ME 3264 Lab 3: Cross-Flow Heat Exchanger 1

ME 3264: Lab 3

Department of Mechanical Engineering, University of Connecticut

Cross-Flow Heat Exchanger

Professor Wilson K. S. Chiu

Spring 2017

Objectives

The objectives of this experiment are:

(a) Understand and practice basic heat exchanger analysis methods.

(b) Measure the hot and cold fluid temperatures and mass flow rates.

(c) Calculate the experimental heat exchanger effectiveness and other performance metrics,

and compare to the predicted effectiveness obtained by the NTU method.

(d) Learn how to apply this analysis to determine cross-flow heat exchanger performance.

Background

A heat exchanger is an apparatus designed to transfer thermal energy from a relatively hot

fluid to a relatively cold fluid [1]. Heat exchangers are widely used in refrigeration, air

conditioning, heating, energy generation and chemical processing. In this laboratory, you will

study a cross-flow heat exchanger such as that used in the radiator of a car, in which the hot

liquid is cooled down by the air flow on the surface of the radiator. In this configuration, heat

transfer occurs between a flow of hot water and a flow of cool air circulating in an orthogonal

direction with respect to each other, so that hot flow and cold flow cross each other (crossed-

flow). This experiment serves as an introduction to the determination of a heat exchanger’s

performance and calculation of the fundamental quantities involved in the operation of a heat

exchanger, such as the heat exchanger’s overall heat transfer coefficient, number of transfer units

(NTU), and effectiveness.

ME 3264 Lab 3: Cross-Flow Heat Exchanger 2

Preparation

a) Reading Material

• Read Chapter 11 of the 7th edition of “Fundamentals of Heat and Mass Transfer” by

Bergman et al. [1].

• Read the Supplemental Sections Chapter 11S.1 of the 7th edition of “Fundamentals of

Heat and Mass Transfer” by Bergman et al. [1].

• Read the “Analysis and Calculations” section of this document (page 6).

b) Homework

1. Exhaust gas from a furnace is used to preheat the combustion air supplied to the furnace

burners. The gas, which has a flow rate of 15 kg/s and an inlet temperature of 1100 K, passes

through a bundle of tubes, while the air, which has a flow rate of 10 kg/s and an inlet

temperature of 300 K, is in cross-flow over the tubes. The tubes are unfinned, and the overall

heat transfer coefficient is 100 W/m2-K. The exhaust and the air may be each assumed to

have a specific heat of 1075 J/kg-K. Using the effectiveness-NTU method, determine the

total tube surface area required to achieve an air outlet temperature of 850 K.

2. Repeat Problem 1 using the LMTD method.

ME 3264 Lab 3: Cross-Flow Heat Exchanger 3

Experimental Description

a) Equipment Description

An educational unit from Edibon [2] shown in Fig. 1 will be used for this laboratory exercise.

 

Base unit: Cross-Flow Heat Exchanger unit:

         

Figure 1: Description and schematic of the cross-flow heat exchanger apparatus [2]. The components of the experiments labeled in Fig.1 are described as the following:

1. A stainless steel tank is used to heat the hot water. It includes:

- Heating element (AR-1).

- Level sensor (SN-1) to guarantee a specific level inside the tank.

- Temperature sensor (ST-16) of the water in the tank.

2. Pressure regulator. The regulator set to 0.7 bar to prevent overpressure in the unit.

!

!

ME 3264 Lab 3: Cross-Flow Heat Exchanger 4

3. AB-1: Variable speed centrifugal pump to pump the hot water.

4. VR-1: Valve to regulate the bypass (hot water). It is pre-set to a recommended flow rate

value for optimal operation.

5. VR-2: Valve to regulate the cold water flow rate (not used in this experiment).

6. SC-1: Hot water flow sensor.

7. SC-2: Cold water flow sensor (not used in this experiment).

8. V-1 and V-6: Hot water control ball valve and drain ball valve.

9. V-2, V-3, V-4 and V-5: Cold water circuit ball valves to set co- or counter-flow (not used

in this experiment).

10. AVE-1: Fan to drive air through the heat exchanger.

11. Radiator.

12. V-7 and V-8: Ball valve to control and drain the hot water.

13. ST-1: Hot water inlet temperature sensor.

14. ST-2: Hot water outlet temperature sensor.

15. ST-3: Air inlet temperature sensor.

16. ST-4: Air outlet temperature sensor.

17. SV-1: Air velocity sensor.

The functions of the base unit are for water heating and pumping, and for the measurement of

the hot water flow rates. Hot water flows through a closed circuit. An electrical heating element

(AR-1) immersed in the tank heats water to a set temperature (ST-16). Water leaves the tank by a

centrifugal pump AB-1. Part of the water enters the exchanger, and part of the wáter returns to

the tank via a bypass. The bypass has a valve to regulate the flow of hot water, VR-1. After the

water is cooled by going through the heat exchanger, it is passed through a flow sensor (SC-1) at

the exchanger’s outlet, then returns to the heating tank. The base unit has two ball valves to

control and drain the hot water: V-1 and V-6.

