Chemical Engineering Laboratory Manual Double-Pipe Heat Exchanger
362L_EXPERIMENT NO - 1 (revised 2012)
DOUBLE-PIPE HEAT EXCHANGER
OBJECTIVE:
The objective of this experiment is to become familiar with the simplest type of heat exchanger construction, the double-pipe heat exchanger. A further objective is to use the exchanger in actual runs and determine the overall heat transfer coefficient, as well as to estimate from empirical correlations the individual coefficients from flow conditions, enthalpy balances, and temperature measurements.
NOTATION:
A - tube surface area (length2)
Ai - tube surface area based on the tube inside diameter (length2)
Ao - tube surface area based on the tube outside diameter (length2)
Di - tube inside diameter (length)
Do - tube outside diameter (length)
hi - convective heat transfer coefficient for the inside surface of the tubes (energy/[time * length2 * absolute temperature])
ho - convective heat transfer coefficient for the outside surface of the tubes (energy/[time * length2 * absolute temperature])
k - thermal conductivity of the tube wall (energy/[time * length * absolute temperature])
L - tube length (length)
R”f,i - fouling factor for the tubes' inside surface (length2 * absolute temperature * time/energy).
R”f,o - fouling factor for the tubes' outside surface (length2 * absolute temperature * time/energy)
U - overall heat transfer coefficient (energy/[time * length2 * absolute temperature])
Ui - overall heat transfer coefficient based on Di (energy/[time * length2 * absolute temperature])
Uo - overall heat transfer coefficient based on Do (energy/[time * length2 * absolute temperature])
THEORY:
The theory and correlations that apply to enthalpy balances and the overall heat transfer coefficient are given in Incropera and DeWitt (1), Chapter 11, or Kreith et.al., (2). For example from thermodynamics, the heat transferred from one fluid to the other, under certain assumptions (you need to identify these) can be expressed by,
(1)
From heat transfer we recognize that the amount of heat transferred from one fluid from another, under certain assumptions (which you must identify) can be expressed by,
(2)
The overall heat transfer coefficient and individual convective heat transfer coefficients can be estimated from physical properties and flow conditions of the stream. The equations needed for estimation of individual convective heat transfer coefficients from temperature measurements and flow conditions are given in Incropera and DeWitt, Chapter 7, 8, 9, and 10, or Kreith, Chapters 4, 6, 7, 8. The overall heat transfer coefficient is related to the individual heat transfer coefficients by the Equation 11.5 from Incropera and DeWitt (or from Kreith, see Chapter 8). The equation in terms of heat transfer resistances including fouling factors is,
(3)
where is the overall heat transfer resistance and can be set
equal to or
and where other symbols are as defined in Incropera and DeWitt (1).
Each value of h, whether ho or hi and the tube wall resistance must be calculated by the appropriate correlation from Incropera and DeWitt (1) or some other suitable source (give source). With these results then the sum of the two scaling or fouling resistances.
(4)
can be estimated.
Section 11.3 in Incropera and DeWitt (1) shows how enthalpy balances and temperature measurements can be used to obtain the overall heat transfer coefficient.
EQUIPMENT:
The equipment is comprised of 2 double-pipe heat exchangers, one located above the other, and associated auxiliary instrumentation. A schematic of a double-pipe heat exchanger (HE) can be found on page 642 of Incropera and DeWitt (1), or Figure 8.2, page 487 in Kreith. The upper HE consists of a 1" O.D. schedule 40 steel pipe (black iron pipe) within a two-inch schedule 80 steel pipe. The lower heat exchanger consists of a one-inch schedule 40 steel pipe within a two-inch schedule 80 steel pipe. The length of each heat exchanger (inner tube) is 11 feet. The outer pipes of these two heat exchangers are (will be covered Summer 2012) insulated with a wrapped blanket of glass fiber.
In each heat exchanger the hot fluid is piped to the annulus and the cold fluid is piped to the inner tube. Steam is routed to the upper heat exchanger, while, heated water (from the inner pipe of the upper HE) can be routed to the lower heat exchanger. The flow rates of steam, hot, or cold water may be determined by the installed flowmeters. The cold fluid in both heat exchangers is tap water.
