Technical
SELECTING THE CORRECT HEAT EXCHANGER FOR AN APPLICATION
By Peter Timm
In the refrigeration and air conditioning industry there are a variety of heat exchangers encountered in general practice .
This article is a simple explanation of the various types and basic applications . These include plain tube , fin and tube , shell and tube and plate types , which can again be broken down depending upon the application .
Plain tube types include evaporative condensers and ice bank coils .
Fin and tube types include a variety of air heating and cooling coils using hot water , steam , chilled water or glycol , and evaporating or condensing refrigerant .
Shell and tube types include shell and tube condensers , direct expansion ( DX ) chillers , flooded chillers and liquid to liquid heat exchangers .
Plate heat exchangers are generally either of a brazed type or cleanable / gasketed type of a stainless steel construction . The brazed type is generally limited to smaller models for both refrigerant evaporator and condenser applications , ( not ammonia ), with larger models being of the gasketed type .
Evaporative condensers comprise a plain tube coil bundled in an air stream with re-circulating water cascading over the tubes in a spray . Hot refrigerant gas enters the tube bundle and is condensed to a liquid by the cooling water enhanced by the evaporative effect of the water spray .
The tube bundle is generally of hot dip galvanised steel or stainless steel construction .
These condensers are relatively compact with a small footprint but are most effective in areas with a low wet bulb temperature . But not in the eastern coastal regions of South Africa where the wet bulb is relatively high .
They are susceptible to corrosion by the increased salts concentration as re-circulated spray water is evaporated and require bleed-off and water treatment to maintain efficiency .
Ice bank coils generally consist of a bare pipe coil submerged in a tank of water . Refrigerant is fed into the coil and evaporates , resulting in ice formation on the outer surface of the tubes . The water in the tank is circulated over the tube bundle and approaches 0 ° C .
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This generally occurs during off peak times when electricity supply is cheaper and the ice formed provides a cooling source during peak electricity supply periods . Tubes would be of copper or stainless steel for industrial applications but copper tubes in water are not allowed in food processing plants due to copper oxide formation at the water / air interface .
Fin and tube coils are generally of plain copper tube construction with rifle bore copper , stainless steel , aluminium and hot dip galvanised steel as alternatives . Aluminium fin with coated aluminium , copper , stainless steel and hot dip galvanised as alternatives are generally available .
Hot water coils are generally of plain copper tube with aluminium fin construction with small coils having 3 / 8 ” OD tubes and larger coils with ½ ” or 5 / 8 ” OD tubes .
½ ” OD tubed-coils tend to be more cost effective than 5 / 8 ” OD tubed-coils .
Chilled water / glycol coils are generally of plain copper tubes with aluminium fins and small coils having 3 / 8 ” OD tubes and larger coils with ½ ” or 5 / 8 ” OD tubes . Again ½ ” OD tubed-coils tend to be more cost effective than 5 / 8 ” OD tubed-coils but with
Ice banks are also part of the heat exchanger family and are used in applications to minimise cooling capacity needs during peak times .
www . hvacronline . co . za RACA Journal I March 2021 49