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 Cold Water thi thmid tho tci tcmid tco

Temperature (oC) 51 48 46 29 35 39 Parallel HX Qc = 1 lpm Qh = 2 lpm Corres. Length (m) 0 0.75 1.5 0 0.75 1.5

Temperature (oC) 51 48 46 31 36 41

Counter HX Qc = 1 lpm Qh = 2 lpm Corres. Length (m) 0 0.75 1.5 1.5 0.75 0

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

Hea

t E

xcha

nger

Qc lp

m

Qh

lpm

mcc

c

mhc

h

(mc)

min

∆t c

∆t h

t hi -

t ci

mcc

c∆t c

mhc

h∆t h

є =

m

ccc∆

t c o

r m

hch∆

t h /

(mc)

min

(t

hi-t

ci)

Parallel Flow

1 2 69.75 139.5 69.75 10 5 22 697.5 697.5 0.45455

Counter Flow

1 2 69.75 139.5 69.75 10 5 20 697.5 697.5 0.5

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

Figure 2 - Parallel Flow Graph

Figure 3 - Counter Flow Graph

Hot Water Cold Water Qcold (lpm) Qho(lpm) thin

oC thmid oC tho

oC tcin oC tcmid

oC tco oC

1 2 66.5 61 59 29 39 46

1 2 61 56 54 29 37 44

1 2 56 52 50 29 36 41

1 2 51 48 46 29 35 39

Corr. Length for temperature (m) 0 0.75 1.5 0 0.75 1.5

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

Hot Water Cold Water Qcold (lpm) Qho(lpm) thin

oC thmid oC tho

oC tcin oC tcmid

oC tco oC

1 2 51 48 46 31 36 41

1 2 56 54 49 31 38 44

1 2 61 58 53 30 38.5 46

1 2 66 62 56 29 39 49

Corr. Length for temperature (m) 0 0.75 1.5 1.5 0.75 0

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

COUNTER FLOW HEAT EXCHANGER

Hot Water Cold Water Qcold

(lpm) Qhot (lpm) thin oC thmid

oC tho

oC tcin

oC tcmid

oC tco

oC

2 1 66 57 49 29 32 38

2 2 66 60 55 29 34 41

2 3 67 62 57 29 35 43

2 4 67 63 59 29 36 44

Corr. Length for temperature

(m) 0 0.75 1.5 1.5 0.75 0

PARALLEL FLOW HEAT EXCHANGER

Hot Water Cold Water Qcold

(lpm)

Qhot

(lpm) thin oC thmid

oC tho

oC tcin

oC tcmid

oC tco

oC

2 1 51 46 43 29.5 31.5 33.5

2 2 51 47 45 29.5 32.5 35

2 3 51 48 46.5 29.5 33 37

2 4 50.5 48.5 47 30 33.5 37.5

Corr. Length for temperature

(m) 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.