HVAC 240 unit 36 Objectives After studying this

HVAC 240 unit 36

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

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

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

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

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

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

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.

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

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

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

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

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.

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

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

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)

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

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

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

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

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

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

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

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.

Top-Air-Discharge Condensing Units • Modern trend in residential equipment Figure 36– 31 In the top-airdischarge units, the fan is usually on the top. – 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

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 slanttype condensers to keep the coil off the ground

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

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

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

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

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

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

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

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

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

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