Carrier AQUASNAP AIR COOLED CHILLERS WITH COMFORTLINK CONTROLS 30RAP010-060 Operation And Service Manual

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Carrier AQUASNAP AIR COOLED CHILLERS WITH COMFORTLINK CONTROLS 30RAP010-060 Operation And Service Manual Manual pdf 42 page image

42Under normal operation, the chiller will maintain a constantleaving fluid temperature approximately equal to the chilledfluid set point. As the cooler load varies, the entering coolerfluid will change in proportion to the load as shown in Fig. 20.Usually the chiller size and leaving-fluid temperature set pointare selected based on a full-load condition. At part load, the flu-id temperature set point may be colder than required. If theleaving fluid temperature was allowed to increase at part load,the efficiency of the machine would increase.Return temperature reset allows for the leaving temperatureset point to be reset upward as a function of the return fluidtemperature or, in effect, the building load.Demand Limit — Demand limit is a feature that allowsthe unit capacity to be limited during periods of peak energy us-age. There are 3 types of demand limiting that can be config-ured. The first type is through 2-stage switch control, which willreduce the maximum capacity to 2 user-configurable percentag-es. The second type is by 4 to 20 mA signal input which will re-duce the maximum capacity linearly between 100% at a 4 mAinput signal (no reduction) down to the user-configurable levelat a 20 mA input signal. The third type uses the CCN Loadshedmodule and has the ability to limit the current operating capaci-ty to maximum and further reduce the capacity if required.NOTE: The 2-stage switch control and 4 to 20-mA input sig-nal types of demand limiting require the energy managementmodule (EMM).To use demand limit, select the type of demand limiting touse. Then configure the demand limit set points based on thetype selected.DEMAND LIMIT (2-Stage Switch Controlled) — To con-figure demand limit for 2-stage switch control set the DemandLimit Select (ConfigurationRSETDMDC) to 1. Thenconfigure the 2 Demand Limit Switch points (Configura-tionRSETDLS1) and (ConfigurationRSETDLS2)to the desired capacity limit. See Table 27. Capacity steps arecontrolled by 2 relay switch inputs field wired to TB6 as shownin Fig. 6 and 7.For demand limit by 2-stage switch control, closing the firststage demand limit contact will put the unit on the first demandlimit level. The unit will not exceed the percentage of capacityentered as Demand Limit Switch 1 set point (DLS1). Closingcontacts on the second demand limit switch prevents the unitfrom exceeding the capacity entered as Demand Limit Switch2 set point. The demand limit stage that is set to the lowest de-mand takes priority if both demand limit inputs are closed. Ifthe demand limit percentage does not match unit staging, theunit will limit capacity to the closest capacity stage.To disable demand limit configure DMDC to 0. SeeTable 27.EXTERNALLY POWERED DEMAND LIMIT (4 to20 mA Controlled) — To configure demand limit for 4 to 20mA control set the Demand Limit Select (Configura-tionRSETDMDC) to 2. Then configure the DemandLimit at 20 mA (ConfigurationRSETDM20) to themaximum loadshed value desired. Connect the output from anexternally powered 4 to 20 mA signal to terminal block TB6,terminals 1 and 5. Refer to the unit wiring diagram for theseconnections to the optional/accessory energy managementmodule and terminal block. The control will reduce allowablecapacity to this level for the 20 mA signal. See Table 27 andFig. 21.LEGENDLWT — Leaving Water (Fluid) TemperatureFig. 18 — Outdoor-Air Temperature ResetLEGENDLWT — Leaving Water (Fluid) TemperatureFig. 19 — Space Temperature ResetCAUTIONCare should be taken when interfacing with other manufac-turer’s control systems, due to possible power supplydifferences, full wave bridge versus half wave rectification.The two different power supplies cannot be mixed.ComfortLink™ controls use half wave rectification. A sig-nal isolation device should be utilized if a full wave bridgesignal generating device is used.LEGENDFig. 20 — Standard Chilled FluidTemperature Control — No ResetEWT — Entering Water (Fluid) TemperatureLWT — Leaving Water (Fluid) Temperature