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Unformatted text preview: Chap 13 Heat Exchangers The EffectivenessNTU Method 1368C When the heat transfer surface area A of the heat exchanger is known, but the outlet temperatures are not, the effectivenessNTU method is definitely preferred. 1369C The effectiveness of a heat exchanger is defined as the ratio of the actual heat transfer rate to the maximum possible heat transfer rate and represents how closely the heat transfer in the heat exchanger approaches to maximum possible heat transfer. Since the actual heat transfer rate can not be greater than maximum possible heat transfer rate, the effectiveness can not be greater than one. The effectiveness of a heat exchanger depends on the geometry of the heat exchanger as well as the flow arrangement. 1370C For a specified fluid pair, inlet temperatures and mass flow rates, the counterflow heat exchanger will have the highest effectiveness. 1371C Once the effectiveness ε is known, the rate of heat transfer and the outlet temperatures of cold and hot fluids in a heat exchanger are determined from ( ) ( ) ( ) max min , , , , , , , , Q Q C T T Q m C T T Q m C T T h in c in c p c c out c in h p h h in h out = = = = ε ε 1372C The heat transfer in a heat exchanger will reach its maximum value when the hot fluid is cooled to the inlet temperature of the cold fluid. Therefore, the temperature of the hot fluid cannot drop below the inlet temperature of the cold fluid at any location in a heat exchanger. 1373C The heat transfer in a heat exchanger will reach its maximum value when the cold fluid is heated to the inlet temperature of the hot fluid. Therefore, the temperature of the cold fluid cannot rise above the inlet temperature of the hot fluid at any location in a heat exchanger. 1374C The fluid with the lower mass flow rate will experience a larger temperature change. This is clear from the relation Q m C T m C T c p cold h p hot = = ∆ ∆ 1375C The maximum possible heat transfer rate is in a heat exchanger is determined from ( ) max min , , Q C T T h in c in = where C min is the smaller heat capacity rate. The value of max Q does not depend on the type of heat exchanger. 1354 Chap 13 Heat Exchangers 1376C The longer heat exchanger is more likely to have a higher effectiveness. 1377C The increase of effectiveness with NTU is not linear. The effectiveness increases rapidly with NTU for small values (up to abo ut NTU = 1.5), but rather slowly for larger values. Therefore, the effectiveness will not double when the length of heat exchanger is doubled. 1378C A heat exchanger has the smallest effectiveness value when the heat capacity rates of two fluids are identical. Therefore, reducing the mass flow rate of cold fluid by half will increase its effectiveness....
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This note was uploaded on 08/25/2009 for the course AET AET432 taught by Professor Rajadas during the Spring '06 term at ASU.
 Spring '06
 Rajadas

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