Tuesday, December 2, 2014

Pumping down and charging

Pumping down and charging

Before an engineer can repair a refrigerant leak in the system it has to be located and for this reason leak detection is the first step. If the system has not lost all its refrigerant charge, leak detection may proceed as described in Chapter 3 provided that there is an adequate test pressure within the system. When the entire refrigerant charge has been lost, the system has to be pressurized.

If the leak is found to be minor and on the low side of the system, ‘pumping

down’ will be necessary before any attempt is made to repair the leak. Leakage from the high side of the system will require the removal of all the refrigerant before repairs are undertaken.

Pumping down

This procedure transfers the refrigerant circulating around the system from the liquid shut-off valve at the liquid receiver to the inlet of the compressor for storage in the condenser and the receiver, thereby making it possible for other tasks to be performed.

The method is as follows:

1 Fit gauges, set the service valves to operating positions and operate the plant.

2 Close the liquid shut-off valve at the receiver.

3 Allow the compressor to operate until a pressure slightly above atmospheric pressure is registered on the compound gauge (3 psi or 0.2 bar). If the system employs a low pressure cut-out switch then the range will need to be altered; note the cut-out pressure beforehand. Modern switches have a small lever which can be moved to override the cut-out point, thus keeping the compressor running.

4 Stop the plant and front seat the suction service valve. The system can now be worked on for a number of repairs.

When it is required to close down a plant for an extended period, the discharge service valve should also be front seated after the gauges have been removed.

Adding refrigerant

When it is necessary to add refrigerant to a system after a loss it indicates that there is a leak in the system. The leak must be located, repaired and the system leak tested. If the leak is found to be on the low side of the system the repair can be made after the system has been ‘pumped down’ to a balance in pressure. Do not draw in any air which contains moisture, especially where hygroscopic ester oils are concerned, (for example R134a). Some oils are not affected and accept the ingress of air and moisture, recovering after a vacuum is drawn with a recovery unit.

Vapour charging

A slight loss of refrigerant is recognized by ‘bubbles’ in the liquid-indicating sight glass installed in the liquid line of a system, a loss of performance, partial frosting of the evaporator and lower than normal operating pressures. Refrigerant may be added in vapour form to the low side of the system if the refrigerant is azeotropic such as R502. This means that it reacts as a single substance refrigerant.

The zeotropic refrigerant blends do not react in a similar manner and they should be added to a system in liquid form. This subject is covered in more detail in Chapter 17.

The procedure for vapour charging is as follows (Figure 10):

1 Fit gauges, set the service valves to operating positions and operate the plant.

2 Obtain a service cylinder of the correct refrigerant: this can be verified from the equipment log, the compressor nameplate or the label on the expansion valve.

3 Connect the yellow hose to the centre connection on the manifold and to the service cylinder.

4 Open the valve on the service cylinder, loosen the connection on the centre hose on the manifold and purge air from the hose.

5 Tighten the hose connection and set the suction service valve to the midway position.

image6 Open the compound gauge valve on the manifold slowly, and regulate the refrigerant into the system at an approximately average suction pressure

(e.g. 30 psi or 2 bar for R12).

7 Observe the liquid-indicating sight glass and, when the bubbles cease, close the compound gauge valve on the manifold. If bubbles return intermittently after a short time, add more refrigerant. When bubbles have ceased completely the operating pressures will have returned to normal and the evaporator will be fully frosted.

A further check on the refrigerant charge is described on pages 33 8.

When vapour charging a system with an azeotropic refrigerant such as R502 the refrigerant cylinder must always be kept in a vertical position to prevent the possibility of liquid refrigerant from entering the compressor. This can create a dynamic pressure when the compressor starts, causing damage to valves or may even break piston connecting rods and damage pistons. The liquid refrigerant will also flush lubricating oil from bearing surfaces.

The main disadvantage of vapour charging to the low side of the system is that it is a comparatively slow process, especially during low ambient temper- ature conditions and when the system requires a large operating charge. A large compressor will quickly reduce the suction pressure. The cylinder will become cold and, if the low pressure condition is prolonged, the cylinder will frost up.

Hermetic and semi-hermetic motor compressors may be suction vapour cooled; that is, they may rely upon suction vapour returning to the compressor

in sufficient volume to cool the motor windings. It is possible that the low volume of vapour entering the compressor during the charging process will not be adequate for motor cooling.

The charging process can be speeded up by applying swabs dipped in warm water to the refrigerant cylinder. Never heat cylinders with a blowtorch or immerse them in boiling or very hot water.

