HVAC 240 unit 36

1. HVAC 240unit 36

2. Objectives • After studying this chapter, you should be able to: • Explain three ways that heat transfers into a structure • Explain refrigeration as applied to air conditioning • Describe the evaporators, compressors, condensers, and metering devices that are used on air conditioning systems

3. Objectives ‏ • List the various types of evaporators found on air conditioning systems • Explain the difference between standard and high efficiency systems • Describe package and split-type air conditioning systems • Note: Equipment will perform at it’s best efficiency when running at full load

4. 36.1 Refrigeration Air conditioning is refrigeration applied to keeping an occupied space at the desired temperature in warmer months Air conditioning equipment removes heat that leaks into the structure and puts it back outside where it came from Many components used for air conditioning are different from those used for commercial refrigeration

5. 36.2 Structural Heat Gain • Heat leaks into a structure by conduction, infiltration, and radiation Direct sun light is the main cause of radiation solar load • Summer solar load on a structure is higher on the east and west sides (fig.36.1) and would be lower on the north and south sides • Attic ventilation can help relieve solar load on ceiling

6. 36.2 Structural Heat Gain • Heat enters by conduction through walls, windows, and doors, and by infiltration through cracks around or through open doors and windows • Infiltrating air has different characteristics (humidity), depending on location, that must be taken into account

7. 36.2 Structural Heat Gain Figure 36-2 A ventilated attic, power fan or natural helps keep the solar heat from the ceiling of the house

8. 36.2 Structural Heat Gain Figure 36–4 The difference between the inside and outside temperatures of a home in Augusta, Georgia, and a home in Phoenix, Arizona.

9. 36.3 Evaporative Cooling • Used primarily in climates where humidity is low • Uses fiber mounted in a frame with water running over the fiber as cooling media • Fresh air is drawn through the fiber and cooled by evaporation to a temperature close to the wet-bulb temperature of the ambient air • There must be an outlet for air that has entered to leave the structure or it will pressurize

10. Evaporative Cooling (cont'd.)‏ Figure 36-5 An evaporative cooler

11. 36.4 Refrigerated Cooling or Air Conditioning • Refrigerated air conditioning components • Evaporator, compressor, condenser, and metering device • Package air conditioning (fig.36.6) • Self-contained; all components in one cabinet with air ducted to and from the unit • Split-system air conditioning (36.7) • Condensing unit outside; removed from evaporator; connected by refrigerant lines

12. 36.5 The Evaporator • Component that absorbs heat into the refrigeration system • All aluminium coils are very difficult to repair leaks • Refrigeration coil made of aluminium or copper with aluminium fins to increase heat exchanger surface • Common evaporator coil types: • A coil(36.8), slant coil(fig.36.9) and H coil(36.10) • Horizontal application A Coils requires a special over flow pan

13. The Evaporator (cont'd.)‏ Figure 36–8 An A coil. Notice that two coils are standing at an angle and touching at the top—shaped like an A.

14. 36.6 The Function of the Evaporator • Transfers heat from air to refrigerant • Absorbing sensible heat lowers air temperature • Absorbing latent heat removes moisture by changing water vapor to condensate, which is drained • Typical room conditions: 75°F • Evaporator coil operates at about 40°F

15. 36.6 The Function of the Evaporator • Air leaves the coil at about 55° • When climate is humid, coil temperature may be lower to remove more humidity • Coil temperature is controlled by airflow across the coil • Airflow affect coil temperature

16. 36.8 Evaporator Application • The evaporator may be installed in the airstream in several different ways • Coil normally operates below dew point temperature and coil cabinet should be insulated to prevent it from absorbing heat from the surroundings • An insulated coil cabinet will not sweat on the outside (fig.36.14)

17. 36.9 The Compressor • Vapor pump that pumps heat-laden vapor from the low-pressure (evaporator) side of the system to the high-pressure (condenser) side by compressing gas • Increases temperature and pressure of gas • Types include reciprocating, rotary, scroll, centrifugal, and screw • Use same designs as similar types used in commercial refrigeration

18. 36.9 The Compressor • Suction gas dumps inside shell near motor and rotor dissipates liquid • Usually located outside with condenser • Hermetic compressors cannot be field serviced and are rarely factory serviced

19. 36.11 Compressor Speeds (RPM)‏ • Modern small and medium size range compressors turn standard motor speeds of 3450 or 1750 rpm • Early compressors were 1750 rpm and were large and heavy • Present compressors use faster motor and more efficient and environmentally friendly refrigerants • Some modern compressors use variable speed electronics

