CSZ ZP 16, Installation, Operation & Maintenance Manual, Page: 34 / 88

GENERAL DESCRIPTION
Page 2-18
SEQUENCE OF OPERATION
CASCADE UNIT - SYSTEM 1 (R-404A) DESCRIPTION
Refer to Refrigeration Diagram in Drawing Section NOTE: Pressure settings are approximate.
The compressor (item 1) will pump compressed R-404A vapor through the discharge line where the high
pressure gauge displays the pressure of the refrigerant. The high pressure switch (item 95 or item 100)
senses the discharge refrigerant pressure and will open a contact in the event that discharge pressure
exceeds 350 psig (24 bar). This contact opening will serve to shut down the unit and will automatically reset
when the pressure drops to 250 psig (17 bar). The discharge refrigerant vapor enters the condenser.
The condenser (item 5) cools the high pressure R-404A vapor and condenses it into a high pressure liquid.
If the condenser is air-cooled, the condenser fan motor will be energized anytime System 1 is running. If
the unit is water-cooled, the discharge pressure regulator (item 7) will maintain the discharge pressure at
approximately 210 PSIG (14.5 bar).
High pressure liquid refrigerant exits the condenser and the liquid fl ows through a receiver (water-cooled
units), fi lter-drier, sight glass, and into the T.E.V. (item 11) where it changes to a low pressure two-phase
refrigerant (10 HP and larger units have minimum and maximum solenoid valves and T.E.V.s). The two phase
refrigerant is cold due to the
fl ashing of refrigerant. It enters the cascade condenser (item 13) where the heat
from system 2 boils the rest of the R-404A into a vapor. The cascade condenser serves as the evaporator
for system 1. If humidity is called for, some of the R-404A refrigerant is diverted to the humidity loop prior to
entering the T.E.V. See humidity loop section below for description.
Superheated R-404A vapor exits the cascade condenser and moves through the system 1 suction line. A
service valve is located near the compressor.
Humidity Loop (Optional Equipment)
When Humidity is called for, the unit runs as a single-stage unit and not a cascade. High-pressure liquid
refrigerant from the condenser, is diverted into two separate paths.
The fi rst path is to the liquid-line solenoid (item H49) that feeds liquid refrigerant to the expansion device (item
H50). The pressure of the refrigerant drops through the expansion device due to friction and the acceleration
of the refrigerant. The refrigerant exits the expansion device a two-phase mixture as some of the refrigerant
has now
fl ashed into vapor. The refrigerant enters the evaporator coil (item 52) and leaves a superheated
vapor to the suction line.
The second path is to the humidity coil solenoid (item H27) that feed liquid refrigerant to the humidity coil
expansion device (item H23). Two-phase refrigerant exits the expansion device and enters the wet coil
(item H25). The wet coil is cold enough to attract moisture from the chamber air, but not allowed to get cold
enough to freeze the moisture on the coil. The temperature of the coil is regulated by a pressure regulating
E.P.R. valve (item H26) and is intended to be a
fl ooded coil by design. The E.P.R. valve is set at 68 PSIG (4.7
bar), which corresponds to a 29
O
F (-1.6°C) wet coil temperature. The warm chamber air keeps the moisture
from freezing on the coil. After the E.P.R. valve there is a check valve (item H28) that keeps refrigerant from
migrating to the wet coil when the humidity mode is inactive. The refrigerant leaves the check valve and
enters the suction line, where it remixes with rest of the refrigerant from the
fi rst path.
CASCADE UNIT - SYSTEM 2 (R-508B) DESCRIPTION
The compressor (item 41) will pump compressed R-508B/R-23 vapor through the discharge line where the
high pressure gauge senses the pressure of the refrigerant. The high pressure switch (item 95) senses the
discharge refrigerant pressure and will open a contact in the event that discharge pressure exceeds 350
psig(29 bar). This contact opening will serve to shut down the unit and will automatically reset when the
pressure drops to 250 psig (17 bar).