Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 179

The cooling tower, a very conventional rejection system, is an air-to-water
heat exchanger. In a cooling tower, the heat and mass transfer processes are
combined with cooling water, which absorbs the heat from the refrigerant
and then rejects the heat to the air. The mass transfer from evaporation
consumes water. This loss is about 5 per cent of the water needed for an
equivalent once-through cooling by a stream of water. Cooling towers are
capable of cooling water until 2.5 to 5.5 K above the ambient wet-bulb tem-
perature. The larger the cooling tower for a given set of water and air flow
rates, the smaller this temperature difference will be.
9.3.2 An overview of chilled water systems
The goal of a chilled water system is to produce and distribute chilled water
with high reliability, low capital cost and high flexibility. The central chill-
ing system can be divided into two loops: the primary system which is also
called the production system - the place where chilled water is produced;
and the secondary system which is also called the distribution system because
its function is to convey the chilled water to the load side. Typically, there
are three pumping configurations applied to chilled water systems: constant
primary-only pumping system ; constant primary/variable secondary pump-
ing system ; and variable primary-only pumping system. For all options, the
distribution system is variable flow using two-way valves or constant flow
using three-way valves at cooling coils. It is well known that the use of three-
way valves wastes energy because of the flow in the distribution system being
greater than necessary in low-load conditions. The distribution pumping
system (secondary pumps for constant primary-secondary variable system
and primary pumps for variable primary system) can be fitted with variable-
speed driven pumps or constant-speed pumps.
9.3.3 Constant primary-only pumping system
This is usually a constant differential pressure-control system. The schematic
diagram of this pumping system for a typical application is illustrated in
Figure 9.2. In this pumping system, a set of pumps serves the chillers and the
load side. Two-way valves are used on the load side and pumps are operated
at a constant speed. The differential pressure bypass valve (DPV) helps in
maintaining a constant chilled water flow through the chillers, which is an
essential condition for steady operation of the chillers.
The controlled variable in this control system is the differential pressure
between the main supply and return pipes. A bypass pipe is installed to link
up the two pipes across which the differential pressure is monitored and
controlled. At the bypass pipe, a flow control valve is installed and this valve
is modulated to open or close, under the control of the differential pressure
controller. When the terminal cooling load decreases, the chilled water flow
rate through the cooling coils will drop because of the modulating of the two-