Experiment No: Objective o determine the effectiveness of shell and tube, cross & plate heat exchangers heory A heat exchanger is an equipment which facilitates the of thermal energy between two or more fluids at different temperatures. Heat exchangers are employed in a variety of domestic, commercial, and industrial applications, such as power generation, refrigeration, air conditioning, process industry, manufacturing industry etc. Classification of heat exchangers is presented in Fig.. Heat Exchangers Heat transfer process Constructional geometry Relative direction of fluid Heat transfer mechanism Direct contact heat exchangers Indirect contact heat exchangers Plate-type Extended surfaces ubular Concentric ubes Shell & ube Parallel Flow Counter Flow Cross Flow Single-phase wo-phase Evaporators Condensers Regenerators Spiral tubes Recuperators Pipe coils Fig. Classification of Heat Exchangers Parallel heat exchanger In a parallel heat exchanger, the two fluid streams (hot and cold) through the heat exchanger in the same direction. he two fluid streams enter at one end of the heat exchanger and leave at the other end. he schematic and the temperature profile of the fluid streams in parallel heat exchanger are shown in Fig.. From the temperature profile (Fig. (b)), it is clear that the temperature difference between the fluid streams decreases from the inlet to the outlet of the heat exchanger. Parallel heat exchangers are rarely employed due to their requirement of large surface area for heat transfer. Examples:Oil heaters, oil coolers, water heaters etc.
t c t (emperature) Cold fluid t h t h Cold fluid t h temp. (t h ) t h t c t c t c Cold fluid temp. (t c ) L (Length) (a) (b) Fig. Parallel Flow Heat Exchanger emperature difference(θ ) = t h t c () emperature difference(θ ) = t h t c () Log mean temperature difference (LMD) = θ θ ln θ θ () Counter heat exchanger In a counter heat exchanger, the two fluid streams in relatively opposite directions. he fluid streams enter at opposite ends. Figure shows the schematic and the temperature profile of the fluid streams for such a heat exchanger. he temperature difference between the two fluid streams remains nearly constant (Fig. (b)). Counter heat exchangers provide the maximum heat transfer rate for a given surface area. Hence, they are the most widely used heat exchangers. t c t (emperature) t h Cold fluid Cold fluid t h t h t c temp. (t h ) t c Cold fluid temp. (t c ) t h t c L (Length) (a) (b) Fig. Counter Flow Heat Exchanger emperature difference(θ ) = t h t c () emperature difference(θ ) = t h t c () Log mean temperature difference (LMD) = θ θ ln θ θ ()
Cross heat exchanger In a cross- heat exchanger, the paths of the two fluid streams through the heat exchanger are usually at right angles to each other. Figure shows a schematic diagram of cross- heat exchanger. he cross heat exchanger in our laboratory is of finned type and the cooling media used is air. Cold fluid (outlet) Baffles (inlet) (outlet) Cold fluid (inlet) Fig. Cross Flow Heat Exchanger Shell & tube heat exchanger his type of heat exchangers has a bundle of tubes enclosed in a shell, usually cylindrical. he tubes are arranged parallel to the shell axis. One fluid stream is passed through the bundle of tubes, while the other fluid stream is s through the shell over the tubes (Fig.). Overall heat transfer between the fluid streams is enhanced by the use of multiple shell & tube passes. With the use of baffles, the shell-side fluid stream is re-routed and made to back-and-forth over the tubes. Plate heat exchanger Plate heat exchangers consist of a number of thin corrugated metal plates arranged together. he fluid streams enter the heat exchanger through frame connections and are then distributed to the plates. he two fluid streams pass through alternate spaces formed between the successive plates. hanks to the corrugations on the metal plates and the small spacing between the plates, the fluid is essentially turbulent which improves the overall heat transfer between the fluid streams. Also, the eddies generated in the clean the heat exchanger surface, thus minimizing fouling. Plate heat exchangers are widely used in industries due to their low cost, easy maintenance, and high thermal efficiency.
Shell ubes Baffle plates (outlet) (inlet) Cold fluid (outlet) Cold fluid (inlet) Fig. Schematic of Shell & ube Heat Exchanger a Effectiveness of a heat exchanger he effectiveness (ϵ) of a heat exchanger is defined as the ratio of the actual heat transfer to the maximum possible heat transfer. = actual heat transfer maximum possible heat transfer (7) Actual heat transfer = Q = m h C ph t h t h = m c C pc t c t c (8) where m h. C ph = C h = hot fluid capacity rate m c. C pc = C c = Cold fluid capacity rate Maximum possible heat transfer = Q max = C h t h t c or = C c t h t c Q max is the minimum of these two values i. e. Q max = C min t h t c (9) a Rajput, R.K., 007,Engineering hermodynamics, Laxmi Publications, New Delhi.