Heat exchangers are typically classified according to the fluid flow arrangement and type of construction. The douple pipe heat exchanger is the simplest construction type. Flow arrangements to be investigated in the lower heat exchanger are parallel (or co-current flow) and counter-current flow. In the parallel flow arrangement the hot and cold fluid flow in the same direction through their respective channels. In the counter-current flow arrangement the hot and cold fluids flow in opposite directions.
Temperatures of incoming and outgoing fluids are measured by thermocouples connected to both a panel mounted local temperature indicator and a computer-based data acquisition system (Opto-22). The thermodynamic condition of the steam entering can be determined by throttling the steam from the supply to a condition that is just saturated or slightly superheated and measuring the temperature and pressure. The pressure in the annulus may be significantly below the inlet steam pressure. Consider why this might happen.
SAFETY:
Hard hats and safety glasses must be worn at all times. Steam is deadly; treat it with the respect it deserves. Thoroughly familiarize yourself with all valving and piping.
START-UP PROCEDURE:
1. Become familiar with the P&ID in Figure 1 (or the expanded P&ID at the end of this document), and be familiar with process flows. Note the upper limits to the flowmeters:
a. Steam flowmeter range is 0 – 500 lb/hr
b. FM1 (upper cold fluid flowrate) range is 0 – 20 gpm
c. FM2 (lower cold fluid flowrate) range is 0 – 13 gpm
2. Turn on power to all sources (located in gray box).
3. Open OPTO 22 software on computer:
a. Open PAC Control Basic 9.0.
b. Go File -> doublepipe.
c. Click debug -> click Stop Strategy -> click Run Strategy.
d. Open PAC Display Runtime Basic 9.0.
4. Ensure that all hand valves are closed. On the cold water lines, open appropriate hand valves downstream of the flowmeters (FM1 and FM2) to allow for the desired flow scheme through the heat exchangers to the drain.
Figure 1. P&ID of Double Pipe Heat Exchanger
5. Open main water inlet ball valve (V1). Open water inlet valves (V3 and V4) to set cold water flowrates to the desired values (watch digital readouts for FM1 and FM2 to ensure you don’t exceed the maximum water flowrates).
6. Have TA or instructor open the steam header valve to supply steam to V2, which should be closed now.
7. Ensure the steam control valve (SV1) and steam flow meter (FM3) are isolated by checking that the block/isolation/bypass valves (SV1-b, SVB-a, SVB-b, V5) are closed.
8. Open the steam flow meter by-pass valve (V5-b). Open hand valves (V6, V7) to ensure an open path for the steam/condensate to the drain.
9. Check that the local pressure gauge near the steam inlet valve (V2) is near 20 psig. If it is, then slowly open valve V2.
10. Very slowly open the by-pass valve (SV1-b) [ensure that steam is bypassing the control valve]. Do this slowly to allow time for the pipes to heat up gradually and to push any liquid condensate through the steam trap to the drain. If you open too fast you may induce “water hammer”. After the system has warmed up, slowly open the hand valve (V5) to the steam flowmeter (FM3). Begin to close the steam flowmeter by-pass valve (V5-b), while watching the digital readout for FM3. If the steam flowrate is approaching the maximum, adjust the steam valve by-pass valve (SV1-b) more closed. You should not allow the steam flowrate to exceed its maximum (500 lb/hr). [Note this procedure is used to avoid damage to the steam flow meter and the piping system.]
11. Turn on the instrument air inlet valve (blue ball valve not shown on the P&ID).
12. Ensure that valves are positioned to allow desired flow through system. Make sure there is a path from inlet water to drain, and a path from the inlet steam to drain.
PROCEDURE:
1. Obtain any important dimensions not already stated for this equipment.
2. Start data collection for T3 by clicking “On” for DATA-LOG. Note: once you start acquiring data, do not open the file where your data is stored, until you stop the data collection. You may corrupt the data file, thus losing the data.
