Mechanical Engineering Science 9 3Rd Year Laboratory Experiment
Mechanical Engineering Science 9 3Rd Year Laboratory Experiment
Mechanical Engineering Science 9 3Rd Year Laboratory Experiment
Contents
Abstract .................................................................................................................................................................................................. 1
Introduction ........................................................................................................................................................................................... 2
Objectives: ......................................................................................................................................................................................... 6
Theory .................................................................................................................................................................................................... 7
Sample Calculation ............................................................................................................................................................................ 7
Equipment .............................................................................................................................................................................................. 8
Procedure ........................................................................................................................................................................................ 10
Results & Discussion............................................................................................................................................................................. 11
Conclusion ............................................................................................................................................................................................ 12
Abstract ................................................................................................................................................................................................ 13
References ........................................................................................................................................................................................... 14
Bibliography ......................................................................................................................................................................................... 15
Introduction
What is a heat exchanger?
Heat exchangers are machines designed for the purpose of transferring heat, typically between two or more fluids at differing
temperatures, without the fluids having direct contact with each other. There are exceptions that use direct contact to facilitate
heat transfer but are less widespread that other conventional types. Depending on the purpose or the environmental conditions,
a heat exchanger can use working fluid combinations of gas-to-gas, liquid-to-gas, or in rarer conditions, liquid-to-gas. Since the
function of a heat exchanger is to transfer heat, it can function both as a heater (heat engine), or a cooler (reverse heat engine).
Heat exchangers are used in a variety of industries, primarily in industries that need strictly regulated temperature conditions,
such as in the oil, gas, petrochemical, and beverage industry, to keep fluids within a safe operating temperature. In most of these
large-scale cases, they are used within industrial plants or refineries, and primarily are used for refrigeration. For more sustainable
or eco-friendly oriented plants or processes, heat exchangers are used to capture heat, and reutilize it in other areas (such as
powering office space heaters for employees), thereby reducing running costs and even increasing overall production efficiency.
There are also smaller scale uses for heat exchangers, such as use within automobiles. Within a car engine, the regulation of oil
temperature and the engine fluid is essential for peak performance and the healthy lifespan of the engine and its parts. The
temperature of a fluid is linked to its viscosity and density, and any deviation out of a favorable range can cause issues in
performance. As such, an intercooler (which functions as a cross flow heat exchanger) is present in order to maintain thermal
equilibrium within the engine bay. Pictured above is an example of a downflow intercooler, which takes in the warm radiator
working fluid from the top, when then flows down the intercooler, cooling down as air flow across the front surface area of the
intercooler, after the heat is extracted from the working fluid, it returns back to the engine bay, and the cycle continues.
During the selection process of a heat exchanger, the parameters that would typically be taken into consideration would be the
construction material, expected pressure and heat characteristics of the surroundings and the working fluid, performance
parameters (temperature setup, flow, pressure gradient), frequency of maintenance, type of working fluids to be used and their
phases, the size of the heat exchanger and availability of resources. Based on these parameters, a specific type of heat exchanger
can be selected. The various types of heat exchangers are summarized below, based on their flow arrangement and physical
structure:
Parallel flow & Counter flow
The most rudimentary and common configuration of a heat exchanger is where the two fluids move in a concentric pipe layout.
Depending on the pipe inflow and outflow arrangements, the fluids either flow in the same direction (parallel flow), or flow in
opposite directions (counter flow).
Figure 2 & 3: Parallel flow, counter flow configurations Source: Engineer’s Edge
One disadvantage of parallel setups is due to the direction of fluid flow. There is a large buildup of temperature difference at the
ends of the system, which causes thermal stress due to the vastly opposing temperature gradients caused at one side of the
tubing. Due to thermodynamic principles of heat exchange, the maximum temperature attainable by the colder fluid will never be
greater that the lowest temperature of the warmer fluid. This is only a detriment if the desired outcome is to increase the
temperature of the cooler fluid.
Counterflow systems are preferred due to its many benefits over a parallel configuration. The temperature gradient between the
fluids over the entire pipe flow is more uniform, resulting in the thermal stresses experienced by the system are greatly reduced.
This uniform temperature difference also results in a more uniform rate of heat transfer from one fluid to another.
This type of heat exchanger uses conduction and convection in order to achieve its goal.
Cross flow
Figure 4 & 5: Finned & unfinned tubular heat exchangers, examples of flow configurations Source: Engineer’s Edge
A crossflow heat exchanger has two perpendicular flows of fluids and typically used with a gas-to- liquid working fluid
configuration. A common example of a crossflow heat exchangers is in steam condensers where the heat is delivered to the cooler
liquid, to convert it into a gas. Two subcategories of crossflow types exist, being finned tubular and unfinned tubular heat
exchangers.
Finned heat exchangers are typically used where air is used as the cooling fluid. The surface area available for heat transfer
determines the efficiency of the heat transfer process. Finned tubular exchangers are optimized for maximizing heat transfer
surface area, thereby being one of the most efficient heat exchangers available.