The heat exchanger consists of a finned radiator. This radiator is placed perpendicular to an

air flow generated by a fan. Heat carried by the hot water from the base unit is transferred to the

air flow. A series of thermocouples measures the inlet and outlet temperature for both the hot

water and air.

ME 3264 Lab 3: Cross-Flow Heat Exchanger 5

Base Unit Specifications

• Stainless steel tank (30 l) equipped with:

- Electrical heating element (3000 W) with thermostat (70 ºC) to heat the water.

- Level switch to control the water level in the tank.

- Temperature sensor to measure the water temperature.

- Stainless steel cover with orifice to visualize the water level.

- Water drain valve.

• Centrifugal pump to pump the hot water. Range: 0-3 l/min.

• 2 flow sensors (range: 0.25-6.5 l/min) or flowmeters, depending on the unit, one for the

hot water and the other for the cold water.

• Pressure regulation valve to prevent the introduction of too much pressure in the

exchangers, tared to 0.7 bars.

• 4 ball valves that, according to their position, allow a parallel or countercurrent flow of

the cold water in the exchanger (not used in this experiment).

• 2 ball valves to control and drain the hot water of the base unit.

• 4 flexible tubes to connect to different exchangers.

Cross-Flow Heat Exchanger Specifications

• Rectangular conduit made of PMMA (800 x 200 x 200 mm).

• Fan range: 0-600 m3/h.

• Radiator located perpendicular to the air conduit:

- Fins made of aluminum.

- Heat transfer area: 35,000 mm2.

• 4 “J” type thermocouples.

• Air velocity sensor. Range: 0- 4 m/s.

• 2 ball valves to control and drain hot water from the exchanger.

ME 3264 Lab 3: Cross-Flow Heat Exchanger 6

Analysis and Calculations

The setup allows for calculation of the amount of thermal energy transferred to the cold air,

qc, from the knowledge of its mass flow rate and temperature rise [1].

𝑞! = 𝑚!  𝑐!,!"# 𝑇!,!"# − 𝑇!,!" (1)

where 𝑚!   is the mass flow rate, 𝑐!,!"#  is the specific heat of air, and 𝑇!,!" and 𝑇!,!"# are the inlet

and outlet temperatures of the cold air, respectively. Similarly, the heat removed from the hot

water can be calculated as:

𝑞! = 𝑚!  𝑐!,!"#$% 𝑇!,!" − 𝑇!,!"# (2)

Due to the heat loss to the ambient, 𝑞!, the heat absorbed by the cold air stream is smaller than

that removed from the hot water stream. The overall energy balance at steady-state implies:

𝑞! = 𝑞! + 𝑞! (3)

It is noted that before the steady-state condition is reached, the above equation is not satisfied

due to the storage of thermal energy within the heat exchanger components. The heat transferred

to the cold air flow, 𝑞!, is the actual heat throughput of the heat exchanger. Since the inlet and

outlet temperatures of the hot and cold fluids are known from the measurements, the log mean

temperature difference (𝐿𝑀𝑇𝐷) can be calculated as [1]

∆𝑇!" =∆𝑇! − ∆𝑇!

ln ∆𝑇!∆𝑇!

=∆𝑇! − ∆𝑇!

ln ∆𝑇!∆𝑇!

(4)

Where ΔT1 and ΔT2 depend on the heat exchanger configuration. For the definition of ΔT1 and

ΔT2 for co-flow and counter-flow heat exchangers, the reader is referred to [1]. The 𝐿𝑀𝑇𝐷

shows the appropriate average temperature difference between the hot water and cold air streams

which drives heat transfer.

Although flow conditions are more complicated in a cross-flow heat exchanger, the

equations derived may still be used if the following modification is made to the log mean

temperature difference [1]:

∆𝑇!" = 𝐹∆𝑇!",!" (5)

ME 3264 Lab 3: Cross-Flow Heat Exchanger 7

That is, the appropriate form of ΔTlm is obtained by applying a correction factor to the value of

the LMTD that would be computed under the assumption of counter-flow conditions, ΔTlm,CF.

Algebraic expressions for the correction factor F have been developed for various shell-and-tube

and cross-flow heat exchanger configurations [1], and the results may be represented graphically.

Selected results are shown in Chapter11S.1 of [1] for common cross-flow heat exchanger

configurations.

At steady-state, the overall thermal resistance of the heat exchanger, R, is defined as [1]

𝑅 =1𝑈𝐴 =

∆𝑇!"𝑞!

(6)

where 𝑈 is the overall heat transfer coefficient and 𝐴 is the overall heat transfer surface area.

