9 The main purpose of commercial HVAC (heating, ventilating, and air conditioning) systems is to provide the peopleworking inside buildings with "conditioned"air so that they will have a comfortable and safe workenvironment."Conditioned" air means that air is clean and odor-free, and thetemperature, humidity, and movement of the air arewithin certain comfort ranges.
10 Many factors affect the way people respond to their work environment.Air quality is one of these factors.The American Society of Heating, Refrigerating andAir-Conditioning Engineers (ASHRAE) has establishedstandards which outline air quality forindoor comfort conditions that are acceptableto 80% or more of acommercial building's occupants. Generally, theseindoor comfort conditions, sometimes calledthe "comfort zone," are between 68 degrees F and75 degrees F for winter and 73 degrees F to 79 degrees Fduring the summer. Both these temperatureranges are for room air atapproximately 50% relative humidity andmoving at velocity of 30 feet per minute or slower.
11 Heat naturally flows from a higher energy level to a lower energy level.In other words, heat travels from a warmermaterial to a cooler material.The unit of measurement used to describe thequantity of heat contained in a materialis a British thermal unit (Btu).
13 1. Fan(s) to circulate the supply air (SA) and return air (RA).2. Supply air ductwork in which the air flows from the supplyfan to the conditioned space.3. Air devices such as supply air outlets and return air inlets.4. Return air path or ductwork in which the air flows back fromthe conditioned space to the mixed air chamber (plenum).5. Outside air (OA) device such as an opening, louver or duct toallow for the entrance of outside air into the mixed air chamber.
14 6. Mixed air chamber to receive the return air and mix it with outside air.7. Filter section(s) to remove dirt and dust particlesfrom the mixed air.8. Heat exchanger(s) such as hot water coil(s), steamcoil(s), refrigerant evaporator(s), or chilled water coil(s) toadd heat to or remove heat from the circulated air.9. Auxiliary heating devices such as natural gasfurnace(s) or electric heating element(s).10. Compressor(s) to compress the refrigerant vaporand pump the refrigerant around the system.
15 11. Condenser(s) to remove heat from the refrigerant vapor and condense it to a liquid.12. Fan(s) to circulate outside air across air-cooledcondenser(s)13. Pump(s) to circulate water through water-cooledcondenser(s); condenser water pump (CWP);and condenser water supply (CWS) and return (CWR).14. Pump(s) to circulate hot water from the boiler(s)through the hot water coil(s) and back orto circulate chilled water from the chiller(s) throughthe chilled water coil(s) and back to the chiller(s).
16 15. For central systems, water or steam boiler(s) as a central heating source.16. For central systems, water chiller(s) as a centralcooling source.17. For central systems, cooling tower(s) with water-cooledcondenser(s).18. Controls to start, stop, or regulate the flow of air,water, steam, refrigerant and electricity.
17 Science: heat is transferred in three possible ways Radiation: an energy source transfers energy in a direct line to a surface (feeling the heat of the sun's light on your skin)Conduction: direct contact with a surface of a different temperature causes energy transfer (burning your finger when you touch a hot oven rack)Convection: energy transfer through fluid motion (hot air rises)
18 condensation on window glass: the cold outside air cools the piece of glass; the warm inside air contacts the cold glass; the moisture contained in the warm air ‘drops out’ of the warm air as water/condensation
19 very cold temperatures can cause warm air inside of a building to become ice on the cold piece of window glass
20 friday, february 10, 2006: the dew point temperatures around the country
21 Heating devices include gas or oil burning central furnacewood stoveheat pumpelectric radiatorsolar panelradiant heating coils in floors, or ceilings (these may be electric, water filled, or air filled)
22 Cooling methods/devices include natural ventilationair movement: fanswindow air conditionercentral air conditioning systemmassive materials that have a long 'thermal lag' time, (such as brick, concrete, stone)
24 Vapor compression refrigeration cycle refrigerators, air conditioners, and heat pumps each provide cooling through this process (vcrc)this process is a true modern miracle, and it is based on the discovery that increased pressure creates heat.
25 A refrigerant is a fluid which vaporizes (boils) at a low temperature.The refrigerant circulates through tubes("refrigerant lines") that travel throughout the heat pump.At point A the refrigerant is a cold liquid -- colderthan the outdoor air.The refrigerant flows to theoutdoor coil (point B). This coil is a "heat exchanger"with a large surface areato absorb heat from the air into the colder refrigerant.The heat added to therefrigerant causes the fluid to vaporize
26 At point C the refrigerant is a cool gas, having been warmed and vaporized by the outdoor air. It is too cool to warm the house, so that's where the compressor(point D) comes in.The compressor raises the pressure of the gas. When thathappens, the gas temperature rises.The indoor coil (point F) is where the refrigerant givesup its heat to the indoor air. A fan blows airpast the indoor coil to distribute heat to the house. This coolsthe refrigerant to the point where much of it condenses,forming a liquid.
27 As it gets colder outside, the heat pump provides less heat. Yet the house needs more heat to keep comfortable.At some outdoor temperature it will be too cold for theheat pump to provide all the heatthe house needs. To make up the difference, heat pumps havea supplemental heating system - usually electric resistance coils
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