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Last Rev.: 11 JUN 08SHELL & TUBE HEAT EXCHANGER : MIME 3470

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Grading Sheet

~~~~~~~~~~~~~~MIME 3470Thermal Science Laboratory~~~~~~~~~~~~~~Laboratory 14~~~~~~~~~~~~~~

Shell-and-Tube Heat Exchanger

~~~~~~~~~~~~~~

Students Names / Section POINTS SCORETOTAL

APPEARANCE, ORGANIZATION, ENGLISH/GRAMMAR5

ORDERED DATA, CALCULATIONS & RESULTS

ORDERED DATA5

CALCULATE HOT & COLD AVERAGED MEAN TEMPS,

5

INTERPOLATED PHYSICAL DATA AT APPROPRIATE TEMPS5

CALCULATE HOT AND COLD FLOW RATES, Cmax, Cmin, and Cr5

CALCULATE TUBE-SIDE HEAT TRANSFER COEFFICIENT5

CALCULATE AVERAGE FLOW AREA ON SHELL SIDE5

CALCULATE SHELL-SIDE HEAT TRANSFER COEFFICIENT5

INTERPOLATE C1 & m BOTH VERTICALLY & HORIZONTALLY5

CALCULATE OVERALL HEAT TRANSFER COEFFICIENT5

CALCULATE NTU5

CALCULATE EFFECTIVENESS5

CALCULATE OUTLET HOT WATER TEMPERATURE5

CALCULATE OUTLET COLD WATER TEMPERATURE5

CALCULATE PERCENTS ERROR5

SUMMARY TABLE OF RESULTS5

DISCUSSION OF RESULTS

HOW GOOD IS THE NTU METHOD?5

EXPLAIN SOURCES OF ERROR5

CONCLUSIONS5

ORIGINAL DATASHEET 5

TOTAL100

COMMENTS

GRADERd

MIME 3470Thermal Science Laboratory~~~~~~~~~~~~~~Laboratory . 14Shell-and-Tube Heat Exchanger

~~~~~~~~~~~~~~

Lab Partners: Name Name

NameName

NameNameSectionExperiment Time/Date:Time, date

INTRODUCTIONMany engineering applications involve a process of heat exchange between two fluids. Heat exchangers are devices used to promote the heat transferred between two fluids; e.g., a car radiator and the condenser units on air conditioning systems. Space heating, air conditioning, power production, and chemical processing are typical areas of application. There are many heat exchanger designs. The laboratory setup for this experiment contains three heat exchanger types: a shell-and-tube exchanger, a concentric tube exchanger, and a tube bank exchanger in cross flow. This particular experiment employs the shell-and-tube type heat exchanger (see Figure 1). A shell-and-tube heat exchanger is constructed of tubes that are attached on each end by a plate, called the tube sheet, through which the tubes pass. One fluid streams into the inlet of the heat exchanger, flows through the tubes, and exits through the tube sheet at the opposite end of the heat exchanger.

A shell encloses the internal volume where the tubes are housed. Another, fluid flows through the shell and heat is exchanged between the tube-side fluid and the shell-side fluid. In a power plant, most heat exchangers are of the shell-and-tube design. The number of passes commonly presents a further description of a shell-and-tube heat exchanger. A single pass means the fluid flows straight through the entire heat exchanger without changing direction and so, in this design, the fluid moves past the length of the heat exchanger only a single time. In a two-pass heat exchanger the fluid in the tubes goes in one end, flows to the other end, reverses direction then flows back to the same end that the fluid entered through a second set of tubes. Thus, the fluid travels the full length of the heat exchanger twice. Similarly, multiple pass heat exchangers are so named because they make many passes. This experiment employs a shell-and-tube heat exchanger consisting of two tube passes and one shell pass. THEORY: HEAT EXCHANGER ANALYSISThermodynamics and the First Law dictate the overall energy transfer in a heat exchanger. There are two widely used methods of heat exchanger analysis, the NTU-Effectiveness method and the Log-Mean-Temperature-Difference (LMTD) method. These are briefly discussed below. Log-Mean-Temperature-Difference (LMTD) Method For a heat exchanger between two fluids with given inlet and outlet temperatures, there are three equations for the rate of heat transfer, Q,

Q=Rate of heat transfer, W

=

=

=

where,

=mass flow rate of fluid j, kg/s

=specific heat of fluid j, J/(kg(K)

T=temperature, (C

i(inlet o ( outlet

U=overall heat transfer coefficient, W/(m2(K)

A=area of surface across heat transfer occurs, m2For known specific heats, U, A, and entering temperatures, the three equations above can be solved for three unknownsT1,o, T2,o, and Q by successive substitution of one of the equations for Q onto another. It is a simple matter to use the log-mean-temperature-difference method of heat exchanger analysis when the fluid inlet temperatures are known and the outlet temperatures are specified or readily determined from the energy balance expressions. The value of (Tlm for the exchanger may then be determined. However, if only the inlet temperatures are known, use of the LMTD method requires an iterative procedure. In such cases, it is preferable to use an alternative approach, termed the NTU-Effectiveness method. NTU-Effectiveness MethodOften, when working with a given heat exchanger one must predict the outlet temperatures given the inlet temperatures. As the dimensions of the exchanger are known, the NTU-effectiveness method is a popular way to perform this task. This is an easy method to calculate the overall heat transfer rate, Q. The number of (heat) transfer units, NTU, is a dimensionless parameter which precipitates form the heat exchanger analysis and is defined as:

