Emerson Fluid Chiller Instrukcja Naprawy Pobierz pdf (Strona 10)

Thermal Expansion Valves

Emerson Climate Technologies

sible to determine that the cause of the trouble call is

because of improper methods of instrumentation rather

than any malfunction of the TXV.

When troubleshooting in mountain areas (such

as Denver, Colorado or Salt Lake City, Utah) use a

Pressure-Temperature chart that has correct readings

such as Emerson Climate Technologies’ 5,000 ft. pocket

chart. Gauge pressures will read lower than they would

at sea level.

TXV Selection

Proper TXV size is determined by the BTU/HR or

tons load requirement, the pressure drop across the

TXV, and the evaporator temperature. Do not assume

that the pressure drop across the TXV is equal to the

difference between discharge and suction pressures at

the compressor. This assumption could lead to incorrect

sizing of the TXV.

The pressure at the TXV outlet will be higher than the

suction pressure at the compressor because of the fric-

tional losses through the distribution header, evaporator

tubes, suction lines, ttings, and hand valves. On rack

systems, the EPR valve also adds substantial pressure

drop.

The pressure at the TXV inlet will be lower than the

discharge pressure at the compressor because of fric-

tional losses created by the length of liquid line, valves

and ttings, and vertical lift. The only exception is if the

TXV is installed considerably below the receiver and

static head built up is more than enough to offset fric-

tional loses. The liquid line should be properly sized for

its actual length plus equivalent length due to tting and

hand valves. Vertical lift in the liquid line adds pressure

drop and thus static head must be included.

The pressure drop across the TXV will be the differ-

ence between the discharge and suction pressures at

the compressor less the pressure drops in the liquid line,

through the distributor, evaporator, and suction line.

ASHRAE tables should be consulted for determining

pressure drops in liquid and suction line.

Here is the procedure for properly selecting a TXV:

1. Determine pressure drop across TXV: using the

maximum and minimum condensing pressures, subtract

the evaporating pressure from each to get the total high-

to-low side pressure drop. From these values subtract

the other possible pressure losses– piping and heat

exchanger losses; pressure drop thru accessories; verti-

cal lift pressure drop; and the pressure drop across the

refrigerant distributor.

2. Consider the maximum and minimum liquid tempera-

tures of the refrigerant entering the TXV and select the

correction factors for those temperatures from the table

below the capacity ratings. Determine the corrected ca-

pacity requirement by dividing the maximum evaporator

load in tons by the liquid correction factors.

3. Select the TXV size from the proper capacity table for

the evaporator temperature, pressure drop available,

and corrected capacity requirement.

4. Select the proper thermostatic charge based on the

evaporator temperature, refrigerant, and whether a

Maximum Operating Pressure (see MOP section) type

charge is needed.

5. Determine connections and whether an externally

equalized model is required. Always use an externally

equalized TXV when a distributor is used.

A solid column of liquid refrigerant is required for

proper TXV operation. Calculate the pressure drop

in the liquid line to determine if there will be enough

subcooling to prevent ash gas. If the subcooling of the

liquid refrigerant from the condenser is not adequate,

then a heat exchanger, liquid subcooler, or some other

means must be used to get enough subcooling to en-

sure solid liquid entering the TXV at all times.

Emerson Climate Technologies has prepared

extended TXV capacity tables. These tables can be

found in the Emerson catalog. Always select a TXV

based on operating conditions rather than nominal TXV

capacities.

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