JOHNSON CONTROLS
22
FORM 201.30-ICOM1 (318)
ISSUE DATE: 3/29/2018
Head Pressure Control
(Applicable to software version Y.WCS.31.05 or
Y.WCS.41.05, and later.)
Head Pressure is the pressure difference between con-
denser and evaporator. It is of a crucial requirement to
maintain sufficient head pressure for normal oil circu-
laiton in lubrication system.
YVWA Chillers are capable of operation with-
in a wide range of condenser water tempera-
tures. If condenser water temperatures meet the
guidelines documented in the Engineering Guide
(FORM 201.30-EG1), then head pressure regulation is
not necessary. However, chiller operational issues may
occur if the condenser water temperature is colder than
usual. A customer facility control may be provided to
ensure adequate head pressure exists.
SECTION 2 - PRODUCT DESCRIPTION
The preferred means is cooling tower bypass circuit,
since this would maintain adequate tube velocity in the
condenser, by regulating the return cooling water to the
chiller. Additionally, a two-way condenser water throt-
tling valve, a variable speed cooling tower pump, or
tower fan control circuit might be employed.
The Head Pressure Control function provides an output
control signal that will respond to a programmed mini-
mum chiller differential pressure.
DIFFERENTIAL PRESSURE =
DISCHARGE PRESSURE - EVAPORATOR PRESSURE
The output to the above equation will be a direct im-
pact. An increasing head pressure differential provides
more water flow to the cooling tower or less tower by-
pass.
FIGURE 6 - HEAD PRESSURE CONTROL SYSTEM
LD26904
Tower
Condenser
Evaporator
Compressor
TXV
ChWP
Motor
Hot
Gas
Cooling Load
CWP
Cooling Tower
Tower
Bypass
Valve
TBV
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