,(1)where

U Overall heat transfer coefficient (W/m2(K)

AArea of heat transfer (m2)

CC=

(2a)

Cold fluid heat capacity rate

CH=

(2b)

Hot fluid heat capacity rate

Cmin=min(CC, CH)

Smaller of the two heat capacity rates (W/K)

Cmax=max(CC, CH)

Larger of the two heat capacity rates (W/K) Note that NTU is a function of geometric and material properties, and the mass flow rates. It does not include any fluid temperatures. Using the calculated NTU, the effectiveness of the heat exchanger, (, can be calculated from tables where the effectiveness formulae for different heat exchanger arrangements can be found. In such tables, another dimensionless term that precipitates from the analyses appears. This is the heat capacity rate ratio, Cr = Cmin/Cmax. For a shell-and-tube exchanger with one shell pass and some multiple of two tube passes, the effectiveness is

. Heat Exchanger Effectivenessis defined as

The maximum heat transfer occurs in the fluid with the least capacity to absorb or give off heat. This is the fluid with the minimum value of = Cmin. If this fluid is the cold fluid, its temperature cannot rise above the hot-side, inlet temperature. Alternately, if the fluid is the hot fluid, it cannot be cooled below the cold-side, inlet temperature. Thus,

.

As the actual heat transfer is the same for both fluidsone gaining thermal energy and the other loosing an equal amountthe actual heat transfer rate is defined by both

and .

These last two relations yield the outlet temperatures desired. LABORATORY PROCEDURE

1.Verify the dimensions and features of Figure 2. 2.Generally, small flow rates will generate better results but may take longer to reach steady state. Also, do not let the air that comes out of entrainment accumulate in shell. Use bleed taps as needed.

3.For a hot water flow of about 15% of the maximum rotameter reading and a cold water flow of about 30%, take inlet and outlet temperatures of both flows until no further changes in tempera-ture are noted. This is the steady-state conditionuse only the associated flow rates and temperatures for calculations. Detailed Computational ProcedureThe NTU method will be described using just one tube; but that single tube could represent an entire tube bundle. The NTU method calculation procedure for a shell-and-tube heat exchanger follows: 1.a.Determine cold and hot water flow rates,and (from rotameter readings), and their specific heats,and(look up values based on the average of the inlet and outlet tempe-ratures). The units of mass flow, , are kg/s and those of specific heat, cp, are J/(kg(K). [NOTE: Some tables list specific heat as kJ/(kg(K)so always check units!!]

Figure 2Experimental apparatus with dimensional data

b.Calculate a temperature specific energy flow known as the heat capacity rate, C, for both the cold and hot flows

.

c.Calculate the heat capacity rate ratio, Cr = Cmin/Cmax.2.Calculate the heat transfer coefficients at the inside and outside surfaces of the tubes, hinside and houtside. These are used to compute the overall heat transfer coefficient, U. (See Figure 3)

Figure 3Heat transfer coefficients at inside and outside tube surfaces

a.Flow Inside Tubes: Even though there are many tubes in the bundle and there are parallel and counter flows in this two-pass exchanger, the calculation may be performed by considering the flow in just one of the tubes with the caveat that one must account for the direction of the flow. That is, half of the tubes are associated with parallel flow and half the tubes are associated with counterflow. Thus, the mass flow in the equivalent tubes is

where, N = total number of tubes.

From simple flow relations, it is known that the velocity inside a single tube is

where, A = cross sectional area of one tube.

Given this velocity, a Reynolds number () can be computed to indicate whether the inside flow is laminar or turbulent. This will most likely be fully-developed, laminar flow. For such with constant surface temperature, Ts, and:

where fluid properties are based on the mean (or bulk) temperature across a cross section, Tm.

If the flow is fully developed, turbulent (Re ( 10,000),

.

Tube-side fluid properties should be evaluated at the average of the mean temperatures, .

b.Shell Flow Outside of Tubes: For the staggered tube arrangement of the experiment shown in Figure 4, use the following expression for the average Nusselt number

. (3) Use Table 1 to determine m and C1. Note in the report which values of m and C1 were used. This relation applies when there are more than 10 tubes in a bundle (NL ( 10), 2000 < ReD,max < 40,000 where ReD,max is defined below, and Pr ( 0.7. average mean temperature of the fluid,, as defined above. ST/D

1.251.52.03.0

SL/DC1mC1mC1mC1m

0.6000.2130.636

0.9000.4460.5710.4010.518

1.0000.4970.558

1.1250.4780.5650.5180.560

1.2500.5180.5560.5050.5540.5190.5560.5220.562

1.5000.4510.5680.4600.5620.4520.5680.4880.568

2.0000.4040.5720.4160.5680.4820.5560.4490.570

