4. Scratch matching marks on tubing stubs and old accumulator.
Scratch matching marks on new accumulator. Unbraze stubs
from old accumulator and braze into new accumulator.
5. Thoroughly rinse any flux residue from joints and paint with
corrosion-resistant coating such as zinc-rich paint.
6. Reinstall accumulator into system with copper slip couplings.
7. Evacuate and charge system.
Pour and measure oil quantity (if any) from old accumulator. If
more than 20 percent of oil charge is trapped in accumulator, add
oil to compressor to make up for this loss.
Step 9—Contaminant Removal
Proper evacuation of a unit will remove non-condensibles and
assure a tight, dry system before charging. The 2 methods used to
evacuate a system are the deep vacuum method and the triple
evacuation method.
DEEP VACUUM METHOD — The deep vacuum method re-
quires a vacuum pump capable of pulling a vacuum of 1000
microns and a vacuum gage capable of accurately measuring this
vacuum depth. The deep vacuum method is the most positive way
of assuring a system is free of air and liquid water.
TRIPLE EVACUATION METHOD — The triple evacuation
method can be used where the vacuum pump is capable of
pumping down to only 28 in. of mercury vacuum, and the system
does not contain any liquid water. The procedure is as follows.
1. Pump the system down to 28 in. of mercury vacuum and allow
pump to continue to operate for additional 15 minutes.
2. Close the service valves and shut off the vacuum pump.
3. Connect a refrigerant cylinder to the system and open until
system pressure is 2 psig.
4. Close the service valve and allow system to stand for 1 hr,
during which time the dry refrigerant will be able to diffuse
throughout the system, absorbing moisture.
5. Repeat procedure 3 times. System will then be free of any
contaminants and water vapor.
Step 10—R134A Refrigerant System Charging
NOTE:
Consult unit information plate for superheat charging
table and unit rating plate for TXV subcooling temperature.
For all approved combinations, system must be charged correctly
for normal system operation and reliable operation of components.
Always wear safety glasses and gloves when handling refrigerants.
If system has lost all charge, weigh in charge using dial-a-charge
or digital scale designed for R134A refrigerant.
System charge should be fine tuned by using the superheat or
subcooling method, whichever is appropriate. These methods are
covered later in this manual.
Step 11—Checking Charge
Superheat charging is the process of charging refrigerant in a
system until the temperature (superheat) of the suction gas entering
the compressor reaches a prescribed value. Small variations of
charge affect suction gas superheat temperatures greatly. There-
fore, this method of charging is very accurate. This method can be
used only on split-system condensing units with fixed restrictor-
type metering devices such as AccuRater. For units using a TXV,
the subcooling method must be used. To charge by superheat, a
service technician will need an accurate superheat thermocouple or
thermistor-type thermometer, a sling psychrometer, and a gage
manifold. Do not use mercury or small dial type thermometers as
they are not adequate for this type of measurement. Then use one
of the following procedures.
SUPERHEAT CHARGING METHOD
1. Operate a unit a minimum of 10 minutes before checking
charge.
2. Measure vapor pressure by attaching a gage to vapor valve
service port.
3. Measure vapor line temperature by attaching a service ther-
mometer to unit vapor line near vapor valve. Insulate ther-
mometer for accurate readings.
4. Measure outdoor air dry-bulb temperature with a second
thermometer.
5. Measure indoor air (entering indoor coil) wet-bulb tempera-
ture with a sling psychrometer.
6. Locate outdoor temperature and evaporator entering air wet-
bulb temperature in Table 1. At this intersection note the
superheat.
7. Locate superheat temperature located in previous step and
vapor pressure in Table 2. At this intersection note vapor line
temperature.
8. If unit has a higher vapor line temperature than charted
temperature, add refrigerant until charted temperature is
reached.
9. If unit has a lower vapor line temperature than charted
temperature, bleed refrigerant until charted temperature is
reached.
10. If outdoor air temperature or pressure at vapor valve changes,
charge to new vapor line temperature indicated on chart.
This procedure compensates for slight variations of indoor airflow.
SUBCOOLING CHARGING METHOD
1. Operate unit a minimum of 15 minutes before checking
charge.
2. Measure liquid service valve pressure by attaching an accurate
gage to the service port.
3. Measure the liquid line temperature by attaching an accurate
thermistor-type or electronic thermometer to the liquid line
near the outdoor coil.
4. Refer to unit rating plate to find required subcooling tempera-
ture for unit. Find the point at which the required subcooling
temperature intersects the measured liquid service valve
pressure on Table 3.
5. To obtain the required subcooling temperature at a specific
liquid line pressure, add refrigerant if liquid line temperature
Fig. 20—Accumulator
A88410
430
°
FUSE
ELEMENT
15
Summary of Contents for 38TN
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