.Buildings.Fluid.DXSystems.Heating.UsersGuide

Information

This package contains models for direct evaporation (DX) system heating coils.

The following DX coil model is available:

Control of the coils

The DX coil model takes an on/off signal as a control input. Because the thermal response of the coil is very fast, it is important to use the room air temperature as the controlled variable, as it has a much slower response compared to the supply air temperature. If the supply air temperature is used, then the control algorithm should be designed to avoid short-cycling.

Coil performance

The steady-state total rate of heating and the Energy Input Ratio (EIR) are computed using polynomials in the air mass flow fraction, condenser air inlet temperature, and outdoor air temperature, as explained at Buildings.Fluid.DXSystems.BaseClasses.CapacityAirSource.

Defrost operation

The coil model calculates the defrost operation of the outdoor evaporator coil analytically using the blocks Buildings.Fluid.DXSystems.Heating.BaseClasses.CoilDefrostTimeCalculations (which calculates the time duration fraction for which the coil is assumed to be in defrost mode)and Buildings.Fluid.DXSystems.Heating.BaseClasses.DefrostCapacity (which calculates the heat transferred from the indoor airstream to the outdoor coil for defrost). The user needs to keep in mind that there is no actual defrost mode operation. The model only calculates a theoretical time fraction (of a constant, assumed timestep) that the coil enters defrost mode, and calculates heat transfer for it.

Coil dynamics

The dynamics of the condenser is approximated by a first order response where the time constant is a model parameter. Hence, the dynamic response is similar to other models of the Buildings.Fluid package and described at Buildings.Fluid.UsersGuide.

Limitations

This model has the following limitations:

• The coil model does not account for fan in the condenser air stream. Fans can be modeled separately using models from the package Buildings.Fluid.Movers. However, if the performance curve for the energy input ratio contains electricity use for a fan, then this is of course reflected by the model output.
• The air must flow from port_a to port_b. If there is reverse flow, then no heating is provided and no power is consumed.

Revisions

• March 19, 2023 by Xing Lu and Karthik Devaprasad:
  First implementation.