Lab 4 manual - heat exchanger

ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

Experiment # 4

Shell & Tube Heat Exchanger

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

Experiment # 4: Shell & Tube Heat Exchanger 1.1.

Objective

Purpose of this experiment is to investigate a shell-and-tube heat exchanger, which uses water for both hot and cold fluid streams. Measurements are to be performed to improve the understanding of heat transfer phenomena under different mass flow rates and temperatures. 1.2.

Introduction

Heat exchangers are devices that facilitate the exchange of heat between two fluids that are at different temperatures while keeping them from mixing with each other. Heat exchangers are commonly used in practice in a wide range of applications, from heating and air-conditioning systems in a household, to chemical processing and power production in large plants. Heat exchangers differ from mixing chambers in that they do not allow the two fluids involved to mix. In a car radiator, for example, heat is transferred from the hot water flowing through the radiator tubes to the air flowing through the closely spaced thin plates outside attached to the tubes. Shell-and-tube heat exchangers contain a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell. Baffles are commonly placed in the shell to force the shell-side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Despite their widespread use, shell- and-tube heat exchangers are not suitable for use in automotive and aircraft applications because of their relatively large size and weight. Note that the tubes in a shell-and-tube heat exchanger open to some large flow areas called headers at both ends of the shell, where the tube-side fluid accumulates before entering the tubes and after leaving them.

Figure 1. The schematic of a shell-and-tube heat exchanger (one-shell pass and one-tube pass)

1.3

Theoretical Background

An energy flow in a heat exchanger is illustrated in Figure 1, where losses have not been included. For an ideal heat exchanger with no losses it is irrelevant whether the hot or cold fluid stream is used for calculation purposes (see Figure 2). The heat transfer rate exchanged ( Q h or Q c ) is calculated from the difference between the inlet and outlet heat transfer rates Q in and Q out .

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

Hot fluid side Q h ,out

Q h ,in

Q h ,c : Rate of heat exchanged Q c ,out

Q c ,in

Cold fluid side Figure 2. Energy flow within a heat exchanger with no heat losses The heat transfer rate is determined from the following equation.

Q = m C p T

(1)

This yields the heat transfer rates exchanged as follows: For hot fluid stream:  =Q    Q h h ,out − Q h ,in = m h C ph (Th ,out − Th ,in )

(2)

For hot fluid stream:  =Q    Q c c ,out − Q c ,in = m c C pc (Tc ,out − Tc ,in )

(3)

Assuming that there is no heat loss to the surrounding medium, and any heat transfer occurs between the two fluids only, the first law of thermodynamics requires that the rate of heat transfer from the hot fluid be equal to the rate of heat transfer to the cold one. That is,

 =Q  =Q  Q h ,c c h

(4)

If the two heat transfer rates in the hot and cold fluid sides are not the same, i.e., heat-exchanger losses are taken into account, the following relation is used.

Q + Q h Q hc ,m = c 2

(5)

The heat transfer rate in a heat exchanger can be expressed in an analogous manner to Newton’s Law of cooling as

Q = U As ΔTlm

(6)

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

with

ΔT1 − ΔT2 ΔT ln 1 ΔT2

ΔTlm =

(7)

Poor insulation causes losses in a heat exchanger. A distinction must be made between two cases when defining efficiency with allowance for such losses. Efficiency is basically defined as the relationship between yield and effort involved. In the case of cooling of a hot fluid, the yield is the heat given off by the hot fluid and the effort involved is the heat to be transported by the cooling medium.

η cool =

Q h Q c

(8)

In the case of heating of a cold fluid, the yield is the quantity of heat absorbed by the cold fluid while the effort involved is the heat to be transferred from the hot fluid.

η heat =

Q c Q h

(9)

The heat transfer effectiveness (ε) is defined as follows.

