Cold Point CNC series Installation, Operation And Maintenance Manual - Page 8

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FIELD CHARGING:

Unit performance, efficiency, and life depends, to a

large extent, on a proper system charge. Time spent on

getting the charge right at start-up will pay off in the

long run.

Operating conditions such as voltage, air flow,

evaporator coil size, and indoor and outdoor

temperature and humidity all have an effect on the

system pressures and superheat conditions.

CNC series units are factory charged for the outdoor

and indoor unit and factory supplied filter drier.

Charge must be added for the interconnecting tubing.

Cold Point supplied evaporator coils are supplied with

non-bleed type thermostatic expansion valves. Use the

sub-cooling method or verify/adjust charge (see page

7).

Use the following 'superheat' and 'sub-cooling'

methods to verify proper charge after adjustments are

made.

SUPERHEAT METHOD FOR FIXED

ORIFACE/ CAPILLARY TUBE INDOOR

UNITS:

For fixed metering devices such as bullet orifices,

pistons, and cap tubes the superheat method will always

be the method used. Measuring superheated vapor

values at the suction service valve is one of the most

important indicators of proper system performance. If

the superheat is too high, the evaporator coil will under

perform. If too low, there is a risk of damaging the

compressor with liquid refrigerant. A fixed metering

device is unable to control super heat; therefore, proper

superheat values can vary greatly with conditions that

affect it, such as outdoor and indoor air conditions. Use

the steps outlined in steps 1 thru 6.

PROCEED as FOLLOWS:

1). Run the unit for at least 10 minutes to allow the

system to stabilize. Then take readings of the indoor

and outdoor air temperatures and suction pressure.

Write them down.

2). On the gauge face, read the equivalent temperature

on the R-22 scale next to the suction pressure reading.

3). Using a thermocouple or probe type thermometer

read the suction line temperature as it enters the service

valve. Make certain that the thermocouple/probe is

tight to the copper line.

4). From the thermocouple thermometer reading

subtract the suction pressure equivalent temperature

(saturated suction).

5). Proceed to the Superheat chart (Fig. 3). Add or

remove R-22 to achieve the superheat reading

indicated by the chart.

IMPORTANT: When connecting the manifold to the

charging bottle be sure to purge the hose of air with a

small amount of R-22.

Wait at least 5 minutes between charge adjustments to

allow the unit to stabilize.

6). When charge adjustment is complete remove the

manifold hoses from the service valves and replace and

tighten service port caps.

Fig. 3

Chart based on 400 cfm/ton indoor airflow and 50% relative

humidity; use only on systems that cool with an fixed oriface or

capillary tube.

* If relative humidity is above 70%

or below 20%, use indoor Wet

Bulb Temperature.