical refrigerant flow rate to accommodate heat losses at least 1.15 times this ratio or 0.0758 kg/kg refrigerant.
from the high-temperature shells and heat gain through insulation for the By constructing flow lines for fluid streams entering and leaving the
evaporator. tower, generator, and condenser, the combined condenser and reflux coil
From the ammonia-water diagram at 507 kPa and 41°C from the loads, as well as the generator load, can be determined with a mass enthalpy
absorber, assuming no subcooling of solution, the strong aqua (SA) has balance calculation.(Figure 22).
a concentration of 49070by mass of ammonia. A 6 to 8070concentration
split allows sufficient flow and adequate wetting of plain horizontal tubes Enthalpy kJ/kg
up to 25 mm in diameter in optimum arranged absorbers when using
gravity feed for large systems. It also ensures reasonable maximum liquid of (point 7) h;o 1468.80
flows for cost-effective exchangers and towers and a practical minimum of (point lO) h v 1450.26
temperature of heat source for the generator. Large splits reduce the flow . of (point 8) hi 361.27
rate, efficiency, and cost-effectiveness of absorbers and exchangers and Difference (lO - 8) I1h lO88.99
raise the required temperature of the heat source, unless a device minimiz-
ing these effects (a solution-cooled absorber) is used. With a 6070split, the Refrigerant flow rate:
concentration of ammonia in the weak aqua (WA) from the generator or
tower will be 43070by mass. At a 30070split, the concentration of WA will 1.03 x lOoo kW/1088.99 = 0.946 kg/s
be 19070by mass ammonia. Use of Equation (70) develops the solution flow
rates: .--------._-----.-----.------.
II
For 6070split: II
0.49X - 0.43 (X - 1) = 1 (!) :CONDEN8ER
0.06 X + 0.43 = 1
or X = 0.57/0.06 = 9.5
X - 1 = 8.5 REFLUX COI.

CD
For 30070split:

