HVACR317 - Refrigeration

1. HVACR317 - Refrigeration Condensers and Subcooling

2. Condensers • The part of the refrigeration system which receives high-temperature, high-pressure vapor from the compressor, and removes heat from the refrigerant until it returns to a liquid state.

3. Condensers • Must be larger than the evaporator. • Must be able to hold 100% of the charge without a receiver.

4. Condensers • Removes heat via: • Heat of evaporation • The heat absorbed from the evaporator. • Heat of compression • The heat produced by the compression process.

5. Condenser Capacity • Factors that affect capacity: • Surface area • The physical contact area of the condenser. • Temperature difference between the • Cooling medium (air or water) • Refrigerant gas

6. Condenser Capacity • Factors that affect capacity, cont’d: • Velocity of the gas The greater the velocity (the faster the gas flows through the condenser) the • better the heat transfer. • greater the capacity. • Material the condenser is made of • The cleanliness of the surface area • Dirty condensers do not transfer heat well.

7. Condenser Capacity • Large Condensers • Good temperature difference between the refrigerant and the cooling medium. • The result is: • Low discharge pressure • Good subcooling

8. Condenser Capacity • Small Condensers • Little temperature difference • The result is: • High discharge pressure • Very little subcooling

9. Condenser Capacity • Dirty Condensers • Little temperature difference • The result is: • High discharge pressure • Very little subcooling

10. Condenser Capacity • The end result of a condenser that is to small or is dirty: • Overheating of compressor • Boiling of oil • Compressor failure

11. Condenser Types • Two Types of Condensers: Air Cooled and Water Cooled • Air Cooled. Use air to cool the condenser as found in most residential or light commercial units. • Water Cooled. Use water to cool the condenser, found in larger commercial and some ice machines, and some geothermal applications.

12. Air-cooled Condensers • Natural Convection • Transfers heat to the surrounding air by means of natural air flow. • Wire Static (Static) • Constructed out of steel tubing interlaced with wire. • Mounted to the back of the unit. • Cooled by ambient air.

13. Air-cooled Condensers • Plate-Static (Warm Wall Condenser) • Tubing is in contact with outer wall. They use the same principle as radiant heat to cool. • The advantage of this type is that it allows the unit to go flush to the wall, and no side-wall heaters are needed. • The disadvantage is that leaks are hard to find and costly to repair.

14. Air-cooled Condensers • Forced Draft Condensers • Use a fan to move air through the coils. • Fins are connected to condenser tubes to provide more surface area for heat transfer.

15. Air-cooled Condensers • The normal temperature difference between the condensing point of the refrigerant and the ambient air is between 20-30° F. • The condensing point of the refrigerant is found by converting the pressure in the condenser to the temperature.

16. Air-cooled Condensers • Subcooling takes place in the last few passes of the condenser. • To measure subcooling, use the condensing point and subtract the temperature of the refrigerant, leaving the condenser (what). • The temperature of the refrigerant is found by placing a thermometer on the outlet of the condenser.

17. Air-cooled Condensers • If the condenser has a fan that enhances air flow, it is a forced draft condenser. • Window A/C • Refrigeration systems • Split A/C systems • All our shop condensers

18. Water-cooled condensers • Water cooled condensers were developed before air cooled condensers. • Water cooled condensers are more efficient than air cooled condensers. • Same principle as air cooled but water is used to extract the heat instead of air.

19. Water-cooled condensers • Water-cooled Condensers come in different types, like: • Tube in tube • Shell and coil • Shell and tube

20. Water-cooled Condensers • Tube in Tube Characteristics: • Made by putting one pipe inside another, larger diameter tube. • The tubes are wrapped into a circle to save space. • The refrigerant is in the outer tube with water in the inner tube.

21. Water-cooled Condensers • Tube in Tube, Cont’d: • The heat is transferred from the refrigerant to the water through the walls of the inner tube. • This style is frequently used in ice machines and geothermal heat pumps. • Mineral deposits will eventually build up in these condensers and will need to be treated or cleaned. • As minerals build up it will decrease water circulation.