In a finned heat exchanger, the fluid is unmixed due to the fins preventing the fluids from traversing across them, acting as a
barrier. However, the unfinned configuration allows the air to mix. Both versions of the crossflow heat exchanger maintain an
unmixed flow in the tube.
This type of heat exchanger uses conduction and convection in order to achieve its goal.
Due to this enhanced flexibility and versatility, they are extensively used in the industry, and make up approximately 90% of the
total heat exchangers used, both commercially and industrially.
Figure 7: Components of a shell-and-tube heat exchanger Source: Çengel, Yunus A, and Afshin J. Ghajar. Heat and Mass Transfer:
The main body comprises of two sets of inlets and outlets for each fluid, one flowing through the tubes and on through the shell
structure. The tube fluid enters from the tube inlet, passes through the tube bundle, and exits through the tube outlet. Similarly,
the shell fluid starts at the shell inlet, passes over the tubes, and exits at the shell outlet. The headers on either side of the tube
arrangement create spaces for the tube fluid to collect, either directly after entry or before departure from the system.
Each tube contains a turbulator, which triggers turbulent flow within the fluid. This disruption reduces the probability of dissolved
solids accumulating the walls of the tube and shell. In essence, turbulent flow has a self-cleaning effect on the tank. A similar
process is triggered within the tube flow section by barriers known as baffles, which maximizes the thermal mixing between the
shell-side fluid and the tube-side fluids by redirecting the water stream multiple times on its path through the heat exchanger.
The shape and structure of these baffles can be altered to produce differing results based on the fluids used and the amount of
heat transfer desired.
The myriads of configurations of Shell-and-Tube heat exchangers can be grouped into three main classifications:
In this heat exchanger, the tube bundle is bent into a “U” shape and is held in place by a tubeplate. The bend allows for
thermal expansion to occur without having the need of using expansion joints. This particular class of heat exchanger is
used in the industry for high contrasting temperature gradients between the two working fluids. An apparent downside
due to the shape of the tubes is the fact that the inner walls are difficult to clean, making maintenance harder than the
other classes of shell-tube exchangers.
• Fixed tube heat exchanger
The fixed tube method uses simplicity in its ideology, to provide the most cost-effective configuration that is also easy to
manufacture. It, however, suffers with handling high temperature gradients between the two fluids due to the fact that
the tubes are fixed in place. This does not allow for significant expansion, as that would result in a risk of damage to the
structural integrity of the exchanger. Hence, the temperate difference between fluids is minimal.
This variant aims to fix the disadvantages raised by the other two types, while also assimilating their advantages. The
left end of the tube bundle is fixed in place, but the other end is free to expand. This allows for fluids with high
temperature gradients like the U-tube exchanger. The layout of the pipe bundle also allows for easy cleaning, making
maintenance easier. This is the best variant with regards to efficiency and maintenance, albeit at a greater monetary
cost.
Objectives:
There are a few objectives the lab focuses on, which are raised in order to understand the relevance of fluid flow in the tube
system, and how varying it can affect real life scenarios. Finding a correlation between the direction of flow and the overall
effectiveness of the heat transfer helps in determining methods to maximize heat exchanger efficiency in the real world. The
objectives can be summarized as such:
1. To determine the effectiveness of the heat exchanger based on the flow arrangement
2. To study effect of fluid temperature on counter flow heat exchanger performance
3. To study effect of fluid temperature on parallel flow heat exchanger performance
Theory
One of the general values of each heat exchanger is the rate of heat transfer. It can be defined by the following formula:
The performance of the heat exchanger is best described by the amount of heat transferred in the system. This can be determined
by using the following equation:
Where 𝑸̇ is the rate of heat transfer, 𝒎̇ is the mass flow rate , 𝒄𝒑 is the specific heat capacity of water, and 𝑻𝒊𝒏 is the inlet
temperature, and 𝑻𝒐𝒖𝒕 is the outlet temperature.
Equation 1 uses mass flow rate in order to calculate the heat transfer. However, during the experiment, values of volumetric flow
rate were measured. Therefore, the values have to be converted to match the equation, and is done via the following:
𝑚̇ = 𝜌 × 𝑄
Equation 2
Where 𝒎̇ is the mass flow rate, 𝝆 is the density of water and 𝑸 is volume flow rate of the fluid.
Sample Calculation
Heat transferred for fluid flows in parallel flow configuration, at 2.67 lpm:
Converting volumetric flow rate to mass flow rate:
For cold water:
2.67 𝑘𝑔
𝑚̇ = ( ) ∗ 1000 = 0.0445 ⁄𝑠
60 ∗ 1000
2.15 𝑘𝑔
𝑚̇ = ( ) ∗ 1000 = 0.0386 ⁄𝑠
60 ∗ 1000
𝑘𝑔 𝐽
𝑄̇𝑤𝑎𝑟𝑚 = 0.0386 ⁄𝑠 × 4186 ⁄𝑘𝑔𝐾 × (333.05 𝐾 − 327.05 𝐾) = 969.48 𝑊
Equipment
Experimental testing was conducted using a centrifugal GUNT W110 heat exchanger and a tube network. The pipe network
contained two pressure gauges, to measure upstream and downstream pressure heads. A variable frequency drive control panel
was used to control the impeller speeds, and a tachometer was used to measure the rotational speed of the impeller.
