Factors affecting heat transfer
The rate of energy transfer (P) of the heat exchanger depends on the temperature difference between the media, the thermal conductivity of the material in the heat exchanger and the area over which the energy is transferred; it can be calculated from the equation
P = kA LMTD
k = heat transfer coefficient (W/(m2 °C))
A = heat transfer area (m2)
LMTD = log mean temperature difference (°C).
The value of k gives the quantity of energy trans-ferred per square metre surface area and degree temperature difference. Various factors affect k; in practice, values of up to 8 kW/(m2 °C) are achieved. kcan be calculated from the following equation:
Here, a1 and a2 are the heat transfer coefficients on each side of the material in the heat exchanger. They give the quantity of energy transferred from a liquid or gas to or from unit area of a fixed mate-rial and per degree temperature difference. The values depend on the condition for convection and conduction and can be improved by optimizing operational conditions.
tp is the thickness of the material that separatesthe two flowing media; increasing tp will decrease k because the heat must be transported a greater dis-tance through the fixed material. Use of a thin fixed material results in a low k value.
l is the heat convection factor for the material;steel and other metals have a high factor, while glass and plastics have a lower factor. This why metal with good conductivity is used for the heat transfer plates.
Rf is the fouling factor, which gives the amountof fouling on the material of the heat transfer plates. Fouling reduces k, and therefore the rate of energy transfer will be reduced. High turbulence close to the surface of the heat transfer plates will reduce the amount of fouling, in addition to improving a1 and a2. Cleaning of the exchange sur-faces will also reduce the fouling, Rf. The reduction in k is because the conductivity of the layer of fouling is low and the thickness of the transfer material is increased.
Manufacturers of heat exchangers normally give kA as a single value. This is because each manufac-turer will have their own design for the heat transfer area to create optimum flow with turbulence; to improve conditions for turbulence and increase the heat transfer area, various patterns are used on the exchange surface, including grooves and corrugations.
The temperature gradient between the warm and the cold side in the exchanger (Fig. 7.5) ensures energy transfer. It is expressed as the LMTD. A log-arithmetic expression is used because temperature equalization between the media through the exchanger may not be linear. LMTD can be calcu-lated from the following equation:
T1=tin(hot water)− tout(heated water)
T2=tout(cooled water)− tin(water to be heated).
If the amount of water and the heat exchange area are the same on both sides of the heat exchanger, ∆T1= ∆T2=LMTD. LMTD will vary depending onwhether it is a parallel-flow or counterflow exchanger.
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