63
(7) Machine Control
Strategy
The unit control panel is designed to reliably achieve and
maintain the required leaving chilled water setpoint. The
control compares the leaving water temperature to the
panel set point, and adjusts generator solution flow, and
the energy applied as needed. Adjusting the generator
solution flow maintains the required leaving water
temperature. Adjusting the energy input creates the
solution concentration needed to sustain the desired chilled
water temperature. These two adjustments are the two
primary control algorithms employed to optimize machine
efficiency.
Simplified Diagram of Leaving Water
Temperature Control and Concentration
Control
Microprocessor
Control System
1) Leaving
Water
Temperature
input
2) Conc. Input
AFD
Control
Energy
Control
1. Leaving Water Temperature (LWT)
The machine panel controls the chilled water leaving
temperature by controlling the refrigerant vapor pressure in
the evaporator. The refrigerant vapor pressure is
determined by lithium bromide concentration and
temperature at the absorber sprays. The absorber spray
concentration is changed by adjusting the generator
solution flow. An adjustable frequency drive (AFD) is used
to change pump speed which determines the absorber
spray and low temperature solution flow.
As the generator solution flow is varied, the corresponding
volume of strong solution leaving the generator changes.
This solution then mixes with recirculated dilute solution to
produce the absorber spray solution.
The absorber spray concentration, temperature, and AFD
pump speed determine the absorber vapor pressure. This
absorber combination controls the refrigerant vapor
pressure. The resulting refrigerant spray temperature
produces the desired leaving chilled water temperature
setpoint.
2. Concentration Control
The concentration control adjusts the energy input for
optimum solution efficiency. The strong solution
concentration is typically maintained at 15 degrees F from
the theoretical crystallization point. This is called the low
temperature heat exchanger (LTHX) temperature margin.
As strong solution concentration is varied by Leaving Water
Temperature solution flow control, the theoretical
crystallization temperature is changed. The control system
adjusts the energy input to maintain the LTHX temperature
margin throughout the load range. The simplified control
illustration below illustrates the two control inputs and
indicates that these inputs must pass through the “limit
control” before the signals pass to the solution pump AFD
and/or energy input control.
In addition to the microprocessor control of Leaving Water
Temperature and generator solution concentration, the
microprocessor monitors conditions during machine
operation that could affect unit reliability. If an unusual
condition is detected control limits will override Leaving
Water Temperature and concentration control requests. The
override limit would prevail when a control output (from
Leaving Water Temperature or concentration control)
request is greater than allowed by the particular limit
schedule. The Limit control will modify the output signal
maintaining limited chiller operation.
When a limit or safety assumes control, the clear language
display will indicate the active limit mode. A limit mode
does not necessarily mean that a problem exists. In many
cases this limit is a normal function such as soft load during
startup. The microprocessor will shutdown the chiller if the
limit or safety condition reaches a level that could cause
damage to the machine.