.AixLib.Fluid.Movers.Compressors.UsersGuide.Approaches

Information

Compressor models implemented in this library often use three different efficiencies to calculate the mass flow rate, thermodynamic change of state and power consumption of the compressor. In the following, all efficiency models implemented in this library are shortly summarised. Furthermore, all compressor models have a parameter to calculate transient behaviour of changing the rotational speed. This approach is also summarised in this information section.

Efficiency modeling approaches

Actually, three different efficiency models are suggested and the modelling approaches of these efficiencies are shortly characterised below:

"Efficiencies" border="1" cellspacing="0" cellpadding= "2" style="border-collapse:collapse;">
Efficiency Formula Comment
Engine ηeng = Q̇ref / Pel Used for calculation of compressor's power consumption
Isentropic ηise = (houtIse - hinl) / (hout - hinl) Used for calculation of thermodynamic change of state
Volumetric ηvol = V̇ide / V̇rea Used for calculation of mass flow rate

These efficiency models are stored in AixLib.Fluid.Movers.Compressors.Utilities.EngineEfficiency, AixLib.Fluid.Movers.Compressors.Utilities.IsentropicEfficiency and AixLib.Fluid.Movers.Compressors.Utilities.VolumetricEfficiency. Therefore, the calculation procedure of the efficiencies are introduced as replaceable models and must be defined by the User. Further information is given in the following sections:

  1. Engine efficiencies
  2. Volumetric efficiencies
  3. Isentropic efficiencies

Naming and abbreviations

In the following, a guideline of naming efficiency models is summarised:

Approach of calculating efficiency _ Valid refrigerants _ Displacement volume _ Type of compressor

  1. Approach: Approach of calculating efficiency, e.g. polynomial or power.
  2. Refrigerant: Refrigerants the efficiency model is valid for, e.g. R134a or R410a.
  3. Displacement volume: Displacement volume the efficiency model is valid for, e.g. 170cm³.
  4. Type: Type of compressor, e.g. scroll compressor.

Transient behaviour

The base model has a parameter useInpFil that is used to model the compressors's transient behaviour while changing rotational speed. Generally, this approach uses the same modeling attempt as the stat-up and shut-down transients introtuced for flow machines (see AixLib.Fluid.Movers.UsersGuide). Therefore, just the parameter's affections are presented here:

  1. If useInpFil=false, then the input signal opeSet.y is equal to the compressor's rotational speed. Thus, a step change in the input signal causes a step change in the rotational speed.
  2. If useInpFil=true, which is the default, then the rotational speed is equal to the output of a filter. This filter is implemented as a 2nd order differential equation. Thus, a step change in the compressor's input signal will cause a gradual change in the rotational speed. The filter has a parameter risTim, which by default is set to 1 second. The rise time is the time required to reach 99.6% of the full rotational speed, or, if the compressor is shut-down, to reach a rotational speed of 0.4%.

Contents

NameDescription
 MechanicEfficiencyMechanic Efficiencies
 VolumetricEfficiencyVolumetric Efficiencies
 IsentropicEfficiencyIsentropic Efficiencies

Revisions

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