Controls, Start-Up, Operation, Service And Troubleshooting - Carrier

Temperature Reset — Temperature reset is a valueadded to the basic leaving fluid temperature set point. The sumof these values is the control point. When a non-zero temperaturereset is applied, the chiller controls to the control point,not the set point. The control system is capable of handlingleaving-fluid temperature reset based on cooler fluid temperaturedifference. Because the change in temperature through thecooler is a measure of the building load, the temperature differencereset is in effect an average building load reset method.The control system is also capable of temperature reset basedon outdoor-air temperature (OAT), space temperature (SPT), orfrom an externally powered 4 to 20 mA signal. An accessorysensor must be used for SPT reset (33ZCT55SPT). The energymanagement module (EMM) is required for temperature resetusing space temperature or a 4 to 20 mA signal.Under normal operation, the chiller will maintain a constantleaving fluid temperature approximately equal to the chilledfluid set point. As the cooler load varies, the cooler fluidtemperature difference will change in proportion to the load asshown in Fig. 18. Usually the chiller size and leaving-fluidtemperature set point are selected based on a full-load condition.At part load, the fluid temperature set point may be lowerthan required. If the leaving fluid temperature were allowedto increase at part load, the efficiency of the machine wouldincrease.Temperature difference reset allows for the leaving temperatureset point to be reset upward as a function of the fluidtemperature difference or, in effect, the building load.To use Water Temperature Difference Reset, four variablesmust be configured. They are: Cooling Reset Type(Configuration→RSET→CRST), Delta T No Reset Temp(Setpoints→COOL→CRT1), Delta T Full Reset Temp(Setpoints→COOL→CRT2) and Degrees Cool Reset(Setpoints→COOL→DGRC). In the following example usingWater Temperature Difference Reset, the chilled watertemperature will be reset by 5.0° F (2.8° C) when the ΔT is2° F (1.1° C) and 0° F (0° C) reset when the ΔT is 10° F.The variable CRT1 should be set to the cooler temperaturedifference (ΔT) where no chilled water temperature resetshould occur. The variable CRT2 should be set to the coolertemperature difference where the maximum chilled watertemperature reset should occur. The variable DGRC shouldbe set to the maximum amount of reset desired. To verify thatreset is functioning correctly proceed to Run Status→VIEW,and subtract the active set point (SETP) from the controlpoint (CTPT) to determine the degrees reset. See Fig. 18 andTable 25.Other, indirect means of estimating building load and controllingtemperatures reset are also available and are discussedbelow. See Fig. 19.To use Outdoor Air Temperature Reset, four variables mustbe configured. They are: Cooling Reset Type (Configuration→RSET→CRST), OAT No Reset Temp (Setpoints→COOL→CRO1), OAT Full Reset Temp (Setpoints→COOL→CRO2) and Degrees Cool Reset (Setpoints→COOL→DGRC). In the following example, the outdoor airtemperature reset example provides 0° F (0° C) chilled waterset point reset at 85.0 F (29.4 C) outdoor-air temperature and15.0° F (8.3° C) reset at 55.0 F (12.8 C) outdoor-air temperature.See Fig. 20 and Table 26.65DEGREES RESET (deg F)4321(EXAMPLE)00246810COOLER FLUID TEMPERATURE DIFFERENCE (deg F)Fig. 18 — Water Temperature Difference Reset34