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Unformatted text preview: Chap 13 Heat Exchangers The Effectiveness-NTU Method 13-68C When the heat transfer surface area A of the heat exchanger is known, but the outlet temperatures are not, the effectiveness-NTU method is definitely preferred. 13-69C 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. 13-70C For a specified fluid pair, inlet temperatures and mass flow rates, the counter-flow heat exchanger will have the highest effectiveness. 13-71C 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 13-72C 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. 13-73C 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. 13-74C 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 = = ∆ ∆ 13-75C 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. 13-54 Chap 13 Heat Exchangers 13-76C The longer heat exchanger is more likely to have a higher effectiveness. 13-77C 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. 13-78C 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