In order to predict the overall heat transfer rates when the outlet conditions of the hot water

and cold air streams are unknown, a method termed the effectiveness-NTU method is employed.

The effectiveness is a quantity that shows how close to the ideal condition a particular heat

exchanger is operating.

By definition, the effectiveness, 𝜀, shows the ratio of actual heat transfer in a heat

exchanger to the maximum possible heat transfer 𝑞!"# [1].

𝜀 =𝑞

𝑞!"# (7)

For any heat exchanger, it can be shown that the effectiveness is a function of the ratio

𝐶!"#/𝐶!"# and 𝑁𝑇𝑈, where the dimensionless quantity 𝑁𝑇𝑈 is the number of transfer units

defined as [1]

𝑁𝑇𝑈 =𝑈𝐴𝐶!"#

=1

𝐶!"#𝑅 (8)

Equations have been developed for calculation of the effectiveness for various types of heat

exchangers. For co-flow, counter-flow, and cross-flow (both fluids unmixed) heat exchangers,

the effectiveness can be calculated, respectively, from the following equations [1]

𝜀 =1− exp −𝑁𝑇𝑈 1+ 𝐶!

1+ 𝐶! (6)

𝜀 =1− exp −𝑁𝑇𝑈 1+ 𝐶!1− 𝐶! exp −𝑁𝑇𝑈 1+ 𝐶!

(10)

ME 3264 Lab 3: Cross-Flow Heat Exchanger 8

𝜀 = 1− 𝑒𝑥𝑝1𝐶!

𝑁𝑇𝑈 !.!! 𝑒𝑥𝑝 −𝐶! 𝑁𝑇𝑈 !.!" − 1   (11)

where 𝐶! = 𝐶!"#/𝐶!"#.

For more detailed analysis of heat transfer and fluid flow analysis of heat exchangers, the

reader may consult Chapter 11 and Supplemental Chapter 11S.1 of Fundamentals of Heat and

Mass Transfer [1].

Experimental Procedure

1. Check that the hot water valves of the base unit (V-1 and V-6) are open, and valves V7 and V-8 of the exchanger are also open.

2. Check that the heating tank is filled with water above the level switch.

3. Switch on the pump and the heating element (unit supply).

4. Set the tank temperature to 65 ºC (ST16).

5. Fix the hot water flow to 1.5 l/min (SC-1).

6. Start the fan and set it to the maximum air flow rate, and keep it constant throughout your experiment. Measure the air velocity at 4 discrete heights to calculate and record the average air velocity.

7. Adjust the tank temperature to a constant 65 ºC.

8. After the experiment has settled, record the temperature and flow rates in the table below.

9. Repeat for the other flow rates shown in the table, 1.7 l/min, 1.9 l/min, and 2.1 l/min.

ME 3264 Lab 3: Cross-Flow Heat Exchanger 9

Results and Analysis

Table for measurement collection during the laboratory exercise:

Measured Test 1 Test 2 Test 3 Test 4

Hot Water Tank

Temperature

ST16 (°C)

65 65 65 65

Hot Water Inlet

ST1 (°C)

Hot Water Outlet

ST2 (°C)

Air Inlet

ST3 (°C)

Air Outlet

ST4 (°C)

Hot Water Flow

Rate SC1 (l/min) 1.5 1.7 1.9 2.1

Average Air

Velocity

SV-1 (m/s)

Considering the measurements above, calculate the following:

• Experimental effectiveness (ε).

• Heat transferred by the hot water (qh).

• Log mean temperature difference between hot water and cold air (ΔTlm).

• Parameters: U⋅A, NTU and Cr.

• Effectiveness obtained by the NTU method (εNTU).

• Hot and cold water temperatures at the exchanger outlet obtained from the experimental

effectiveness (Th,o and Tc,o).

• Correction factor F for cross-flow heat exchanger analysis.

ME 3264 Lab 3: Cross-Flow Heat Exchanger 10

Test 1 Test 2 Test 3 Test 4

ε

qh (W)

ΔTlm (K)

U⋅A (W/K)

NTU

Cr

εNTU

Th,o (°C)

Tc,o (°C)

F

Discussion

1. Comment on the values obtained, comparing the experimental effectiveness (ε) with the

effectiveness obtained by the NTU method (εNTU). How does the value of qh compare to qc?

Do you expect them to be equal? If qh ≠ qc , what are the some of the causes?

2. Compare the average temperature values at the exchanger outlet with those obtained by using

the experimental effectiveness.

3. Comment on the impact of the correction factor F on the calculations.

References

[1] T.L. Bergman, A.S. Lavine, F.P. Incropera, D.P. Dewitt, Fundamentals of Heat and Mass Transfer, 7th ed., Wiley, Hoboken (2011), chapter 11; and Supplemental Sections Chapter 11S.1, downloaded at http://www.wiley.com/college/incropera. [2] Edibon Practical Exercises Manual for Unit TCIF, September 2014.