20. 36.12 Cooling the Compressor and Motor • Lack of cooling results in damage and breakdown of compressor oil • Reciprocating and Scroll compressors: suction gas cooled • Newer compressor motors can run at higher temperatures because of better winding materials and higher-quality lubricating oils • Technician should determine that discharge temperature is correct (not over 225°F)‏ • Some serviceable semi-hermetic compressors are air cooled and have ribs to dissipate heat

21. 36.13 Compressor Mountings • Welded hermetic reciprocating compressors have rubber mounting feet outside and mount compressor on springs inside • Older compressors mounted on springs outside with compressor pressed into shell • New compressors have vapor space inside shell and motor-temperature sensor inside

22. 36.15 The Scroll Compressor • Uses two spirals, one stationary and one orbiting (fig.36.24,25,26) • The two scrolls form multiple individual “pockets” where refrigerant is trapped • The pockets get smaller as refrigerant passes through the compressor • Many compression stages occur simultaneously; highly efficient

23. 36.16 The Condenser • Component that rejects heat from the air conditioning system • Most equipment is air cooled and rejects heat to the air • The coils are made of copper or aluminum and have aluminum fins to increase the heat transfer rate • New coil designs are used in some units to make them lighter and more efficient

24. Side-Air-Discharge Condensing Units • Older units discharge air out the side of the unit • Fan and motor are located under top panel • Noise generated inside cabinets discharged in airstream • Heat from the condenser coil can be hot enough to kill plants it blows on This condensing unit discharges hot air out of the side.

25. Top-Air-Discharge Condensing Units • Modern trend in residential equipment • Hot air and noise discharge from the top into the air • Motor on top should be protected with a rain shield • Bearings mounted in vertical position need thrust surface Figure 36–31 In the top-air-discharge units, the fan is usually on the top.

26. Condenser Coil Design • Vertical condenser coils may become clogged with grass and dirt • Coils must be clean to operate efficiently • Bottom rows of coil may be used for subcooling • Lowers condensed refrigerant temperature for greater efficiency • Some manufacturers use horizontal or slant-type condensers to keep the coil off the ground

27. High-Efficiency Condensers • The times, and the federal government, demand higher efficiency • Improve efficiency by lowering head pressure • Physically larger coils → greater surface area → lower head pressure → lower compression ratio and less power consumed • Some units use two-speed condenser fans • One speed for mild weather; one for hot

28. 36.17 Expansion Devices • Meter flow of refrigerant to evaporator • Most common types are fixed-bore and TXV • TXVs are more efficient because they allow the evaporator to reach peak performance faster • Refrigerant pressures in TXV systems typically do not equalize during the off cycle, requiring compressors to use high-starting torque motors

29. Cabinet Design Cabinets are usually located outside, so they need waterproofing Aluminium is lightweight, but susceptible to rust in salty environments Most small equipment is assembled with self-tapping screws made of weather resistant sheet metal Fasten screws tightly when installing equipment in the field to prevent rattling

30. 36.19 Air-Side Components • Consist of supply air and return air systems • Airflow is normally 400 cfm/ton in average climates, 350 cfm/ton in coastal areas, and 450 cfm/ton in desert areas • Air usually leaves handler at 55°F and returns at 75°F • Ductwork running through unconditioned space must be well-insulated • Cool air distributes better from diffuser located high in the room

31. 36.20 Installation Procedures • Package systems (whole air conditioner in one cabinet)‏ • Equipment is located outside the structure so service can be done outside • Completely factory assembled and charged with short, efficient refrigerant lines • To install, mount on a firm foundation and connect ductwork and electrical service

32. 36.20 Installation Procedures‏ • Split systems • Locate condenser and evaporator as close together as possible • Install interconnecting piping (suction line and liquid line)‏ • Suction line should be insulated • Take care handling equipment • Watch out for insects, sharp objects, weak ceilings, and lines carrying refrigerant

33. Summary • Heat leaks into a structure by conduction, infiltration, and radiation • Air conditioning involves the cooling of an occupied space for comfort purposes • Evaporative cooling is used primarily in areas where the humidity is low • Air conditioning systems are typically package units or split-type systems

34. Summary‏ • Air conditioning systems typically use the same four major components as commercial refrigeration systems • Common evaporator types include the slant coil, “A” coil, and the “H” coil • The most common condensing unit type is a top-air discharge unit

35. Summary ‏ • Higher efficiency condensers have larger surface areas than standard efficiency coils • Expansion devices commonly found on air conditioning systems are the thermostatic expansion valve and the fixed-bore metering device • TXVs are more efficient than capillary tubes

36. Summary ‏ • Typically, 400 cfm of air are needed for each ton of refrigeration • Air conditioning unit are designed to at its best efficiency when running at full load