3.0000.3100.5920.3560.5800.4400.562 0.4280.574

Figure 4Staggered tube arrangement

is defined for the maximum velocity occurring within the tube bank, Vmax, which occurs at one of two locationseither in way of A1 or A2 (see Figure 4). The maximum velocity will occur at A2 if. The factor of 2 results from the bifurcation experienced by the fluid moving form the A1 to the A2 planes. In this case, , otherwise it occurs at A1 and .

Note: The average velocity of flow over the tube is not constant as the shell is not wall-sided but circular. Thus, one needs to use some average value of area. To use the relations for staggered tube arrangements, a free-stream, shell-side, fluid velocity must be determined. As the sides of the shell are circular, this free-stream velocity varies. Thus, an average free-stream velocity must be determined based on an average width of the shell, wavg. This can be obtained from simple integration as

Multiplying this with the distance between baffles gives an average cross-sectional area, Aavg, for the flow and the average velocity, Vavg, can be determined from = AavgVavg. 3.a. Calculate the overall heat transfer coefficient, U

where, t = the tubing thickness

Then NTU is

Now, the heat exchanger effectiveness, (, can be determined. For one shell pass and two tube passes the effectiveness is

.PHYSICAL PROPERTIESAs the liquid (water) is moving, it must be under a slight pressure. This experiment is interested in the properties of liquid water density and specific heat which are both functions of temperature and pressure. However, at low pressures, one may assume that density and internal energy are approximately equal to their saturated liquid values at the same temperature; i.e., ((T, p) ( (f(T) and u(T, p) ( uf(T). Thus, density can be defined. Enthalpy is, h(T, p) ( hf(T) + [p psat(T)]/(f(T). At a room temperature of, say, 70(F (~21(C), psat = 0.02487bar. Compared to atmospheric pressure of 1.01325bar, this is small and negligible. Thus, h(T, p) ( hf(T) + p/(f(T). At the temperature assumed, the density of water is 998kg/m3. At small pressures, say 2atm = 2.02bar, p/(f(T) = 0.202 kJ/kg while hf(T) = 88.14 kJ/kg. Thus, a fair approxi-mation of enthalpy is h(T, p) ( hf(T). Finally, the definition of specific heat is (h = c(T) (T; thus, C (T, p) ( C f(T).FOR THE REPORT1.Be sure to clearly state/show the calculations along with any assumptions made on the Mathcad worksheet in the order appearing on the grading sheet. Of course, you may have other intermediate calculations.

2.Indicate sources of error in equations as they apply to the shell-and-tube heat exchanger in the lab, as well as sources of error in the measurements. 3.Discuss how good is the NTU method.

ORDERED DATA, CALCULATIONS, and RESULTS The object below is reduced to 70% of full size.

DISCUSSION OF RESULTSDiscuss how good is the NTU method. Indicate sources of error in equations as they apply to the shell-and-tube heat exchanger in the lab, as well as sources of error in the measurementsCONCLUSIONS

APPENDICES

APPENDIX AData Sheet for Shell-and-Tube Heat Exchanger Lab

Time/Date:___________________________

Lab Partners:______________________________________________________

______________________________________________________

Verify supplied dimensions given in Figure 2. Is anything else needed?

Is the hot flow on the tube side or shell side? ______________ Rotameter max flow rate: ________________RunCold Volumetric Flow Rate,

( % of max rotameter rating)Hot Volumetric Flow Rate,

( % of max rotameter rating)Hot Outlet Temperature,

(C)Hot Inlet Temperature,

(C)Cold Outlet Temperature,

(C)Cold Inlet Temperature,

(C)

1

2

3

4

5

APPENDIX BPHYSICAL PROPERTIES TABLE

SD

ST

SL

SL = 0.475(

ST = 0.548(

SD = 0.548(

The width of the flow course varies & thus the average velocity

IMPORTANTWhen using the Heat Exchanger Performance Test Bench, there are some important items to remember for your safety and the safety of others.

1.Make sure the proper inlet and outlet valves are open before the heat exchanger is operated. Failure to do this will pressurize the system and rupture the heat exchanger seams. As a rule of thumb, do not close any of the outlet ball valves more than half way. In particular, make sure the outlet valves that allow the water to go to the drain are open prior to turning on water.

2.For meaningful data, bleed taps will need to be opened and closed to allow air to escape while the experiment is going on. Outlet valves may be closed SLIGHTLY to help keep the heat exchanger full.

5 Baffles, 1.2( thick. Equally spaced to form 6

chamber. 23 tube penetrations per baffle.

Shell: 5( OD

4.5( ID

Hot water

outlet

thermometer

Cold water

outlet

thermometer

Hot water inlet thermometer

Cold water

inlet

thermometer

Vavg, T(

30 Tubes, each 0.25( diameter

neglect wall thickness

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

Distance between Tube Sheets, 16-1/8(

(inside face to inside face)

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

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