Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSHEAT PUMPSGENERALA heat pump is a r efrigeration system tha t pr ovides bothheating and cooling within the same unit. In the heating mode,the pump deli vers heat fr om a hea t sour ce to the conditionedspace. In the cooling mode , the pump r emoves heat fr om aspace and transfers it to a heat sink. Heat pumps use standardrefrigeration components (compr essor, expansion v alve,evapor ator, and condenser) and a r eversing valve to r everserefrigerant f low thr ough the coils. A refrigerant r eversing valveswitches at chang eover to con vert the condenser to an e vapor atorand vice v ersa.Figur e 36 shows heat pump c ycles. In the cooling c ycle,refrigerant f low uses the outdoor hea t exchang er coil as thecondenser to reject heat from the space, and the indoor coil asthe evapor ator. In the hea ting cycle, flow of air and r efrigerantis reversed making the outdoor coil the sour ce of heat. Theindoor coil becomes the condenser and pr ovides hea t for thespace. When outdoor air temper atur es ar e too cold to pr ovideenough hea t tr ansfer, electr ic resistance hea ting elements canbe used to pr ovide supplemental hea t. An alter native method isto r everse indoor and outdoor airf low acr oss the coils andelimina te the r eversing valve.Heat pumps ar e typicall y classified by the hea t sour ce at the“outdoor” coil. The common air -to-air hea t pump uses outdoorair as its hea t sour ce dur ing the hea ting cycle. A water -to-airheat pump uses w ater as the hea t sour ce dur ing the hea tingcycle. The water suppl y may be a well or a lak e. In the coolingmode, the outdoor coil r ejects hea t and the air or w ater becomesthe heat sink.In commer cial applica tions, a closed-loop or r unar ound w atersuppl y may serve multiple units (F ig. 37). This system r elies onload di versification. Some units ma y be cooling while other z onesoper ate in the hea ting cycle. In this case , the water loop is a sour ceto heating units and a sink to cooling units, transferring heat fromone to the other . Water-loop temper atur es ar e maintained betw een21°C and 32 °C to pr ovide an adequa te hea t sour ce or hea t sink. Acentr al boiler to gether with a c hiller and/or cooling to wer temperthe water in the loop dur ing peak hea ting and cooling per iods.For a discussion of centr al r egulation and contr ol of water pumphydr onic loops, refer to Chiller , Boiler, and Distr ibution SystemContr ol Applica tions section.HEATPUMPTHERMOSTATCOMPRESSORCHILLER ORCOOLINGTOWERREFRIGERANTREVERSINGVALVEBOILERINDOORCOILOUTDOORCOILPUMPEXPANSIONVALVE FORCOOLINGC3022INDOORCOILCHECKVALVECOOLING CYCLECOMPRESSOREXPANSIONVALVE FORHEATINGCHECKVALVEHEATING CYCLEREFRIGERANTREVERSINGVALVEOUTDOORCOILC3021Fig. 36. Refrigerant Flow in Heat PumpCooling and Heating Cycles.OPERATIONFig. 37. Heat Pump Closed-Loop System.Small and medium-sized heat pumps usually heat and coolindoor air and ar e contr olled fr om a space ther mosta t. Lar geheat pumps usuall y pr ovide war m and c hilled water and ar econtr olled by a chilled water temper atur e contr ol.Space ther mosta ts ar e two-position and usuall y multista ge.They typicall y pr ovide automa tic chang eover, switching theheat pump between heating and cooling as required to maintainspace temper atur e. The first sta ge of the ther mosta t cycles thecompr essor. If the system is in a hea ting mode , additionalthermostat heating stages will bring on supplementary resistanceheat if the heat pump cannot meet the load. In the cooling mode,ther e is only one sta ge tha t cycles the compr essor. If desir ed,chang eover betw een hea ting and cooling can be man ual r atherthan automa tic. The ther mosta t can ha ve separ ate hea ting andcooling settings or a single setting with a f ixed deadbandbetween heating and cooling.ENGINEERING MANUAL OF AUTOMATIC CONTROL 421