ClimaCool UCR 30, 50 and 70 tons Manual - Manual (Page 25)

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Brazing Procedures for Head Pressure Control
Valve
Any of the commonly used brazing alloys for high side
usage are satisfactory. However, when soldering or brazing,
it is very important that the internal parts be protected
by wrapping the valve with a WET cloth to keep the body
temperature below 250°F for the LAC. Also, when using high
temperature solders, the torch tip should be large enough
to avoid prolonged heating of the copper connections. And,
always direct the fl ame away from the valve body.
CAUTION: Inert gases must be added to the system
carefully through a pressure regulator. Unregulated gas
pressure can seriously damage the system and endanger
human life. Never use oxygen or explosive gases.
The table below lists the maximum valves each valve can
withstand without damage. Precautions must be taken to
keep test or operating pressures below these values.
Type Valve
Maximum Rated
Pressure PSIG
LAC-10 680
Determining Amount of Charge
When “refrigerant side” head pressure control is utilized on
a system, one of the most important factors is determining
the total system refrigerant charge. While on most
packaged units the amount of charge is listed on the unit,
the required charge for a fi eld built-up system cannot be
listed by the manufacturer. Charge is usually added when
the system is started up until “proper” system performance
is reached. However, if the system is to function properly
year-round, the correct amount of extra charge must be
calculated ahead of time.
Completely Flooded Condenser
The easiest method is to calculate the volume of the
condenser coil and then use the density factor of the
refrigerant shown in Table 1 to fi gure the pounds of
refrigerant necessary to completely fl ood the condenser
coil at the appropriate ambient. The factors involved in
calculating the extra pounds of refrigerant are:
1. Length of tubing and return bends in condenser
2. Minimum ambient temperature at which systems will be
required to function
3. Tubing size and wall thickness
4. Refrigerant
The primary point to remember in selecting the proper
density factor is that when the liquid drain valve LAC is
throttling, the refrigerant temperature will be at the same
temperature as the ambient.
EXAMPLE: Calculate the extra refrigerant charge necessary
for a refrigerant R-410A, roof mounted remote condenser
(40°F evaporator and a minimum condensing temperature
of 90°F) with compressor unloading to 50% of full
compressor capacity. To determine the equivalent length of
tubing in a condenser, proceed as follows: First, count the
number of tubes and multiply this by their length.
Example: 150 tubes x 7.55 feet = 1132.5 feet
Next, count the return bends and multiply them by the
factor shown in Table 1.
Example: 150 bends x .250 for 1/2 inch bends = 37.5 feet
Then add this 37.5 feet to the 1132.5 feet for a total of
1170 feet
The system uses a 30 hp condensing unit with a condenser
coil containing 1170 equivalent feet of 1/2 inch tubing tubes
and return bends. Assume a design temperature of minus
20°F minimum ambient. From Table 1 we fi nd the density
factor necessary to calculate the pounds of extra refrigerant
to completely fl ood the condenser at minus 20°F: 1170 feet x
.102 pounds/foot = 119 pounds.
Partially Flooded Condenser
On many systems it isn’t necessary to completely fl ood the
condenser to maintain suffi
cient operating head pressure
(equivalent to approximately 90°F condensing temperature)
because of a milder climate. Therefore, a second method
is available. The additional information found in Tables
2 and 3 can be used to fi gure more closely the charge
necessary to properly fl ood the condenser for suffi
cient
head pressure at various minimum ambient temperatures.
(The multipliers are applied to the extra refrigerant charge
that was calculated in the method to completely fl ood the
condenser.)
EXAMPLE: Our example calls for a compressor equipped
with capacity unloading at roughly 50% using hot gas
bypass/combination liquid injection circuits. Since the chiller
controller will likely call for unloading during low ambient,