Selecting the correct heat exchanger for an application

Selecting the correct heat exchanger for an application

Print

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.

PlateHeat


A very simple indication on the structure of a plate heat exchanger. Photo by Wikimedia Commons


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.

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.

ShellTube


A simple schematic of a shell and tube heat exchanger. Photo by Research Gate


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 higher pressure drop, larger glycol coils with 5/8”od tubes may be a better choice.

Evaporator coils tend to be constructed of rifle bore copper tubes for normal refrigerants, but for ammonia and CO₂ coils, plain stainless steel tubed-coils with ½”OD tubes are very common.

Copper tubes are not suitable for ammonia usage and the high pressures in CO₂ systems require stainless steel tubes.

It should be noted too that coils operating on R410a, require greater tube wall thickness due to the higher pressures encountered.

Aluminium fins are the general norm while coated aluminium or copper fins are optional for corrosion resistance.

Condenser coils are constructed with either 3/8” OD or ½”OD rifle-bore copper tubes for normal refrigerant usage with smaller bore tubes gaining ground due to cost effectiveness and reduced gas charge for the same duty.

Again, coils for R410a refrigerant usage require a greater tube wall thickness due to the higher pressures encountered.

Aluminium fins are the norm with coated aluminium fins being preferred in coastal conditions.

Steam coils are commonly constructed with plain copper tubes and aluminium fins.

For low pressure applications up to approximately 7-barg, normal copper tube wall would suffice but for applications above 7-barg and up to approximately 12-barg, thicker wall tubing is required and for still higher pressures ½”OD stainless steel tubes should be considered.

SANS 10147 requires pipes and coils operating at or above 50kPa to be engineered as pressure equipment (piping) and minimum tube walls to be calculated. This would include evaporator coils, condenser coils and steam coils. Reference to and compliance with SANS 10147 is obligatory by law.

Air cooled condensers employ condenser coils as above, together with either axial or centrifugal fans to condense hot refrigerant gas to a liquid. Units can have horizontal coils with vertical air flow, vertical coils with horizontal air flow or V-coils with vertical air flow.

Horizontal coil units are not affected by winds which adversely affect both vertical or V-coil units, particularly at the coast, where prevailing winds are encountered. It is a bit of a myth that V-coil units have a smaller required footprint than horizontal coil units. Horizontal coil units have a shadow plus a free space around the unit for ingress of air, generally something like 500mm or 1 000mm depending on model whereas V-coil units have a smaller shadow but require about 1 500mm all round to allow air ingress and prevent recycling of discharge air with entering air.

The main disadvantage of air cooled condensers is that performance is dependent on ambient air temperature and units perform worse at higher ambient temperatures when the refrigeration load is generally increased. Wet evaporative pads can be added then to reduce the net air on coil temperature but this will also adversely affect the air flow.

Shell and tube condensers have tubes in a shell fixed into tube plates and condense hot discharge refrigerant gas into a liquid when colder water flows in the tubes where it is heated and then cooled in a cooling tower.

These are very compact and efficient units and are generally employed where units are housed in a basement and air cooled units cannot be considered. Connection to a cooling tower on the roof of a building is relatively simple. Maintenance is generally related to scale build up in the tubes and water treatment is recommended to maintain performance.

Land based units generally employ copper tubes with enhanced external surface while marine units have enhanced external surface cupro-nickel tubes to allow cold sea water to be used. 

Click here for the latest issue of RACA Journal