Heat Exchanger Lab Report

Experiment no. 4 Concentric Tube Heat Exchanger Objective: 1. To study the working principle of parallel flow and counter flow heat exchangers. 2. To study effect of fluid temperature on counter flow heat exchanger performance 3. To study effect of fluid flow rates on heat exchanger performance

Theory: A heat exchanger is a piece of process equipment in which heat exchange takes place between two fluids that enter and exit at different temperatures. The primary design objective of the equipment may be either to remove heat from a hot fluid or to add heat to a cold fluid. Depending upon the relative direction of fluid motion, shell-and-tube heat exchangers are classified as parallel flow, counter flow, cross flow. In parallel flow, the hot and cold fluids flow in the same direction and therefore enter the exchanger on the same end and exit the exchanger on the same end. In counter flow, the two fluids flow in opposite directions and thus enter the exchanger and exit the exchanger from opposite ends. Cross flow heat exchangers will not be analyzed as a part of this laboratory experiment.

Figure 1 - Diagram of Parallel and Counter Flow Configurations

Observations:

Hot Water thmid

tho

tci

Cold Water tcmid

48

46

29

35

39

0

0.75

1.5

0

0.75

1.5

51 0

48

46

31

36

41

0.75

1.5

1.5

0.75

0

thi

Parallel HX Qc = 1 lpm Qh = 2 lpm

Temperature ( C) Corres. Length (m)

51

Counter HX Qc = 1 lpm Qh = 2 lpm

Temperature (oC) Corres. Length (m)

o

tco

thi - tci

є= mccc∆tc or mhch∆th / (mc)min (thi-tci)

∆th

mhch∆th

∆tc

mccc∆tc

(mc)min

2

69.75 139.5

69.75

10

5

22

697.5 697.5

0.45455

1

2

69.75 139.5

69.75

10

5

20

697.5 697.5

0.5

mhch

1

mccc

Qh lpm

Parallel Flow Counter Flow

Qc lpm

Heat Exchanger

Table 1 - Raw Data for Parallel Flow and Counter Flow Effectiveness Calculation

Table 2 - Calculation for Parallel Flow and Counter Flow Effectiveness Calculation

Figure 2 - Parallel Flow Graph

Figure 3 - Counter Flow Graph

Qcold (lpm)

Hot Water thin oC

Qho(lpm)

1 2 1 2 1 2 1 2 Corr. Length for temperature (m)

66.5 61 56 51 0

Cold Water

thmid oC 61 56 52 48

tho oC 59 54 50 46

tcin oC 29 29 29 29

tcmid oC 39 37 36 35

tco oC 46 44 41 39

0.75

1.5

0

0.75

1.5

Table 3 - Raw Data for Water Temperature Variation in Parallel Flow

Qcold (lpm)

Hot Water thin oC

Qho(lpm)

1 2 1 2 1 2 1 2 Corr. Length for temperature (m)

51 56 61 66 0

Cold Water

thmid oC 48 54 58 62

tho oC 46 49 53 56

tcin oC 31 31 30 29

tcmid oC 36 38 38.5 39

tco oC 41 44 46 49

0.75

1.5

1.5

0.75

0

Table 4 - Raw Data for Water Temperature Variation in Counter Flow

COUNTER FLOW HEAT EXCHANGER Qcold (lpm) Qhot (lpm) 2 1 2 2 2 3 2 4 Corr. Length for temperature (m)

Hot Water

Cold Water

thin oC 66 66 67 67

thmid oC 57 60 62 63

tho oC 49 55 57 59

tcin oC 29 29 29 29

tcmid oC 32 34 35 36

tco oC 38 41 43 44

0

0.75

1.5

1.5

0.75

0

PARALLEL FLOW HEAT EXCHANGER Qhot Qcold (lpm) (lpm) 2 1 2 2 2 3 2 4 Corr. Length for temperature (m)

Hot Water o

o

Cold Water o

o

thin C 51 51 51 50.5

thmid C 46 47 48 48.5

tho C 43 45 46.5 47

tcin C 29.5 29.5 29.5 30

tcmid oC 31.5 32.5 33 33.5

tco oC 33.5 35 37 37.5

0

0.75

1.5

0

0.75

1.5

Table 5 - Raw Data for Flow Rate Variation in Counter & Parallel Flow

Figure 4 & Figure 5 - Temperature Graphs for Varying Flow Rates of Qhot with Constant Qcold

Figure 6 & Figure 7 - Temperature Graphs for Varying Flow Rates of Qhot with Constant Qcold

Analysis: From the data in Table 1, the general characteristics of parallel flow and counter flow heat exchangers can be observed. In the parallel flow configuration, the exit temperature of the hot fluid must be higher than the exit temperature of the cold fluid. This is supported by the data taken. In the counter flow configuration, the exit temperature of the hot fluid must be higher than the entrance temperature of the cold fluid, but it does not necessarily need to be higher than the exit temperature of the cold fluid. This is also supported by the data, even though in this case

the exit temperature of the hot fluid is still hotter than the exit temperature of the cold fluid. From the calculations resulting in overall effectiveness, it is shown that the counter flow heat exchanger is more effective than the parallel flow heat exchanger. This supports generally held knowledge and experimental data concerning the two types of heat exchanger, governed by the Clausius Statement. Additionally, in the counter flow heat exchanger, had the exit temperature of the cold fluid been hotter than the exit temperature of the hot fluid, the effectiveness would have been even higher, reflecting common data in many textbooks. From the data in Table 3 and Table 4, the temperature differences under constant flow rates are shown. Under constant flow rate conditions, the ratio between temperature differences is also constant. If there is a rise in the temperature difference of the hot fluid, there will also be a rise in the temperature difference in the cold fluid. This is governed by a special case of the First Law of Thermodynamics. In this case, the energy is transferred from hot to cold fluids with constant mass flow rates. Therefore the ratio between temperature differences does not change even though the numerical values of the temperature differences may change. From the data in Table 5, the temperature differences under different flow rates are shown. In this case, the ratio between temperature difference in the hot fluid and temperature difference in the cold fluid changes with respect to the flow rates. This is governed by the First Law of Thermodynamics. In this case, the energy removed from the hot fluid is the energy added to the cold fluid. The higher the flow rate of a fluid, the lower the temperature change in that fluid will be. The opposite is also true, the lower the flow rate of the fluid, the higher the temperature change in the fluid will be.

Conclusions: The heat exchanger apparatus follows the basic laws of thermodynamics and this can be shown experimentally. From all of the parallel flow configurations, the exit temperature of the hot fluid is always hotter than the exit temperature of the cold fluid. This supports the Clausius Statement in which heat may not spontaneously transfer from a colder body to a hotter body. From the other experiments that hold flow rates constant or vary the flow rates, it is clear that the First Law of Thermodynamics and conservation of energy applies to the heat exchanger apparatus. In practical application, the counter flow configuration is preferred for its higher effectiveness. This experiment did show that this configuration does in fact have a higher effectiveness than the parallel flow configuration. Additionally, the counter flow configuration is also capable of have a cold fluid exit temperature that is higher than the hot fluid exit temperature. This was not shown experimentally, however from the data collected it is clear that the flow rates were too high to achieve this desired result. If the experiment were repeated with lower flow rates, it would be possible to demonstrate a situation where the exit temperature of the cold fluid is hotter than the exit temperature of the hot fluid.