Leaving Chilled Water Temperature Control - Trane HORIZON ABDA Manual

(8) Leaving Chilled Water
Temperature Control
The unit control maintains leaving water temperature
setpoint by varying the refrigerant vapor pressure in the
evaporator. The leaving water temperature is third in the
hierarchy of control, after safety functions (first) and
machine protection (second).
The unit control monitors leaving chilled water temperature
changes via a temperature sensor. By modulating solution
flow and solution concentration, the potential to produce
chilled water is modified to meet changes in leaving chilled
water temperature. When no other limits with higher
precedence are operating, the unit control system
maintains the leaving chilled water temperature over the
capacity range of the chiller. See Figure 16.
The lithium bromide concentration and temperature from
the absorber sprays determine the absorber vapor pressure
and the resulting refrigerant vapor pressure and
temperature (NOTE 1). The refrigerant water vapor
absorption rate is a function of absorber solution
temperature and concentration. The condenser cooling
water temperature is critical to the stability of the absorber
solution cycle. Evaporator capacity is then only a function
of absorber spray concentration. Absorber spray flow is
changed by adjusting the solution pump speed with an
adjustable frequency drive (AFD) control.
Figure 16. Temperature control monitoring flow results
Absorber
Spray
Flow
Absorber Spray Concentration
Absorber
Flow
The solution concentration is changed by adjusting the
speed of the LTSP using an AFD. Solution flows and
concentrations are adjusted any time the demand for
chilled water changes or the temperature of the entering
tower water changes. In summary, varying the lithium
bromide solution flow adjusts the amount of strong
solution mixed with dilute solution to develop the absorber
spray concentration.
The refrigerant vapor dilutes the lithium bromide as the
sprayed solution is cooled by the condenser water flowing
through the absorber tubes. Diluted solution is recycled to
the generators to reclaim the refrigerant and concentration
in the lithium bromide. Cooling capacity is the rate at which
refrigerant is vaporized from the evaporator. To keep the
cycle in balance, the rate at which refrigerant is vaporized
must equal the rate at which refrigerant vapor is recovered.
NOTE 1) There is two ways to change absorber spray
concentration — change concentration from the generator
or change the flow from the generator. The most efficient
mode of operation is to maintain a constant concentration
from the generator and vary the amount of solution being
concentrated. This is the method used to control.
Controlling capacity by modulating solution flow rate also
yields the fastest responding system. Concentration control
is tied to leaving water temperature control to anticipate
changes in the machine cycle. Any change in solution flow
is matched by a corresponding change in heat input. The
net result is to minimize disturbances to concentration
control.
Evaporator Loop
Cooling Water
To Generator
Generator Flow
65