= C h t h t h C min t h t c or (0) = C c t c t c C min t h t c () Determination of overall heat transfer coefficient o determine the overall heat transfer coefficient U for a given heat exchanger, we use the following relation: NU = U A C min () where, NU = Number of ransfer Units Dimensionless W U = Overall heat transfer coefficient m K A = Heat transfer surface area (m ) C min = Minimum of C h or C c (kj/k) In the present study, steam is condensing while passing through the heat exchanger. Hence, C h. hus, the capacity ratio, C r = C min / C max = 0. For such a case, NU can be calculated using the following relationship between ϵ and NU b : NU = ln( ϵ) () Hence, from equations () and (), we have U = C min A ln( ϵ) () b Kays, W. M., and London, A.L., 98,Compact Heat Exchangers, McGraw-Hill, New York.
Procedure. Fire the boiler as per instructions in the boiler manual. Wait till it operates satisfactorily at desired and mass rate.. Check the setting of reducing valve (PRV), it should reduce the of steam to around bar, confirm it from the reading of the gauge after the PRV on the main steam header.. Separate inlet and outlet valves for the steam are provided for cross, plate type, and shell & tube heat exchangers. Open the outlet valves first and then the inlet valves for letting steam into the heat exchangers.. In case of cross heat exchanger, measure the air velocity at the inlet to the blower with the help of a digital anemometer.. In case of plate type or shell & tube type heat exchanger, note the water rate to the heat exchanger from the digital display.. Note down the of steam from gauge on steam header. Note the steam rate, temperature of steam at inlet of the heat exchanger, steam condensate and temperature of incoming and outgoing water from the digital display. 7. Vary the air rate (in case of cross heat exchanger) with the help of damper at the inlet of the blower and note down the readings. ake at least - readings in such manner. 8. Vary the water rate (in case of plate and shell & tube heat exchangers) by turning the water inlet valve to the respective heat exchanger and note down the readings. ake at least - readings in such manner. 9. Determine the effectiveness and overall heat transfer coefficient for the three heat exchangers.
Bypass Line Safety Valve Water Supply to Water Inlet Header Water Inlet Header P P BOILER Vortex Flow Meter rap Pressure reducing Valve (PRV) Air inlet to Blower Blower Air outlet to ambient CROSS FLOW HX PLAE HX SHELL & UBE HX Main Header Condensate Header LEGEND: or : Control Valve : Flow Line Discharge to ambient Water Outlet Header o hermo-compressor P : Water Flow Line : Air Flow Line Flash ank : emp. Sensor rap P : Pressure Gauge Fig. Schematic layout of heat exchangers with boiler setup o condensate discharge tank
Observations For cross heat exchanger Heat transfer surface area, A = m Cross-section area of the air inlet, A c = m Air velocity (m/s) inlet t h ( o C) condensate t cond ( o C) Air inlet t c ( o C) Air outlet t c ( o C) For Plate type heat exchanger Heat transfer surface area, A = m Water rate (LPH) inlet t h ( o C) condensate t cond ( o C) Water inlet t c ( o C) Water outlet t c ( o C)
For shell & tube heat exchanger Heat transfer surface area, A = m Water rate (LPH) inlet t h ( o C) condensate t cond ( o C) Water inlet t c ( o C) Water outlet t c ( o C) Calculations. Calculation of effectiveness (ϵ) Use Eq. () to calculate ϵ.. Calculation of overall heat transfer coefficient (U) Use Eq. () to calculate U. Results For cross heat exchanger Air rate, m c Effectiveness, ϵ Overall heat transfer coefficient, U (W/m.K)
For Plate type heat exchanger Water rate, m c Effectiveness, ϵ Overall heat transfer coefficient, U (W/m.K) For shell & tube heat exchanger Water rate, m c Effectiveness, ϵ Overall heat transfer coefficient, U (W/m.K) Further reading: Kern, D.Q., 9, Process Heat ransfer, McGraw-Hill, okyo. Kays, W. M., and London, A.L., 98, Compact Heat Exchangers, McGraw-Hill, New York. Kakac,, and Liu, H., 00, Heat Exchangers: Selection, Rating, and hermal Design, CRC Press LLC, Boca Raton, Florida.