The variable refrigerant flow (VRF) DX heating coil model uses performance information at rated conditions along with performance curves for variations in total capacity, energy input ratio and part load fraction to determine performance at part-load conditions. The impacts of defrost operation is modeled based a combination of user inputs and empirical models taken from the air-to-air heat pump algorithms in DOE-2.1E.
The VRF DX heating coil input requires an availability schedule, the gross rated heating capacity and the rated air volume flow rate. The rated air volume flow rate should be between 0.00008056 m3/s and 0.00002684 m3/s per W of gross rated heating capacity.
Two performance curves are required. The first performance curve defines the heating capacity as a function of indoor air dry-bulb and outdoor condenser entering air wet-bulb temperature. This curve is specified in the variable refrigerant flow air-to-air heat pump object. The second performance curve is specific to each VRF DX heating coil and defines the change in heating capacity as a function of air flow fraction. These performance curves are further discussed below.
A unique auto-assigned name for an instance of a VRF DX heating coil. Any reference to this DX coil by another object will use this name.
Select the DX heating coil availability schedule. Schedule values of 0 mean that the DX heating coil is off. A schedule value greater than 0 indicates that the coil can operate during the time period.
This numeric field defines the volume air flow rate (in m3/s or ft3/s) across the DX heating coil at rated conditions. The value entered here must be greater than 0. The rated air volume flow rate should be between 0.00004027 m3/s and 0.00006041 m3/s per W of gross rated heating capacity. The gross rated heating capacity and the gross rated COP should be performance information for the unit with outside air dry-bulb temperature of 8.33 C, outside air wet-bulb temperature of 6.11 C, heating coil entering air dry-bulb temperature of 21.11 C, heating coil entering air wet-bulb temperature of 15.55 C, and the rated air volume flow rate defined here.
This numeric field defines the total, full load gross heating capacity (in W or btu/hr) of the DX coil unit at rated conditions (outside air dry-bulb temperature of 8.33 C, outside air wet-bulb temperature of 6.11 C, heating coil entering air dry-bulb temperature of 21.11 C, heating coil entering air wet-bulb temperature of 15.55 C, and a heating coil air flow rate defined by field rated air flow volume below). The value entered here must be greater than 0. The gross total heating capacity should not account for the effect of supply air fan heat.
Select the curve that defines the heating capacity ratio modifier as a function of indoor dry-bulb temperature or indoor dry-bulb and outdoor wet-bulb temperatures. This is a linear, quadratic or cubic performance curve if the heating capacity is solely a function of indoor dry-bulb temperature (i.e., the indoor terminal units weighted average inlet air dry-bulb temperatures). Without specific manufacturers data indicating otherwise, the use of a single independent variable is recommended for this coil type. If, however, the user has reason to believe the heating capacity is both a function of indoor dry-bulb temperature and outdoor wet-bulb temperature (and has manufacturers data to create the performance curve), a bi-quadratic equation using weighted average indoor dry-bulb temperature and condenser entering air wet-bulb temperature as the independent variables may be used. See the Engineering Reference for more discussion on using this input field.
Note: The choice of using either outdoor dry-bulb temperature or outdoor wet-bulb temperature as the independent variable in this performance curve is set in the parent object VRF Outdoor Unit.
Select the curve that defines that parameterises the variation of heating capacity as a function of the ratio of actual air flow rate across the heating coil to the rated air flow rate (i.e., fraction of full load flow). The curve must be a linear, quadratic or cubic performance curve. The output of this curve is multiplied by the gross rated heating capacity and the heating capacity modifier curve (function of temperature) to give the gross heating capacity at the specific temperature and air flow conditions at which the coil is operating. The curve is normalized to have the value of 1.0 when the actual air flow rate equals the rated air flow rate.