Complete list of equipment is as follows:
1. GUNT W110 heat exchanger
2. Tube network (one pair for warm fluid, one for cold fluid)
3. Temperature gauges (to measure inlet, outlet, and middle temperatures)
4. Temperature controls (to regulate the warm water inlet temperature)
5. Flowmeters (to measure flow rate at inlet and outlet)
6. Control panel (containing switches for power supply, and water delivery)
7. Valve-flow controls (to regulate flow rate of warm and cold sources)
8. Sink valve (used as the source of the cold water)
9. Exit pipe (for dispersing the cold water into the sink or the reservoir)
10. Water reservoir (to collect excess water dispersed form the cold water flow)
Figure 11 displays the entire laboratory setup, including the pipe and tube network, pump and sink, pressure gauges and
rotameter. The tubes and fittings are also seen, mounted on the heat exchanger. The figure also shows the temperature control
panel, voltage response analyzer and WILO pump. A comprehensive list, accompanied with a label describing each figure is
present below:
Figure 12: Control panel, power & water supply switches Figure 13: Temperature controller
Procedure
There is a range of flow rates for both the working fluids to choose from. When a certain flow rate and warm water temperature
combination has been chosen, the laboratory procedure can begin:
1. The tubes connecting the two flows of water are arranged to produce the counter flow configuration (where the
direction of working fluids are in the opposite direction)
2. The heat exchanger is switched on, by turning the red knob on the control panel clockwise, providing power to
the system.
3. The sink valve is opened, to supply the cold water flow. A water reservoir has been set up, in order to collect
excess water, in addition to an exit pipe that leads into the sink.
4. Next, the predetermined warm water temperature range is selected by entering the desired value into the
temperature controller.
5. The pump supplying warm water is then switched on, via the black switch labelled “pump”, directly above the
power supply knob. This starts circulating warm water through the system.
6. The two fluids are left to flow for between 3 to 4 minutes, before any readings can be plotted. This is done in
order to ensure the system reaches steady state flow.
7. The first set of readings are plotted for the highest with the flow rate in the system being the maximum.
8. The flow rates for both fluids are set to 2 litres per minute. The procedure in step 7 is repeated.
9. The flow rate for the hot fluids is lowered to 1 litre per minute, while the cold flow remains at 2 litres per
minute. The procedure in step 7 is repeated.
10. The flow rates for both fluids are lowered to 1 litre per minute. The procedure in step 7 is repeated.
11. The flow of both fluids are temporarily halted, in order to change the layout of flow.
12. The tubes connecting the two flows of water are arranged to produce the parallel flow configuration (where the
direction of working fluids are in the same direction)
13. Steps 2 through 10 are repeated, to derive all necessary values for the parallel flow configuration.
14. The heater, pump, and sink valve are all turned off. Then, the main power can be turned off. All water in any
pipe or tubing are to be drained. All valves are closed, ending the experiment
15. A final check in conducted to ensure the entire setup has been switched off, and that there are no leaks or
malfunctions of any kind.
Counter flow
Table 2: Tabulated values for counter flow
Parallel Flow
Heat Transferred (Watts)
1500
1000
500
0
0 0.01 0.02 0.03 0.04 0.05
Mass Flow Rate (kg/s)
Counter Flow
Cold Circuit Hot Circuit
Heat Transferred (Watts)
1200
700
200
Conclusion
This experiment focused on the effectiveness of various fluid flow configurations of the overall effectiveness of the heat transfer
of a shell and tube exchanger. This was achieved through running a series of tests, where the flow of both warm and cold fluids
were changed, and the heat exchanged was measured. These values were tabulated, and graphs of the mass flow rate versus the
heat transferred for both warm and cold circuits were plotted, for parallel flow and counter flow. These graphs revealed the trend
that the heat transfer is directly proportional to the increase of mass flow rate increases, which corresponded to the expected
relationship denoted in literature. The values also confirmed that the heat exchanger follows the laws of thermodynamics, as the
exit temperature of the warm fluid flow in both parallel and counter flow would always be greater than the exit temperature of
the cold fluid flow. This was observed in both configurations.
It was concluded that parallel flow was more effective that counter flow. This, however, is untrue, based on literature. This
discrepancy can be held up to the unreliability of flow rate produced by the cold-water system, and the fact that only sole readings
of each data plot were noted.
Abstract
Mass Flow Rate Vs Heat Transfer of warm water flow
Parallel flow Counter flow
Mass Flow rate kg/s Heat Transfer W Mass Flow rate kg/s Heat Transfer W
0.0383 993.3 0.0355 834.96
0.0333 770.0 0.0333 728.0
0.0167 469.0 0.0167 476.0
0.0167 406.0 0.0167 413.0
Bibliography
Çengel, Yunus A, and Afshin J. Ghajar. Heat and Mass Transfer: Fundamentals & Applications. New York: McGraw-Hill,
2011.