Technical
HEAT EXCHANGERS
PART 2
By Marius La Grange , general manager , Thermocoil
A basic introduction to fin and tube designs .
“
Heat exchangers ( HEs ) transfer heat from one fluid to another without the fluids coming in direct contact ” – ( ASHRAE , 2008 , Chapter47 ). An exchange of heat energy taking place between at least two fluids . A fluid of course being a vapour or a liquid or a combination of both .
TYPES OF FIN AND TUBE HEAT EXCHANGERS COMMONLY USED
Due to the relative ease of manufacturing fin and tube designs , these are used in a variety of HVAC & R applications . Not limited to these of course , but these would be the most commonly found in operation .
• Dry coolers Similarly , to an air-cooled condenser , a dry cooler is an air-cooled device making use of ambient air to remove heat from the enclosed fluid . The cooling of a secondary coolant circulated through the internal portion of the coil with the heat transfer towards the ambient air drawn across the surface area . The internal fluid could of course be in a superheated gaseous state of which the temperature is reduced .
• Gas-coolers ( CO₂ systems ) Systems operating with CO₂ ( R744 ) as refrigerant offers substantial direct and indirect benefits to the environment hence these applications have grown exponentially , and the technology is now well established .
R744 is a natural refrigerant like R717 but it typically operates and higher pressures when compared to R717 or most synthetic refrigerants . In trans-critical applications a gas-cooler is used to lower the discharge temperature from a compressor with ambient air . In warm ambient conditions the discharge gas simply experiences a reduction in heat energy . A distinct property of CO₂ is its relatively high critical temperature of 31 ° C .
Ambient conditions above 31 ° C are common in many parts of the world so condensing with ambient air is not possible but the discharge gas from the compressors could be significantly reduced . Operating pressures of up to 120bar is common for gas-coolers . In low ambient conditions a gas-cooler would function very similarly to an air cooled condenser with high pressure condenser liquid draining from the outlet of the coil .
MATERIALS COMMONLY USED Heat exchangers could be made with tubes only with no fin material in specific applications like ‘ tank coils ’ where an evaporator coil might be used to cool a fluid .
• Tube materials In the case of the fin and tube coil the tubes are mostcommonly copper with a few tube diameters options on offer . There are many reasons for this , but the major ones would be that copper is a very good conductor of heat ( second best behind gold ) and it is also rather easy to work with and weld .
The operating pressures of any HCFC or HFC type system is well within the safe pressure limits of copper tubing options . As is the case with refrigeration tubing the imperial designation is still commonly used . 3 / 8 ”, 12mm and 5 / 8 ” is very common globally ( don ’ t ask me about the logic ). ( If you have not reached your thirtieth birthday yet 1 inch is 25.4mm ).
Depending on the fluid circulated within the coil , the type of copper tube is selected . In the case of water tubing with smooth inner sidewall would be selected . Applying fluid dynamic laws one can understand that laminar fluid flow would reduce the pressure drop from inlet to outlet .
In the case of DX applications tubing with an ‘ inner groove ’ is on offer as well . This could be used in evaporator or condenser applications . Various inner groove patterns are on offer , but they all aim to increase the inner surface area . The inner surface area could be up to 68 % bigger with an increase in heat transfer of as much as 30 % compared to a similar application with smooth bore tubbing used ( Onan , Ozkan , and Ceran 2013 ).
Copper tubes and return bends are gas welded with a suitable filler rod after the materials are correctly prepared .
Stainless Steel ( SS ) is also very commonly used to make tube and fin type heat exchangers . The main reason in such cases would be the required operation pressures and the type of refrigerant used . In the case of a trans-critical CO₂ ( R744 ) systems the operating limit of a gas cooler might be 120 Bar , as mentioned previously , making the SS tubing an obvious choice . Grade 304SS ( A2 ) is most common .
In the cases of systems operating with Ammonia ( R717 ) as refrigerant SS is also a good choice since the Ammonia
www . hvacronline . co . za RACA Journal I May 2021 49