A set of chilled water primary pumps can be modeled. Note that this set of pumps should only be modeled if there is a separate building chilled water loop (i.e. secondary loop). If there is a single set of pumps that flows chilled water through the chillers (or heat exchanger) AND out to the building cooling coils, only the “building/secondary” pumps should be defined, and the primary pumps left blank.
For chilled water pumps, the constant speed pumps (primary loop for the chillers) take as inputs the pump design sizing (nameplate horsepower), rated flow rate, pump efficiency and operating head. All of these inputs are optional for the user in the sense that they can either be auto-sized using rule-of-thumb defaults according to the chiller size, or have typical defaults available. Building/secondary pumps are handled the same way in the model, with the exception that they are assumed to be variable speed and have an additional input of minimum part load ratio (with default of 25% available). A constant speed pump can be modeled by entering 100% for this field.
The secondary or building loop pumps take as required inputs the design sizing (horsepower) and the part load ratio for stage-up, which should be assumed to be 100% if not otherwise known. Optional inputs include the water flow rate (autosizing available), as well as efficiency, minimum part load ratio and pump head (defaults available).
Condenser water temperature control: The setpoint for the condenser water entering temperature (cooling tower discharge water temperature) can be set either to a constant value or can be reset as a constant offset from the outdoor air wet bulb temperature with low limits that are typically around 60 to 65°F, but vary by chiller manufacturer The design wet bulb approach temperature is a measure of the effectiveness of the cooling towers in cooling the water to a temperature approaching the outdoor air wet bulb temperature. It is recommended to use 5°F. A temperature offset for the reset can then be specified as greater than or equal to the design approach temperature.
Cooling towers should be defined for water-cooled chillers. There are two required fields for cooling towers: the cooling tower fan type (one-speed, two-speed, and variable speed) and part load ratio for stage-up. The latter should be assumed to be 100% unless otherwise known. Fan design sizing (horsepower) and cooling tower rated sizing (tonnage) are optional fields that can otherwise be autosized. These parameters are often hard to find either in the field or from other documentation sources.
Four additional types of control need to be defined for the operation of the chilled water loop:
- Cooling Coil Lockout: Some buildings use an outdoor air temperature-based lockout on the cooling plant/cooling coils. If so, this lockout can be set here.
- Chilled water temperature control: The setpoint control for the chilled water temperature must be specified. The setpoint can be set as constant or a linear reset can be specified to control the chilled water temperature setpoint based on the average chilled water valve command
- Condenser water temperature control: The setpoint for the condenser water entering temperature (cooling tower discharge water temperature) can be set either to a constant value or can be reset as a constant offset from the outdoor air wet bulb temperature.
- Chilled water differential pressure control: The setpoint for the hot water loop differential pressure can be set to a constant value, or can use a linear reset based on the AHU chilled water valve command.
- Chilled water secondary pump on/off control: Some building chilled water pumping systems may shut down completely when there is no demand for chilled water at the building level, whereas others may stay on at all times, and others may stay on any time the outdoor air temperature is above a threshold temperature. If the control is unknown, it is recommended to have the pumps shut off when there is no demand.