Emerson ZB Copeland Scroll Service Manual download pdf (Page 15)

ZB Series

Installation and piping arrangement

The connecting pipe (high-pressure rubber pipe or cupper tube

ø10x1mm) for the vacuum pump should be as short as possible

with no bends or kinks. The evacuating capacity will evidently

reduce owing to the kinks and joints. Also, it should be noted that

the indicated value of the vacuum gauge is unlikely to comply with

the vacuity at the end of the system, because the vacuum gauge is

usually placed on the vacuum pump. Hence, the evacuating time

should be extended to ensure uniform vacuity throughout the

system. A vacuum pump whose evacuating rate is 40-50 l/min is

enough to meet the demands of medium/small-sized equipment.

Larger equipment should be connected with a tube whose inner

diameter is above 10mm or with a copper-tube (ø12x1mm,

ø15x1mm) equipped with a correspondingly large-sized

evacuating valve. Evacuation should be carried out with a two-

stage vacuum pump. The vacuity must be measured by a vacuum

gauge instead of a conventional gauge. The system should be

evacuated at least twice until the vacuity is 2mbar (1.5torr). Then,

the vacuum should be broken with dry nitrogen, followed by

evacuation of the whole system including the compressor. The

vacuity reaches 0.7mbar (about 0.5torr) by means of the third

evacuation. The refrigerant may finally be charged in the system to

bring gauge pressure to 0.15bar.

Caution: In order to prevent motor damage, do not start the

compressor or conduct any electric test under vacuum. Do not

allow the compressor to run under vacuum condition.

It is important to be careful and exact when evacuating and

drying. Air remaining in the system in the process of installation

will lead to rise in discharge temperature and the lubricating oil

carburizes, thereby affecting the quality of lubricating oil and

causing compressor failure. The moisture and air will generate

acids and corrode the metal resulting in deterioration of

lubricating oil quality. These phenomena will be quick to form

under high-pressure and high-temperature conditions.

Charging of refrigerant

The refrigerating equipment can only be charged with a designated and

selected refrigerant. The running effectiveness depends on the correct

charge quantity of refrigerant. If the charged quantity of refrigerant is

insufficient, refrigerant in the evaporator will be insufficient as well, resulting

in a drop of suction pressure and discharge efficiency, thereby causing

the motor to overheat. If the quantity of refrigerant charge is in excess,

there is excess liquid refrigerant in the condenser as well, causing a rise in

condensing pressure and compressor failure arising from liquid return.

The liquid refrigerant should be charged from the high-pressure and

low-pressure side simultaneously. Most of the charging quantity should

be from the high-pressure side. The refrigerant cylinder should be weighed

before and after charging to determine the physical charged quantity.

A basic method used to charge liquid refrigerant is to make the

refrigerant pass through a filter-drier placed in the charging pipe. The

refrigerant is charged via a cut-off valve or a charge valve with a joint to

the liquid reservoir.

A more common way to determine the charge quantity is observe the

liquid refrigerant flow in the sight glass of the liquid piping. When the

refrigerant flow is visible, it can be assumed that the refrigerant is

charged properly, because the normal operation of the expansion valve

depends on the uninterrupted flow of liquid refrigerant. The emergence

of foam indicates insufficient refrigerant.

However, the use of sight glass as an effective tool to determine the

charged quantity of refrigerant is not entirely advisable. It may be

noted that, at times, foaming may still be observed from the sight

glass, in spite of sufficient refrigerant charge. One of the reasons is that

there is a vent above the sight glass in the liquid piping which results in

foam when the liquid pressure decreases. Also, any quick change in

condensing temperature, such as from the condenser fan not starting,

can cause this sudden evaporation. The criteria to judge whether or

not the charged quantity of the refrigerant is adequate are the super

heat temperature of the return gas and the sub cooling temperature of

liquid refrigerant.

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Emerson ZB Copeland Scroll Service Manual Page 15

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