HEAT TRANSFER OF DOUBLEPIPE HEAT EXCHANGERINTRODUCTIONThe heat transfer principlesin fluids is often used in industrial applications to ensure safe and effectivecontrol in heating and cooling applications. Heat exchangers are devices thattransfer heat energy to heat or cool the incoming or outgoing fluid.
The twostreams have different temperatures and heat characteristics and are in 2different pipes. Heat transfer in heat exchangers is through radiation,convection and conduction. Heat conduction occurs through the pipe wall fromthe fluid of high temperature to the fluid of low temperature.
The major heattransfer method is through convection from one fluid pipe to the other. Thefluid in the pipe wall is combined and mixed with into one fluid stream hencelosing the heat.AIMS AND OBJECTIVESThe major aim ofthis experiment is to use a double sided heat exchanger to investigate heattransfer of water with different temperature gradients.
The flowing waterstream is either co-current or counter-current configurations.The following arethe specific objectives of the experiment:1. Tostudy the basic principles of heat transfer in heat exchanger with a solid wallseparating the 2 fluids streams.2. Tooperate a heat exchanger running with either co-current or counter-current flowin different hot or cold water flowrates.
3. Tocarry out mass balances and overall efficiency4. Tocalculate the heat transfer efficiencies at the investigated flow rates THEORYFig1: Schematic diagram of HT30XC heat exchanger TheHT30XC double sided heat exchanger module comprises of a controlled heatersection, heat exchanger rig, control valves, feedback circuits and userinterface section. Water from the reservoir is heated at 50 then pumped via the inner stainless tube whilethe pipe with cold water at ambient temperature flows between the inner hotstream tube and the outer acrylic tube to enable heat transfer. The heat istransferred from the hot stream to the cold stream leading to temperaturechange between the 2 water tubes. The valves helps to control the flow lengthof the heat exchanger allowing several heat exchanger tube configurations to beused in the experiment.APPARATUS AND EQUIPMENT1.
HT30XCdouble pipe heat exchanger2. Flowcontrol valves3. Feedbackcircuitry and sensory components4. Heater5.
Pump6. TheArmfield Armsoft softwarePROCEDUREEquipment setupa) TheArmfield Armsoft software was loaded for the HT36 fitted to the HT30XC and thecounter-current option of the flowing water streams selected from the start upscreenb) The’power on’ icon on the software’s mimic diagram screen was clicked to switch onthe unit from stand by to on. The green light illuminated on the front ofHT30XCc) Tocontrol the cold water flow in the heat exchanger, the flow rate was set usinga control valve adjusted from 0% that is fully closed to 100%, fully open inequal intervals of 1% as shown below.Fig2: Adjusting the cold water flowd) Tocontrol the hot water flow in the inner stainless tube, the ‘Flow’ button onthe software display was clicked to enable access to the PID controller of theHeat exchanger while setting hot water flow rate.e) Toset the hot water temperature, the ‘Heater’ button of the Heat exchangersoftware to enable access to the PID controller that sets the hot water flowrate.f) Toconfigure the heat exchanger for counter current flow, the “Counter-software exercisefor counter- or co- current flow was selected to ensure that cold water inletsupply always enters the heat exchanger in the same end in concurrent flow.i) For counter current flow,the hot water is configured to enter at the opposite direction using the hotwater pump in the heat exchanger module.
g) Thenumber of active heat exchanger tubes were chosen by closing each manual valveson the heat exchanger. The valve taps were closed by turning the handle at aright angle in the clockwise direction. The appropriate valves were openedbyturning the valves’ knobs as shown in the figure below.Fig 3: Opened valveconfiguration for counter current operationEXPERIMENT PROCEDUREa) Thenumber of active tubes in the heat exchanger were selected in the software’s “Numberof Tubes” icon in the left of the display box.b) Thecold water flow rate was set according to 0.25 by adjusting the arrows on theside of the cold water flow rate displayboxc) Thecold water flow inlet temperature T6 was checked on the display box on the mimic diagram screend) Thehot water temperature controller was set to a set point at approximately 30 above the cold water inlet temperaturee) Thefollowing control parameters were set as follows: proportional band was set to5, integral time was set to 200s and the derivative action was set to 0. Thevalues were changed accordingly if the control parameters did not match theabove control values.f) Theicons ‘Apply’ and ‘Ok’ on the display box were clicked to close the windowg) Thehot water flow was set to 1L/min then the controller box icon ‘flow’ wasclicked and a set point was typed.
The control parameters were set and checkedto be as follows: Proportional band was set to 100%, integral time was set to3s while the derivative time was set to 0s.h) The’Automatic’ icon on the top right of the display window was clicked followed by’Apply’ and ‘Ok’ to close the window.i) Theheat was allowed to stabilize while monitoring the temperatures on the mimicdiagram display.
j) The’Go’ icon on the top toolbar was selected to record data after the temperatureis stabilized. The data of T1, T2, T3, T4, T5, T6, T7, T8, T9, T10 and the actual cold and hot water flow rates and were recorded.k) Thecold water control valves were adjusted to give 0.5L/min using the arrowbuttons on the side of the display box.l) Anew results icon was created.