ε=

Q actual heat transfer rate = a maximum heat transfer rate Q max

(10)

with

Q max = C min (Th ,in − Tc ,in )  c C pc where C c = m

(11)

 h C ph are the heat capacity rates of the cold and the hot fluids, and C h = m

respectively, while Cmin is the smaller of Ch and Cc. The number of transfer units (NTU) is expressed as

NTU =

UAs C min

(12)

Two types of flow arrangement are possible in a shell-and-tube heat exchanger; parallel flow and counter flow. In a parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction (Figure 3a). In counter flow, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions (Figure 3b).

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

(a) Parallel flow

(b) Counter flow

Figure 3. Different flow regimes and associated temperature profiles in a shell-and-tube heat exchanger 1.4

Equipment

The heat exchanger unit is shown in Figure 4, while a schematic diagram of the heat exchanger in a counter flow argument and a picture of the heat exchanger to be used in the experiments are illustrated in Figures 5 and 6, respectively. The following four experiments may be performed using this unit. a) b) c) d)

Shell-and-tube heat exchanger (Exp.# 7) Plate heat exchanger (Exp. # 8) Tubular heat exchanger Jacketed vessel with stirrer and coil

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

Figure 4. Layout of the base unit

Master switch 1 Potentiometer for mixer speed (accessory) 2 Pressure switch for cold water supply valve 3 Tank water low indicator 4 Rocker switches for mixer 5 Heater 6 Temperature regulator 9 Pump 10 Display for flow rate 11 Connector for PC data acquisition card 12 2x cold water connectors with quick action hose couplings 13 2x hot water connectors with quick action hose couplings 14 Temperature in the heat exchanger 15 inlet temperature 16 Display change over switch cold water/hot water circuit 17 control valve for cold water flow rate 18 Control valve for hot water flow rate 19 Tank drain, cold water feed and drain 20 Base 21 Collecting tray

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

Figure 5. Schematic diagram of the heat exchanger in a counter flow arrangement

Position of Hot Water Circuit T1: Inlet temperature T2: Middle temperature T3: Outlet temperature

Position of Cold Water Circuit T4: Inlet temperature T5 : Middle temperature T6: Outlet temperature

Figure 6. A photo of the shell-and-tube heat exchanger

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

1.5

Operating Instructions and Procedure

During the experiments, a step-by-step procedure given below will be followed. • • • • • • • • • • • •

1.6. • •

Check water level in tank and top up if necessary. Switch on master switch. Set desired hot-water temperature at temperature controller Switch on heater from an ambient temperature of 20oC to 60oC requires approx. 20 min, while heating up start with bleeding procedure. Set counter-current by connecting hoses with base apparatus. Only change coldwater hoses! Otherwise there is a danger of scalding! Set a high cold-water flow rate with flow-control valve. Allow water to run until no more bubbles are visible. Switch on pump. Use flow-control valve to set high hot-water flow rate. Carefully open bleeder valve for hot-water flow and allow water to run for a short while Set desired flow rates at flow-control valves. Wait until the temperatures fluctuate by less than 1oC per minute. For this purpose it is sufficient to observe the two outlet temperatures at thermometers T3 and T6. Take temperature readings and enter them in the worksheet together with the set flow rates for countercurrent. Safety Instructions

There is a risk of electric shock. Always unplug first. Do not touch heated surfaces during or at the end of an experiment or place them near to items sensitive to heat. The heat source plate will become hot, up to 150°C!

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

1.7

Worksheet for Experimental Data WORKSHEET FOR EXPERIMENTAL DATA

Experiment #: 7

Experiment: Shell & Tube Heat Exchanger Parallel (Uniform) Flow Hot water Run #

Inlet temperature (oC)

Outlet temperature (oC)

T1=Th,in

T3=Th,out

Date:

Cold water Volumetric flow rate (l/min)  V

Inlet temperature (oC)

Outlet temperature (oC)

