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. |

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.

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.

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.

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.

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 *X _{w,o}*, using the
bypass factor model. This humidity is compared to the humidity at
the evaporator inlet

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.

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 ⁄ c _{p}*,
which is computed in
Buildings.Fluid.HeatExchangers.DXCoils.BaseClasses.UACp.

Once the ratio *UA ⁄ c _{p}* is known, the bypass
factor is a function of the current mass flow rate only. (Under the
assumption that the velocity dependence of

This model has the following limitations:

- For air-cooled DX coil models, they do not account for fan in the evaporator or 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 condenser 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 cooling is provided and no power is consumed.

- February 28, 2017 by Yangyang Fu:

Added desciption about water-cooled DX coil models. - September 24, 2012 by Michael Wetter:

First implementation.

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