Vilter VSM 71-701 Operation And Service Manual Download Page 230

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BULLETIN 10-11 /

Page 7

3. Wax

— Certain systems are contaminated with small amounts

of wax which will precipitate at low temperatures in systems
with Refrigerants 22 or 502. Since the TEV represents the first
cold point in the refrigeration cycle, wax is most likely to form
at the valve port.

It is sometimes difficult to observe the wax in a valve because it
may exist in solid form only at very low temperatures. By the
time the valve has been taken apart, the temperature has
increased enough to cause the wax to melt and thus become
difficult to detect. When wax is suspected, it can usually be
detected on the pin and seat by packing the valve in dry ice
while disassembling.

Remedy

— Clean the valve with solvent before reassembling

the valve. The Sporlan HH style Catch-All Filter-Driers have a
special activated charcoal desiccant that is designed to remove
wax in the liquid line before it causes trouble. Therefore, to
prevent wax problems, use these HH style driers (e.g., C-415-
S-HH) on all low temperature systems using Refrigerants 22 or
502.

4. Refrigerant shortage

— See•All or sight glass in the

liquid line will show bubbles when the system is short of refrig-
erant charge. Before adding more refrigerant however, be sure
the bubbles are not produced by other causes (See Paragraphs

A-2

and

A-5

A lack of refrigerant charge may also be detected by a hissing
sound at the TEV. Some systems not equipped with a liquid line
sight glass will have test cocks or other devices for checking the
refrigerant level in the receiver.

Remedy

— Add enough refrigerant to obtain desired result.

5. Gas in the liquid line

— As explained in Paragraphs

A-2

and

A-4

, liquid line vapor can be produced by a partially

plugged strainer or drier and by a shortage of refrigerant charge.
In addition, gas in the liquid line can be caused by air or other
non-condensable gases in the system or by excessive pressure
losses in the liquid line as a result of:

■

Long or undersized line.

■

Liquid line vertical lift.

Remedy

— Verify the correct liquid line size for the equiva-

lent length and system tonnage. Consult liquid line sizing data
published in many manufacturers’ catalogs and in textbooks. If
undersized, repipe with the correct size.

Determine amount of vertical lift, and obtain the resulting
pressure loss from Table 3, Bulletin 10-9. Using the subcooling
calculation example provided in the "subcooling" section of
Bulletin 10-9, find required subcooling necessary to prevent
gasification with the existing pressure losses. Provide the
necessary subcooling by using one of the methods described on
Page 1.

6. Misapplication of internally equalized valve or

incorrect location of external equalizer

— If the

pressure drop through the evaporator exceeds the predetermined
values shown in Table 1, Bulletin 10-9, an externally equalized
valve must be used. When an externally equalized valve is used,
the equalizer connection should be made at a point in the suction
line that will reflect the pressure existing in the line at the bulb
location.

Remedy

— Replace internally equalized valve with one

having an external equalizer.

If external equalizer is installed incorrectly, change to correct
location. See Page 3.

7. Insufficient pressure drop across valve

— One of the

factors that influence expansion valve capacity is the pressure
drop that exists between the inlet and outlet. Anything
contributing to a reduction in this pressure drop will reduce valve
capacity. Abnormally low condensing pressures, excessive liquid
line pressure losses (even with adequate subcooling), undersized
distributor nozzle or distributor tubes may also be responsible for
a very low net pressure drop across the valve port.

Remedy

— Remove source of pressure loss, or install valve

with adequate capacity at the reduced pressure drop. If inlet
pressure to valve is low due to low condensing pressure, raise
pressure.

If the refrigerant distributor nozzle is undersized replace with
correct size. See Bulletin 20-10.

8. Dead thermostatic element or wrong thermo-

static charge

— If the element has partially or completely

lost its thermostatic charge, the valve will be unable to feed
sufficient refrigerant or will remain closed. A wrong charge may
cause insufficient feed also.

Remedy

— Replace the element if it is dead. If charge is

incorrect, replace with proper selective charge. See Bulletin 10-9.

9. Charge migration (CP series, ZP series, and VGA

charges only)

— In order for valves with these charges to

maintain control at the bulb, the bulb must be kept at a lower
temperature than the element (diaphragm case). If the thermo-
static charge does migrate to the element because of a lower
element temperature, the valve will throttle.

Detection

— Warm the element with a cloth saturated with

hot water. If this produces more refrigerant feed and reduces the
superheat to normal, charge migration is responsible for the
starved evaporator.

Causes

—

■

Insufficient pressure drop between the valve outlet and bulb
location, possibly due to an oversized distributor nozzle or no
nozzle at all.

■

Excessive pushrod leakage, which allows the leaking refrig-
erant to cool the diaphragm case before passing into the
equalizer line. This is a rare occurrence and should be
carefully checked before arriving at this conclusion.

■

Cold location of TEV, or condensate drippage on the
diaphragm case.

Remedies

—

■

Install distributor nozzle correctly sized in accordance with

nozzle sizing procedure given in Sporlan Bulletin 20-10.

■

On valves with packed pushrod construction, remove element

and tighten the pushrod packing nuts.

■

Relocate the TEV away from cold outlet air, or condensate
drippage.