T4=Th,in

T6=Th,in

hot

Volumetric flow rate (l/min)  V cold

1 2 3 4 5 Average

WORKSHEET FOR EXPERIMENTAL DATA

Experiment: Shell & Tube Heat Exchanger Counter Flow Hot water

Run #

Inlet temperature (oC) T1 = Th,in

Outlet temperature (oC) T3 = Th,out

Experiment #: 7 Date:

Cold water

Volumetric flow rate (l/min)  =V  V 1

hot

Inlet temperature (oC) T4 = Th,in

Outlet temperature (oC) T6 = Th,in

Volumetric flow rate (l/min)  =V  V 2

cold

1 2 3 4 5 Average

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

1.8

Performing Calculations Symbol

Unit

Equation no.

2

Results

As

m

Tm,h (or Tav,h)

o

C

Tm,h = (Th,in +Th,out)/2

Tm,c (or Tav,c)

o

C

Tm,c = (Tc,in +Tc,out)/2

Cph

kJ/kg.oC

Cpc

kJ/kg.oC

ρh

kg/m3

ρc

kg/m3

 m h

kg/s

Using EES @Tm,h and 101.325 kPa Using EES @Tm,c and 101.325 kPa Using EES @Tm,h and 101.325 kPa Using EES @Tm,c and 101.325 kPa   h = ρh V m h

c m

kg/s

  c = ρc V m c

 Q h  Q

kW

From Eq. (2)

kW

From Eq. (3)

 Q hc , m

kW

From Eq. (3)

η cool

-

From Eq. (8)

c

ΔTln U

o

C

kW/m2. oC W/m2. oC

0.02

From Eq. (7) (Draw temperature profile) From Eq. (6)

Calculate the effectiveness and NTU values using the following relation (It is assumed that the heat exchanger is well-insulated and the same measured values for all the fluid temperatures and the mass flow rate of the hot fluid are

 /[C (T  c,cor = Q used). m h pc c , out − Tc ,in )] Ch

kW/ oC

 h C ph Ch = m

Cc,cor

kW/ oC

 c,cor C pc C c,cor = m

Cmin

kW/ oC

The smaller of Ch and Cc,cor

 Q max

kW

From Eq. (11)

 Q a

kW

 =Q  Q a h

ε

-

From Eq. (10)

NTU

-

From Eq. (12)

c

-

c = Cmin/Cmax

NTUgraph

-

Obtain NTU from Figures 23-26 on page 1059 of Ref. [2]

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ENGINEERING LABORATORY MANUAL for ENGR3930U-Heat Transfer

1.9

Analysis

a) Perform all the calculations given in the tabulated form (Section 1.8). b) Compare the experimental overall heat transfer coefficient with the representative values given in the literature. c) Comment on the experimental effectiveness and NTU values. d) Discuss whether the effectiveness–NTU method can be used instead of the log mean temperature difference (LMTD) method applied here or not. Nomenclature

As c C Cp  m NTU

Q

T U  V

: Heat transfer area (m2) : Capacity ratio (-) : Heat capacity rate (kW/oC or kW/K) : Specific heat capacity (kJ/kg.oC or kJ/kg.K) : Mass flow rate (kg/s) : Number of transfer units (-) : Heat transfer rate (kW) : Temperature (oC or K) : Overall heat transfer coefficient (W/m2K or kW/m2K) : Volumetric flow rate (m3/s)

Greek Letters ∆Tlm : Logarithmic main temperature difference (oC or K) η : Efficiency (-) ε : Effectiveness (-) Subscripts a : Actual c : Cold c : Corrected cool : Cooling cor : Corrected h : Hot heat : Heating in : Inlet lm : Logarithmic mean m : Mean max : Maximum min : Minimum out : Outlet p : Constant pressure s : Surface References

1. Equipment for Engineering Education, Instruction and Operation Manuals, Gunt Hamburg Germany 02/98. 2. Cengel, Y. A., and Turner, R. H. 2005. Fundamentals of Thermal-Fluid Sciences, 2nd Edition, McGraw-Hill.

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