Fault Detection and Diagnostics for Rooftop Air Conditioners

th
where the i output variable
y i
is the calculated value,
β oi ,
- β 9,i
are the regression
T chwi
coefficients, is the chiller water inlet temperature, T is the cooling water
temperature into the condenser, and Z are variables indicating the number of cylinders
in operation. Regression coefficients were determined using a multivariate leastsquares
method applied to 8000 operating data points which were assumed to be faultfree.
The R
2 statistic of the regression indicated that the fit to the data was quite
accurate for almost all of the variables which were used in the FDD routine.
To create residuals to use in fault diagnosis, a black box model was used by Stylianou
and Nikanpour (1996) to predict values for internal temperatures and pressures. The
model uses two input variables, the water inlet temperature into the condenser and
evaporator, and a simple linear equation for all of the output variables as given by
y i = β 0,
i + β1,
iTchwi
+ β2,
i
T
cwi
cwi
th
where the i output
y i
is the calculated value, β 0
- β 2
are the regression coefficients,
T chwi
is the chiller water inlet temperature, and T
is the cooling water temperature
into the condenser. The parameters of the model were determined using a
multivariate least-squares analysis. The results showed an excellent fit to the
experimental data, although the data only covered a relatively small range of
evaporator inlet water temperatures (50º-59ºF) and condenser inlet water temperatures
(72º-93º F).
In addition to developing the FDD method considered in his thesis, Rossi (1995) also
suggested some models for rooftop air conditioning units for use with his method.
The performance of a rooftop air conditioner with fixed flow rates is a function of the
condenser and evaporator inlet air temperatures and the moisture content of the
evaporator inlet air. The moisture content can be represented using a relative
humidity, dew point temperature, or wet bulb temperature. There are two modes of
operation for a typical cooling coil, wet and dry. The coil is defined as wet when
water is being removed from the air stream in addition to heat. When the coil is wet,
the energy transfer to the coil is driven primarily by the difference between the
temperature of the evaporating refrigerant and the wet bulb temperature of the air,
independent of dry bulb temperature alone. When the coil is dry, the heat transfer is
driven by the difference between the coil and the dry bulb temperature of the air,
cwi
9