0.49X- 0.19(X-1) = 1
0.30X + 0.19 = 1
or X = 0.81/0.30 = 2.7
X - 1 = 1.7 TOWER

The SA is heated to within 1.7 °C of equilibrium in the tower to avoid

flashing in the feed valve to the generator. Consequently, overpressuriz-
ing the SA is avoided, reducing the energy and pressure requirements for
the SA pump. The equilibrium temperature of 49070aqua at 1460 kPa is
80.5 DC. Therefore, the SA temperature to the tower is 78.8 DC. For the 6070
split, the 43070WA leaves the tower or generator at an equilibium temper- GENERATOR I
ature of 90.5 DC. For the 30070split, the 19070WA leaves the tower or gener- Q G ••••••
~ •••
~
I ••••
ator at an equilibrium temperature of 144.4 dc. Enthalpy values to the aqua II
are as follows: III
I1._.
6070Split, 30070Split,
Enthalpy kJ/kg kJ/kg
CD
of (1) hi 166.2 509.1
of (4) hi 112.4 112.4
of (3) hi -65.9 -65.9
Fig. 22 Enthalpy Balance Method
 Thermodynamics and Refrigeration Cycles 1.25
Generator Heat
water with a temperature about 3°C above that for the lithium
For 6010 split: bromide-water cycle described earlier. For 6.7 °C chilled water and
Heat Out kJ/s a lower ammonia evaporator temperature, the procedure in the
Q, 0.946 x 361.27 341.8 example should be used to adjust the cycle's operating tempera-
Qr 0.946 x (1486.2 - 361.27) 1064.8 tures and COP.
Qr 0.167 x 0.946 (1486.8 - 361.27) 177.8
Subtotal of Q, + Qr + Qr 1584.4 REFERENCES
8.5 x 0.946 x 166.2 1336.4
Aker, lE., R.G. Squires, and L.E Albright. 1965.An evaluation of alcohol-
Total heat out 2920.8 salt mixtures as absorption refrigeration solutions. ASH RAE Trans-
Heat In actions 71(2):14.
Qf 9.5 x 0.946 x 112.4 1010.1 Albright, L.E et al. 1960. Solubility of R-11,R-21, and R-22 in organic
Qg Difference, generator heat 1910.7 solvents containing an oxygen atom. ASH RAE Transactions 66:423.
Albright, L.E et al. 1962. Solubility of chlorfluormethanes in nonvola-
For 30010 split: tile polar organic solvents. AICHE Journal 8:668.
Heat Out kJ/s Albright, L.E and A.A. Thieme. 1961.Solubility of refrigerants 11,21,and
22 in organic solvents containing a nitrogen atom and in mixtures of
Subtotal of Q, + Qr + Qr 1584.4 liquids. ASHRAE Journal 3:71.
Qwa 1.7 x 0.946 x 509.1 818.7
Benedict, M., G.B. Webb, and L.c. Rubin. 1940.An empirical equation
Total heat out 2403.1 for thermodynamic properties of light hydrocarbons and their mix-
tures. Journal of Chemistry and Physics 4:334.
2.7 x 0.946 x 112.4 287.1 Benedict, M. 1937. Pressure, volume, temperature properties of nitrogen
Difference, generator heat 2116.0 at high density, I and II. Journal of American Chemists Society
59(11):2224.
Absorber Load· BIytas, G.C. and E Daniels. 1962. Concentrated solutions of NaSCN in
For 6010split: liquid ammonia. Journal of American Chemical Society 84:1075.
Heat In Briggs, S.w. 1971.Second law analysis of absorption refrigeration. AGA
kJ/s
and IGT Conference on Natural Gas Research and Technology,
Point 11 0.946 x 1450.26 1371.9 Chicago, IL.
Point 2 8.5 x 0.946 x (-33.1) -266.2 Brown and Associates. 1956. Unit operations, 6th ed. John Wiley and
Subtotal heat in Sons, Inc., New York, 325.
1105.7
Heat Out Buffington, R.M. 1949.Qualitative requirements for absorbent-refrigerant
combinations. Refrigerating Engineering 4:343.
Point 3 9.5 x 0.946 x (-65.9) -592.2 Cooper, H.w. and J.C. Goldfrank. 1967.B-W-RConstants and new corre-
Qa Difference, absorber load 1697.9 lations. Hydrocarbon Processing 46(12):141.
For 30010 split: Haseltine, lD. and E.B. Qvale. 1971.Comparison of power and efficiency
Heat In of constant-speed compressors using three different capacity reduc-
kJ/s tion methods. ASHRAE Transactions 77(1):158.
Point 11 0.946 x 1450.26 1371.9 Hesselberth, J.F. and L.E Albright. 1966.Solubility of mixtures of R-12
Point 2 1.7 x 0.946 x 225.9 363.3 and R-22 in organic solvents oflow volatility. ASHRAE Transactions
Subtotal heat in 72(1):198.
1735.2
Heat Out Hirschfelder, J.0. et al. 1958.Generalized equation of state for gases and
liquids. Industrial and Engineering Chemistry 50:375.
Point 3 2.7 x 0.946 x (-65.9) -168.3 Hust, lG. and R.D. McCarty. 1967. Curve-fitting techniques and appli-
Qa Difference, absorber load 1903.5 cations to thermodynamics. Cryogenics 8:200.
Heat Balance Hust, J.G. and R.B. Stewart. 1966. Thermodynamic property computa-
tions for system analysis. ASHRAE Journal 2:64.
6010 Split 30010 Split
Heat In IGT. 1964. Physical and thermodynamic properties of ammonia-water.
kJ/s kJ/s
Institute of Gas Thchnology, Research Bulletin No. 34, Chicago, IL.
Evaporator 1030.1 1030.1 Jacob, X., L.E Albright, and W.H. Tucker. 1969. Factors affecting the
Generator 1910.7 2116.0 coefficient of performance for absorption air-conditioning systems.
2940.8 3146.1 ASHRAE Transactions 75(1):103.
Heat Out Jennings, B.H. and EP. Shannon. 1938.The thermodynamics of absorp-
Condenser tion refrigeration. Refrigerating Engineering 35(5):338.
1064.8 1064.8
Reflux coil Keenan, lH., EG. Keyes, P.G. HilI, and lG. Moore. 1969. Steam tables:
177.8 177.8
Absorber 1697.9 Thermodynamic properties of water including vapor, liquid, and solid
1903.5 phases. John Wiley and Sons, Inc., New York.
2940.5 3146.1 Macriss, R.A. 1968.Physical properties of modified LiBr Solutions. AGA
1030.1 1030.1 Symposium on Absorption Air-Conditioning Systems, February.
COP = Macriss, R.A., SA. Weil, and W.E Rush. 1969.Absorption refrigeration
1.03 x 1910.7 1.03 x 2116.0
system containing solutions, of monomethylamine with thiocyanate.
or 0.523 0.473 U.S. Patent 3,458,445, July 29.
'See Figure 21. Martin, J.J. and Y. Hou. 1955. Development of an equation of state for
gases. AICHE Journal 1:142.
The most cost-effective cooling water system, condensers, and Patel, Y.P. 1969. A thermodynamic analysis of a two-stage compression
absorbers determine the cooling tower water flow rate. hi the refrigeration cycle. Worcester Polytechnic Institute, Worcester, MA.
ammonia-water cycle, a 0.114.m3 /s cooling water rate raises the Qvale, E.B. 1971a.The development of a mathematical model for the study
temperature 1O.9°C for a 6010 split and 11.7 °C for a 30% split in of rotary-vane compressors. ASH RAE Transactions 77(1):225.
concentration. This is about a 25% greater range than for the Qvale, E.B. 1971b.Parametric study oftheeffect of heat exchange between
lithium bromide-water cycle operating at the same evaporator the discharge and suction sides of refrigerating compressors. ASHRAE
temperature. Transactions 77(1):152-57.
A large conventional flooded water cooler in the ammonia Roberson, J.P. et al. 1966. Vapor pressure of ammonia and methylamine
in solution for absorption refrigeration system. ASHRAE Transactions
system of the example using plain steel tubes produces chilled 72(1):198.
1.26 1993 Fundamentals Handbook (81)

Rush, W.E 1968. The stability of LiBr-LiSCN solutions in water. AGA Whitlow, E.P. and 1.S.Swearingen. 1958.An improved absorption refriger-
Symposium on Absorption Air-Conditioning Systems, Chicago, ation cycle. Gas Age 122(9):19.
February.
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