B. logarithmic mean temperature difference is big, small terminal temperature difference in tube and shell heat exchanger, two kinds of fluid flow in tube side and shell side, respectively, on the whole is wrong flow, logarithmic mean temperature difference correction coefficient is small, and the plate heat exchanger is more and flow or countercurrent flow mode, the correction coefficient is usually at around 0.95, in addition, cold and hot fluid flow in the plate heat exchanger is parallel to the heat transfer surface, no side stream, thus makes the plate heat exchanger at the end of the temperature difference is small, the water heat exchanger can be less than 1 ℃, shell and tube heat exchanger is usually 5 ℃.
C. cover an area of an area small plate heat exchanger structure is compact, per unit volume of heat exchange area of tube and shell of 2 ~ 5 times, also don't like tube shell type want to reserve a spare tube bundle service place, so for the same in heat, plate heat exchanger which covers an area of about 1/5 ~ 1/10 of tube and shell heat exchanger.
D. easy to change the heat transfer area or process combination, as long as the increase or decrease a few pieces of board, can achieve the purpose of increase or decrease heat exchange area; Change of plates or replace a few pieces of plate, can achieve the required process combination, adapt to the new working condition of heat transfer, shell and tube heat exchanger heat transfer area is almost impossible to increase.
E. light weight of the plate heat exchanger plate thickness of only 0.4 ~ 0.8 mm, the thickness of heat exchange tube and tube heat exchanger is 2.0 ~ 2.5 mm, tube and shell of shell much heavier than the framework of plate heat exchanger, plate heat exchanger is generally about 1/5 of the weight of tube and shell.
F low price of the same material, under the same heat transfer area, prices of plate heat exchanger tube and shell type low about 40% ~ 60%.
G. convenient plate heat exchanger heat transfer plate is made by stamping processing, high degree of standardization, and can be mass production, usually use handmade tube heat exchanger.
H. easy cleaning frame type plate heat exchanger as long as the bolt looseness pressure, can loosen plate beam and plate for mechanical cleaning, heat transfer process of the need to constantly cleaning equipment is very convenient.
I. only heat transfer plate shell heat loss small plate heat exchanger plate is exposed to the atmosphere, so the heat loss can be neglected, also do not need to heat preservation measures. Heat exchanger of shell and tube heat loss is big, need to heat insulation layer.
J. smaller capacity is 10% ~ 20% of tube and shell heat exchanger.
K. the pressure loss of per unit length is big because the gap between the heat transfer surface is small, the heat transfer surface has concave and convex, therefore is bigger than traditional smooth pipe pressure loss.
L. not easy scaling due to internal fully turbulent, so not easy to scale, the scale coefficient is only of tube and shell heat exchanger 1/3 ~ 1/10.
M. shoulds not be too great work pressure, medium temperature is unfavorable and exorbitant, may let the cat out of the plate heat exchanger using gasket seal, pressure should not be commonly more than 2.5 MPa, medium temperature should be below 250 ℃, otherwise may leak.
N. easily blocked due to the plate between the channel is very narrow, usually only 2 ~ 5 mm, when the heat transfer medium containing larger particles or fiber material, easy to jam between channels.
1.4 the application of plate heat exchanger
A. refrigeration: used for condenser and evaporator.
B. hvac: use in the middle of the boiler heat exchanger, high-rise building intermediate heat exchanger, etc.
C. chemical industry: soda ash industry, synthetic ammonia, alcohol fermentation, resin composite cooling, etc.
D. metallurgical industry: aluminate liquor heating or cooling, steelmaking process cooling, etc.
E. machinery industry: all kinds of quenching liquid cooling, reducer lubricating oil cooling, etc.
F. power industry: high voltage transformer oil cooling, oil cooling generator bearing, etc.
G. papermaking industry: bleaching process heat recovery, heating wash slurry, etc.
H. textile industry: viscose rayon aqueous alkali cooling, boiling nitrocellulose cooling, etc.
I. the food industry: juice sterilization cooling, animal and plant oil heating or cooling.
J. grease technology: soap base ambient pressure, heating or cooling of various process with liquid.
K. central heating: the heating power plant waste heat area, heat the bath water.
L. other: petroleum, medicine, shipbuilding, desalination, and geothermal.
1.5 should be paid attention to in the model selection of the plate heat exchanger
1.5.1 plate type selection
Plate type or corrugated type should be based on the actual needs of the heat exchange places. For large flow pressure drop allows small, should choose small resistance of the plate type, vice choose resistance of plate type. According to the fluid pressure and temperature, determine the removable type choice, or brazing type. Plate type is unfavorable choose veneer board piece area is too small, in order to avoid board piece of too many, small flow rate between plate, heat transfer coefficient is too low, for larger heat exchanger more attention should be paid to the problem.
1.5.2 process and selection of runner
Process refers to a medium plate heat exchanger in a set of parallel flow, flow direction and flow within the dow plate heat exchanger, consisting of two adjacent plate medium flow channel. In general, several port in parallel or in series of fee that is connected to form a different combination of cold and hot media channels.
Process combination shall be computed according to the change of heat and fluid resistance, in satisfies the requirement of process conditions under the determined. Try to make the cold, hot water flow passage of convection heat transfer coefficient is equal or close to, to get the best heat transfer effect. Because the heat transfer surface on both sides of the convective heat transfer coefficient is equal or close to the heat transfer coefficient for larger values. Although velocity varies among different plate, plate heat exchanger in the change of heat and fluid resistance calculation, is still at an average velocity to calculate. Due to the "U" shape of the single process over all fixed on the pressure plate, tear open outfit is convenient.
1.5.3 check pressure drop
In design, selection of plate heat exchanger and generally have a request for the pressure drop, so deal with it to check. If check pressure drop exceeds allowed pressure drop, redesign to type selection calculation, until meet the technological requirements
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