3. Perform three (3) runs on the upper exchanger to cover a wide range of water flow rates in the inner tube, while manually keeping the steam condition constant so far as possible (the automatic control valve should not be “on”, rather utilize the manual bypass valve). Measure water flow rate and steam flow rate, temperature and pressure, and inlet and outlet conditions (temperature) of both streams.
4. After you have finished the runs on the top exchanger, switch the steam control valve (SV1) into automatic mode, open the block valves (SVB-a, SVB-b) slowly. Adjust the setpoint on OPTO system for the outlet cold water side of the top heat exchanger (T3) to the desired value (should be between 140 - 175 oF). Slowly close the by-pass valve (SV1-b) around the automatic steam control valve (SV1). You should have now set the controller for T3 (outlet temperature of the cold stream) to a desired constant temperature. This becomes the inlet constant temperature hot water to the annulus of the bottom heat exchanger.
5. Perform three (3) runs on the lower exchanger in the co-current flow mode using hot water in the annulus and cold water in the inner tube. Vary and measure water flow rates over a wide range in the colder stream and use one rate in the hotter stream. Measure all necessary temperatures.
6. Perform three (3) countercurrent runs on the lower exchanger continuing to use the hot water in the annulus and cold water in the inner tube. Measure rates and temperatures as in (5).
7. Use ASPENPLUS to model one of your runs.
SHUTDOWN PROCEDURE:
1. Stop data collection by clicking “Off” for DATA-LOG.
2. To find data, open the Student Data folder -> Copy the PID Data file onto a jump drive or email it to yourself -> Open on own computer with Excel.
3. Open the original PID Data file straight from the Student Data folder. Delete all data from file except the headings. Save file.
4. Close PAC Display Runtime Basic. In PAC Control Basic 9.0, click Stop Strategy and then close out.
5. Close steam inlet gate valve (V2).
6. Leave the cold water flowing through the system until all temperatures are below 45 oC (113 oF). Then close the inlet cold water valve ¼ turn valve (V1). Leave the remaining valves as they are.
7. Close instrument air inlet ball valve.
8. De-energize the system by turning off the control power in the gray box.
REPORTING:
In all cases you must report the results of your calculations.
1. Calculate enthalpy balances on all runs. If there is a difference between the enthalpy lost from the hot stream and that gained by the cold stream, explain this discrepancy.
2. Calculate the overall heat transfer coefficients for all runs from the enthalpy change and the measured temperature differences.
3. Calculate the individual heat transfer coefficients, ho and hi, for all runs using the appropriate empirical correlations (indicate the correlation/reference used).
4. Calculate the tube wall resistances.
5. Estimate the sum of the two fouling resistances.
6. Compare the fouling resistances and the overall heat transfer coefficients with those in the literature and discuss any differences. Watch the units on the literature fouling resistances to insure you are comparing factors defined in a similar fashion.
7. Discuss ALL results in light of the theory.
8. Discuss the ASPENPLUS analysis of one of your runs.
REFERENCES:
1. F.P Incropera and D.P DeWitt, "Fundamentals of Heat and Mass Transfer", 4th Edition, John Wiley and Sons, 2002.
2. Frank Kreith, Raj M. Manglik, Mark S. Bohn, “Principles of Heat Transfer”, 7th ed., Cengage Learning, Stamford, CT, USA (2011).
FOR ADDITIONAL INFORMATION:
1. R.K. Sinnott, “Coulson and Richardson’s Chemical Engineering,” Revised 2nd Edition, Volume 6, Butterworth and Heinemann, 1993.
2. G.F. Hewitt, G.L. Shires, and T.R. Bott, “Process Heat Transfer,” Begel House/CRC, 1994.
2. McCabe, Smith and Harriott, "Unit Operations of Chemical Engineering", 4th edition, pp. 275-343, McGraw-Hill Book Co, 1985.
3. McAdams, W. H., "Heat Transmission", 3rd edition, pp. 184-201, McGraw-Hill Book Co, 1954.
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