m) Thecorrect number of tubes selected was checked and confirmed on the mimic diagramn) Theheat was allowed to stabilize while monitoring the temperatures on the mimic diagram.o) Theprocess was repeated as the first experiment using the ‘Go’ iconp) toinvestigate other tube configurations of the hot and cold flow rates, thevalves were appropriately set and a new results sheet was createdq) Theheater controller window was opened and the controller adjusted from’automatic’ to ‘Off.’r) Thehot water pump controller window was opened and the set point changed to 0L/mins) Thecold water flow control was set to 0%, fully open.t) Themanual flow rate for the number of tubes under investigation were closed and the valves for the next number ofactive tubes configurations were opened for investigationu) Alldata was saved in an excel sheet.RESULTSTube 1 Temperature change Calculating hot fluidreynolds numberCalculatingcold fluid reynolds number Calculating energyemitted (Qe) and energy absorbed (Qa)Calculating heat power,mean temperature efficiency Table 2:Temperature values Calculating temperaturechange Calculating Qe, Qa andheat powerCalculatingefficiencies and transfer coefficientsCalculating hot fluidReynolds number Calculating cold fluidReynolds number DISCUSSIONFig 4: The graph ofTemperature change with change in flow rate Fig 5: The graph ofTemperature change against change in flow rate From the graphs above,as the flow rate increase, the temperature change decreases.
This is because there is no enoughtime for the water in the tubes to exchange heat for both countercurrent andco-current flow.The increase in thenumber of tubes in the heat exchanger increases the effective area of heattransfer in the HT30XC. However, in an ideal situation, the heated waterflowing in the heat exchanger may loss energy to the surrounding by radiation.This energy loss is neglected such that there’s an assumption that negligibleheat energy is lost from the already cooling water in the long tubesconfigurations. From the data sheet, the effective heat transfer area Where is the mean diameterL is the length of thetubeN is the number oftubes usedThe effect ofcirculating the hot water in the outer annulus would lead to an increase inarea of heat transfer and hence an increased heat transfer co-efficient sincethe hot water flows through both tubes.
Since heat energy istransferred from the hot water tube to the cold water acrylic tube, Qe is theHeat energy lost to the cold water while Qa is the heat energy gained/absorbedby the cold water. Qe is negative since it is energy required to absorb heatenergy in the heat exchanger. Theoretically, for an ideal heat exchanger theamount of heat emitted by hot fluid should be equal to the amount of heatabsorbed by cold fluid. However this is not the case in our experiment due toheat losses and gain in the tubes by radiation.A positive change in flowrate in the hot region decrease the surface area for heat exchange since thehot water travels at a greater velocity hence increasing the volume covered.
Adecrease in flow rate leads to slow heat transfer speed.Comparing the overallheat transfer co-efficient with the heat efficiency of cold and hot waterstreams. u Efficiency hot stream Efficiency cold stream 418.7541 11.
195652 -91.63043 429.71782 20.875421 -44.78114 405.24385 24.
94382 -14.1573 488.71838 30.156951 -3.475336 277.13957 4.
2025862 -87.60776 259.32109 3.9173014 -89.44505 532.24847 18.130631 -16.
89189 677.88177 20.022497 -9.786277 240.
98171 2.8784648 -89.23241 283.17427 2.792696 -89.
90333 615.98064 13.218391 -23.
90805 586.26647 12.984055 -23.12073 The overall heattransfer co efficient is greater than the percentage efficiency of the hotstream. The efficiency in the cold region is negative since it gains heatenergy from the hot stainless steel tube. The overall heat transferco-efficient increases with increase in the number of tubes in the heatexchanger.After determining theoverall heat transfer co efficient, the Reynold’s number is calculated which isthe dimensionless measure of the inertial and viscous flow forces in thestreams. In the inner stainless tube, if the Reynolds number is less than 2000,the flow is laminar flow.
If the Reynolds’s number is 10,000, the flow ispredicted to be turbulent.Heat transfercoefficient increases with increase in Reynolds number. This is due to the factthat localized secondary flow is formed in the tube heat exchanger. Secondaryflow increases turbulence evidenced from increased Reynolds number resultinginto increased heat transfer between tubes.CONCLUSIONThe experiment on thedouble-pipe heat exchanger was a success.
It was experimentally shown that hotwater or any fluid flowing through via the inner tube transfers heat energy tothe cold water flowing in the double outer pipes until the temperature of the 2regions stabilizes. This can be altered by change in the inlet temperature orthe fluid’s flow rate. The stabilization process starts again and with time, anew stabilized value of the final temperature is recorded.
It was evident that theinlet and outlet temperatures can never be constants, but with significantchanges in the stream temperatures and flow rates in an experimental studyleads to a relative temperature stability. An increase in thenumber of u shaped tubes in the heat exchanger increases the effective area forheat transfer. More so, the increase in flow rate increases the speed taken forheat transfer. REFERENCES1. Denver’s,N., Fluid Mechanics, McGraw Hill, (1991).
2. Incropera,F.P., D.
P. DeWitt, Fundamentals of Heat and Mass Transfer, John Wiley &Sons, Inc., pp. 460, 582-612. (1996).3. Redding,Alyssa M., Shell-and-Tube Heat Exchanger, Project 1, and Laboratory Manual.
Standardsof the Tubular Exchange Manufacturers Association, 6th ed., TubularExchanger Manufacturers Association, New York, 1978.