Model for a steady-state or dynamic cooling tower with variable speed fan using the York calculation for the approach temperature at off-design conditions.
To compute the thermal performance, this model takes as parameters the approach temperature, the range temperature and the inlet air wet bulb temperature at the design condition. Since the design mass flow rate (of the chiller condenser loop) is also a parameter, these parameters define the rejected heat.
For off-design conditions, the model uses the actual range temperature and a polynomial
to compute the approach temperature for free convection and for forced convection, i.e.,
with the fan operating. The polynomial is valid for a York cooling tower.
If the fan input signal y
is below the minimum fan revolution yMin
,
then the cooling tower operates in free convection mode, otherwise it operates in
the forced convection mode.
For numerical reasons, this transition occurs in the range of y ∈ [0.9*yMin, yMin]
.
The fan power consumption at the design condition can be specified as follows:
fraPFan_nominal
can be used to specify at the
nominal conditions the fan power divided by the water flow rate. The default value is
275 Watts for a water flow rate of 0.15 kg/s.
PFan_nominal
can be set to the fan power at nominal conditions.
If a user does not set this parameter, then the fan power will be
PFan_nominal = fraPFan_nominal * m_flow_nominal
, where m_flow_nominal
is the nominal water flow rate.
In the forced convection mode, the actual fan power is
computed as PFan=fanRelPow(y) * PFan_nominal
, where
the default value for the fan relative power consumption at part load is
fanRelPow(y)=y^{3}
.
In the free convection mode, the fan power consumption is zero.
For numerical reasons, the transition of fan power from the part load mode
to zero power consumption in the free convection mode occurs in the range
y ∈ [0.9*yMin, yMin]
.
To change the fan relative power consumption at part load in the forced convection mode,
points of fan controls signal and associated relative power consumption can be specified.
In between these points, the values are interpolated using cubic splines.
This model is similar to the model Cooling Tower:Variable Speed
that
is implemented in the EnergyPlus building energy simulation program version 6.0.
The main differences are
y
.
This model requires a medium that has the same computation of the enthalpy as Buildings.Media.Water, which computes
h = c_{p} (T-T_{0}),
where
h is the enthalpy,
c_{p} = 4184 J/(kg K) is the specific heat capacity,
T is the temperature in Kelvin and
T_{0} = 273.15 Kelvin.
If this is not the case, the simulation will stop with an error message.
The reason for this limitation is that as of January 2015, OpenModelica
failed to translate the model if Medium.temperature()
is used
instead of
Water.temperature()
.
EnergyPlus 2.0.0 Engineering Reference, April 9, 2007.
FRWat0
, as this variable
can take on values that are bigger than 1.Medium.temperature()
with
Water.temperature()
in order for the model
to work with OpenModelica.
Added an assert
that stops the simulation if
an incompatible medium is used.
each
keyword for fanRelPowDer
.
Added regularization in computation of TAppCor
.
Removed intermediate states with temperatures.
Evaluate=true
.
allowReverseFlow=false
.
Avoided the use of the conditionally enabled variables sta_a
and
sta_b
as this was not proper use of the Modelica syntax.
Q_flow
and mXi_flow
.
mWat_flow_nominal
since it is equal to m_flow_nominal
,
which is the water flow rate from the chiller condenser loop.