22. Water-cooled Condensers • Shell and Tube Condenser Characteristics: • There is a shell, or a container that contains the refrigerant; the tube is running through the refrigerant with a high rate of water. • The heat exchanges from the refrigerant to the water and condenses to a liquid, and comes out of the bottom in the liquid line. • Also known as ‘water boxes.’

23. Water-cooled Condensers • Shell and Tube Condensers, Cont’d.: • The end caps can be removed so that the tubes can be brushed out and cleaned to prevent mineral build up.

24. Water-cooled Condensers • Shell and Coil Condenser Characteristics: • Same idea as shell and tube, but a little less expensive. • The water line is coiled down the center. • Hard to clean if minerals build up.

25. Water-Cooled Condensers, General • What happens to the water? • Water-cooled condensers must have a way for the water to reject the heat. • Some are called wastewater systems – water is used once then wasted. • Some are called re-circulated – water is used to cool the condenser, then the water is cooled and re-circulated.

26. Water-Cooled Condensers, General • Cooling Towers • Use is limited by the outdoor wet-bulb temperature. • Require yearly cleaning, consistent water treatment and air movement.

27. Water-Cooled Condensers, General • The refrigerant will normally condense at 10° F higher than the leaving water temperature. • A re-circulating water system will circulate about 3 gallons of water per minute per ton of refrigeration.

28. Water Regulators • The water regulating valve will regulate the flow of water through the condenser. • The water regulating valve is controlled by the head (discharge) pressure of the compressor. • As the load increases so does the head pressure and so does the water flow.

29. Water Regulators • Always adjust a water regulator to the manufacturer’s recommended flow rate in gallons per minute.

30. Additional Notes on Condensers • A properly functioning condenser will de-superheat, condense and sub-cool the refrigerant. • When a condenser is placed in a low temperature environment, a low ambient (fan cycling) control can be used to cycle the fan on and off, thus maintaining head (high side) pressures.

31. Additional Notes on Condensers • Low ambient temperatures seriously affect the efficiency of air-cooled condensers.

32. Subcooling • What is subcooling? • A temperature of a liquid when it is cooled below its condensing temperature. • Subcooling is the sensible heat removed from the liquid after the change of state has taken place. • Subcooling is used to determine if the condenser has the proper level of refrigerant.

33. Subcooling • The complete condensing of refrigerant should occur in the bottom quarter of the condenser. • Any heat that is removed after this point is subcooling.

34. Subcooling • Subcooling is needed to maintain proper system balance. • Normal subcooling is between 15-20° F. • Depending on how efficient the condenser is, subcooling may be a little higher or lower.

35. Subcooling • If the subcooling is low and you have a high head pressure • Possible dirty condenser • Check condenser fan operation • Look for overcharged condition • Could have non condensibles in it • Possible restriction before evap coil • If the subcooling is high • Low charge (could be overcharged) • Check comp amps, if lower than RLA rating than low charge

36. Subcooling • Subcooling can be accomplished by placing the liquid line and the suction line in direct contact with each other. • This is usually done in small or low-temperature units. • This higher subcooling results in a more efficient unit.

37. Subcooling • The liquid in the liquid line will be cooled below its condensing temperature. • Also the suction line may be warmed slightly to boil any refrigerant that may be present before entering the compressor. • The lower the temperature in the liquid line, the greater the heat removal capacity.

38. Subcooling • This greater subcooling can also be achieved by one system cooling another so that the system can more efficiently reach lower temperatures. • This is known as a cascade system. • Cascade systems are usually required for ultra-low temperature operations.

39. Measuring Subcooling • Determine the condensing temperate. • Use your gauges and convert the high side. • This should be the ambient temperature plus 30-35 degrees for air cooled systems. • Measure the condenser outlet or the liquid line temperature.

40. Measuring Subcooling • Subtract the liquid line temperature from the condensing temperature. • The result is subcooling.

41. Subcooling Example