.Buildings.Fluid.HeatExchangers.DXCoils.UsersGuide

Information

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

The following six DX coil models are available:

DX coil condenser DX coil model Properties Control signal
Air-cooled Buildings.Fluid.HeatExchangers.DXCoils.AirCooled.MultiStage Coil with multiple operating stages, each stage having a constant speed. Each stage has its own performance curve, which may represent the coil performance at different compressor speed, or the coil performance as it switches between cooling only, cooling with hot gas reheat, or heating only. Integer; 0 for off, 1 for first stage, 2 for second stage, etc.
Air-cooled Buildings.Fluid.HeatExchangers.DXCoils.AirCooled.SingleSpeed Single stage coil with constant compressor speed Boolean signal; true if coil is on.
Air-cooled Buildings.Fluid.HeatExchangers.DXCoils.AirCooled.VariableSpeed Coil with variable speed compressor with lower speed limit. If the control signal is below the lower limit, the coil switches off. It switches on if the control signal is above the lower limit plus a hysteresis. By default, the minimum speed ratio is minSpeRat and obtained from the coil data record datCoi.minSpeRat. The hysteresis is by default speDeaBanRat=0.05. Real number; 0 for coil off, 1 for coil at full speed.
Water-cooled Buildings.Fluid.HeatExchangers.DXCoils.WaterCooled.MultiStage Coil with multiple operating stages, each stage having a constant speed. Each stage has its own performance curve, which may represent the coil performance at different compressor speed, or the coil performance as it switches between cooling only, cooling with hot gas reheat, or heating only. Integer; 0 for off, 1 for first stage, 2 for second stage, etc.
Water-cooled Buildings.Fluid.HeatExchangers.DXCoils.WaterCooled.SingleSpeed Single stage coil with constant compressor speed Boolean signal; true if coil is on.
Water-cooled Buildings.Fluid.HeatExchangers.DXCoils.WaterCooled.VariableSpeed Coil with variable speed compressor with lower speed limit. If the control signal is below the lower limit, the coil switches off. It switches on if the control signal is above the lower limit plus a hysteresis. By default, the minimum speed ratio is minSpeRat and obtained from the coil data record datCoi.minSpeRat. The hysteresis is by default speDeaBanRat=0.05. Real number; 0 for coil off, 1 for coil at full speed.

Control of the coils

The DX coil models take as a control input the stage of operation, an on/off signal, or the speed of the compressor. Because the thermal response of the coil is very fast, it is important to use as the controlled variable the room air temperature, as the room air temperature has a much slower response compare to the supply air temperature. If the supply air temperature is used, then the control algorithm should be such that short-cycling is avoided.

Coil performance

For air-cooled DX coils, the steady-state total rate of cooling and the Energy Input Ratio (EIR) are computed using polynomials in the air mass flow fraction (relative to the nominal mass flow rate), the evaporator air inlet temperature and the condenser air inlet temperature. These polynomials are explained at Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.CoolingCapacityAirCooled.

For water-cooled DX coils, the steady-state total rate of cooling and the EIR are computed using polynimials in the air mass flow fraction (relative to the nominal mass flow rate), the water mass flow fraction (relative to the nomina water mass flow rate), the evaporator air inelt temperature and the condenser water intet temperature. These polynomials are explained at Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.CoolingCapacityWaterCooled.

Evaporation of accumulated water vapor

If a coil dehumidifies air, a water film builts up on the evaporator. When the compressor is off, then this water film evaporates into the air stream. For coils that short-cycle, this significantly decrease the dehumidification capacity of the coil. The accumulation and reevaporation of water on the evaporator coil is explained at Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.Evaporation.

Coil dynamics

Two dynamic effects are modeled: The accumulation and reevaporation of water at the evaporator, and the thermal response of the evaporator. The dynamics of the evaporation is described at Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.Evaporation. The dynamics of the evaporator 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.

Sensible heat ratio

The coil models two separate performances, one assuming a dry coil, and one assuming a wet coil. The dry coil is modeled using Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.DryCoil and the wet coil is modeled using Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.WetCoil. Both use the same model Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.CoolingCapacityAirCooled to compute the cooling capacity, but the wet coil uses the wet-bulb temperature of the air inlet instead of the dry bulb temperature to compute the coil performance. The wet coil model computes the humidity of the leaving air Xw,o, using the bypass factor model. This humidity is compared to the humidity at the evaporator inlet Xi. If Xw,o-Xi > 0 the coil is assumed to be dry, otherwise it is wet. This test is implemented in Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.DryWetSelector in such a way that the transition between wet and dry coil is differentiable.

The split between sensible and latent heat ratio is computed using the apparatus dew point. This calculation is implemented in Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.ApparatusDewPoint. Once the appartus dew point is known, the sensible to latent heat ratio can be determined as shown in the figure below.

image

The method used is the bypass factor method, which assumes that of the leaving air, a fraction is at the same condition as the entering air, and the other fraction is at the apparatus dew point. This computation requires the ratio UA ⁄ cp, which is computed in Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.UACp.

Once the ratio UA ⁄ cp is known, the bypass factor is a function of the current mass flow rate only. (Under the assumption that the velocity dependence of UA can be neglected.)

Limitations

This model has the following limitations:

Revisions


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