A rapid charger is available. This device, fitted between the hose and the cylinder, acts as a restrictor or limit valve, maintaining a higher constant pressure in the cylinder and preventing early frosting. The vapour passing through the device is less superheated when it reaches the compressor, so less time is taken to charge the system.

Liquid charging

This method is not always acceptable to manufacturers, since it involves putting refrigerant liquid into the high side of the system. If the compressor discharge valves are not seating properly there is a danger of liquid refrigerant entering the compressor cylinders, which can cause damage due to dynamic pressure when the compressor starts. The liquid cannot be compressed without creating high pressure.

Some large condensing units are equipped with liquid charging valves on the receiver. Smaller hermetic and semi-hermetic units have service valves located on the receivers, and these can be employed for charging.

Liquid charging is carried out when a system is commissioned, or when it has been completely discharged of refrigerant.

Compressors should be at rest when the system is being liquid charged.

A simple method is by gravity, and should be performed carefully for the initial liquid charge (Figure 11a):

1 Fit gauges and connect to a refrigerant cylinder as shown.

2 Fully open the charging valve or service valve.

3 Open the valve on the refrigerant service cylinder and invert the cylinder.

As long as the pressure in the service cylinder is greater than that in the system, the liquid will flow. The flow will be audible.

4 When the flow has ceased, allow gauge pressures to stabilize, close the valve on the service cylinder and operate the plant.

5 Complete the charging in vapour form to the low side of the system.

To eliminate the possibility of compressor damage, the following procedure may be adopted (Figure 11b):

image1 Run the compressor for a few revolutions with the discharge service valve fully front seated until a pressure of 20 to 30 psi (1.5 to 2.0 bar) is registered in the pressure gauge. This will be compressor cylinder pressure.

2 Stop the compressor.

3 Invert the refrigerant cylinder and open the valve or open the liquid charging valve as appropriate.

Table 1 Initial charges (kg) for various compressors and refrigerants and for flooded and dry evaporators

Compressor capacity kW

R12

Flooded

Dry

R22

Flooded

Dry

R502

Flooded

Dry

0.5

1.4

0.7

1.4

0.7

1.4

0.7

0.75

2.7

1.4

2.7

1.4

2.7

1.4

1.0

4.0

2.0

4.0

2.0

4.0

2.0

1.5

5.5

2.7

5.5

2.7

5.5

2.7

4 Open the liquid charging valve or service valve on the receiver.

5 Crack off the discharge service valve. High pressure from the compressor on the surface of the liquid refrigerant in the cylinder will force the liquid into the condenser and receiver.

6 Set the discharge service valve to the operating position, close the liquid charging/service valve and the refrigerant cylinder valve, and operate the plant.

7 Complete the charge in vapour form.

With water cooled condensers the pressure will normally be greater in the refrigerant cylinder than in the system.

As a guide to refrigerant charges, Table 1 gives approximations for the initial charges to be administered to systems employing both flooded and dry expansion evaporators.

Liquid charging is to be carried out more frequently since the new and replacement refrigerants become available. The current and more efficient method is when the refrigerant is added via the liquid receiver as shown in Figure 11b.

Refrigerant cylinders and fill ratio

Refrigerant bulk cylinders and some service cylinders are fitted with two shut- off valves, which are colour coded and marked to indicate delivery of either liquid or vapour.

Two tubes supply refrigerant to the valves (Figure 12). One terminates at the top of the cylinder above the liquid level, and when the vapour valve (red) is opened it will discharge vapour only. The other tube from the liquid valve

image(blue) terminates almost at the bottom of the cylinder, and when the valve is opened it will discharge liquid until the cylinder is virtually empty.

Modern cylinders have capacities, maximum fill quantity and identification of the contents stamped on the cylinder. Older types may only have the total weight and the tare (empty cylinder weight). The total weight is the weight of water contained in the cylinder if it was completely full (the water capacity) plus the tare.

A refrigerant is a volatile substance, and is more responsive than water to density changes at different temperatures. For this reason a cylinder should never be completely filled with refrigerant.

A simple calculation can be made to determine the amount of refrigerant which can safely be transferred into a service cylinder. A safe fill for any refrigerant is 80 per cent of the water capacity. For example, assume the total weight of a cylinder to be 24 kg and the tare to be 4 kg. Then the water capacity is 24 - 4 D 20 kg. The maximum fill is 80 per cent of 20 kg, i.e. 16 kg.