Cooline ASQ Series, Руководство пользователя

Страница: 44 / 50

4
3
•
Control of Return Water Temperature: Return water temperature is one of the most important operating values for a
chilled water system. It tells the operator just how good a job the control system and coils are doing in converting
energy from the chillers to the air or water systems that are cooling the building. This is such a basic criterion that it
should be addressed early in the design of a chilled water system. The proper method of controlling return temperature
is through the correct selection of control valves and cooling coils. In conclusion, one of the designer’s most important
tasks is the selection of a sound temperature differential that will provide maximum possible system efficiency. The
second step in this process is to ensure that the differential is maintained after the system is commissioned.
•
The water system should be configured to distribute the water efficiently with a minimum use of energy-wasting
devices. These devices are listed here:
a) Three-way temperature control valves
b) Balancing valves, manual or automatic
c) Pressure-reducing or pressure-regulating valves
•
The piping should be designed without
a) Reducing flanges or threaded reducing couplings
b) Bullhead connections (e.g., two streams connected to the run connections of a tee with the discharge on the branch
of the tee)
•
The friction for the piping should be calculated for all pipe runs, fittings and valves.
•
Cooling coils should be selected with a high enough water velocity in the tubes to avoid laminar flow throughout the
normal load range imposed on the coils.
•
Coil control valves and their actuators should be sized to ensure that they can operate at all loads on the system
without lifting the valve head off the valve seat.
•
Expansion tank should be provided to so that water volume changes can be accommodated. Expansion tanks are
generally connected to the suction side of the pump - lowest pressure point.
•
Pumps in parallel must always operate at the same speed. There may be some exceptional cases where parallel
pumps are operated at different speeds, but only experienced designers should make evaluations for such a proposed
operation. Also, it is better to use pumps of the same size when operating them in parallel. Variable speed pumps
should be controlled so that pumps operating in parallel never have more than one percent difference in actual oper-
ating speed. Mixing of constant and variable speed pumps in parallel operation is wrong and leads to disastrous
results.
•
Distribution pumps should be selected for maximum efficiency at the design condition and within the economic con-
straints of the project. Distribution pumps should be added and subtracted to avoid operation of pumps at points of
high thrust and poor efficiency. Pump sequencing should achieve maximum possible system efficiency.
•
Differential pressure control (bypass) valves should never be installed at the pump discharges.
•
Check valves should be provided in pump discharges when pumps are operating in parallel. Pump discharge check
valves should be center guided, spring loaded, disc type check valves and should be sized so that the check valve is
full open at design flow rate. Generally this will require the check valve to be one pipe size smaller than the connecting
piping.
•
Circuiting Chilled water to Multiple Chillers : There are fundamentals for the circuiting of chillers that should not be
violated in order to achieve maximum efficiency. Some of these are:
a) Design the piping arrangement so that energy consumption of chillers is not increased.
b) Arrange the piping so that all chillers receive the same return water temperature.
c) Ensure that the required design water flow through the coolers is always maintained.