Chiller System Control; Introduction; Vapor-Compression Refrigeration; Vapor-Compression Cycle - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER SYSTEM CONTROL

INTRODUCTION

A chilled water system consists of a refrigeration system (water
chiller), a chilled water distribution system, loads cooled by the
chilled water and a means of dissipating the heat collected by the
system. The refrigeration system cools water pumped through it
by a chilled water pump. The chilled water flows through the
distribution system to coils in air handling units or terminal units.
Heat is removed from the refrigeration system using water or air.
For chilled water control within AHU systems, see the Air
Handling System Control Applications section.
Chilled water systems are used in many buildings for cooling
because of their flexibility and operating cost compared with direct
expansion (DX) cooling coil systems. Typically chilled water is
generated at a central location by one or more chillers and
distributed to coils in air handling system (Fig. 1). The quantity
and temperature of the water supplied must be sufficient to meet
the needs of all fan systems. Since the chilled water system is the
major user of energy in many buildings, energy costs should be a
consideration in chilled water plant configuration.
HEAT
WATER
REJECTION
CHILLER
EQUIPMENT
OPEN
COOLING
TOWER
Fig. 1. Typical Water Chilling System.
A chilled water system can provide hot water for a heating
load when a simultaneous heating and cooling load exists. It
can be used with a chilled water, ice tank, or phase change
material thermal storage system to lower the peak load demand
and allow use of a smaller chiller. It can use the system cooling
tower during light load conditions to supply cool water to the
system without running the chiller, if the outside air WB
temperature is low enough.
Chiller capacity controls are usually factory installed by the
chiller manufacturer. The BMCS usually stages chillers on and
off, provides chiller controls with a chilled water temperature
setpoint, and controls the condenser water system. Chillers are
usually controlled from their leaving water temperature; except
that chillers using reciprocating compressors are often controlled
from their entering water temperature, since staging and loading
in steps causes steps in the leaving water temperature.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
compression or absorption. In addition, those using the vapor-
compression cycle are referred to by the type of compressor:
centrifugal or positive displacement. A positive displacement
compressor can be either reciprocating or screw for this discussion.
See related ASHRAE and chiller manufacturers manuals for
detailed information of chiller cycles.

VAPOR-COMPRESSION REFRIGERATION

VAPOR-COMPRESSION CYCLE

refrigeration system. When the compressor (Fig. 2) starts, the
increased pressure on the high side and the decreased pressure
on the low side causes liquid refrigerant to flow from the receiver
to the expansion valve. The expansion valve is a restriction in
the liquid line which meters the refrigerant into the evaporator.
It establishes a boundary between the low (pressure) side,
including the evaporator and the high (pressure) side, including
the condenser and the receiver. The compressor is the other
boundary. The liquid refrigerant in the evaporator boils as it
BUILDING
AIR
absorbs heat from the chilled water. The refrigerant leaves the
HANDLERS
evaporator and enters the compressor as a cold low-pressure
gas. The refrigerant leaves the compressor as a hot high-pressure
gas and passes through the condenser where it is cooled by the
condenser water until it condenses and returns to the receiver
AIR HANDLERS
as a liquid. The cycle is the same regardless of the compressor
type or refrigerant used.
and thermostatic. The constant pressure valve is suitable only
when the load is constant. It is essentially a pressure regulator
C602-1
which maintains a constant pressure in the evaporator.
loads, such as those found in HVAC systems. It has a remote
temperature sensing element which is usually installed on the
suction line between the evaporator and the compressor. It is
set to adjust the expansion valve so there is a small amount of
superheat in the suction line refrigerant. Superheat means that
all of the liquid has evaporated and the vapor has been heated
above the evaporation temperature by the water or air being
cooled. This prevents liquid from entering the compressor.
used with centrifugal compressors while, a direct expansion chiller
evaporator (Fig. 4) is used with positive displacement compressors.
In both cases the condenser is a large pressure cylinder (shell)
with tubes connected to inlet and outlet headers. In the flooded
shell and tube type evaporator, the shell is about 80 percent filled
with refrigerant and the chilled water flows through the tubes.
Heat from the water evaporates the refrigerant surrounding the
tubes which cools the water. The refrigerant vapor rises to the top
of the shell and into the refrigerant suction line.
299
Chiller types are classified by type of refrigeration cycle: vapor-
The vapor-compression cycle is the most common type of
Two common types of expansion valves are constant pressure
The thermostatic expansion valve is used for varying cooling
A flooded shell and tube chiller evaporator (Fig. 3) is usually
ENGINEERING MANUAL OF AUTOMATIC CONTROL