Tuesday, December 2, 2014

New and replacement refrigerants

 New and replacement refrigerants

For the long term replacement of CFCs and HCFCs, various manufacturers have developed products designed to meet the requirements of the industry.

In October 1990 ICI was the first company to introduce HFC 134a, an alternative to CFCs used in refrigeration and air conditioning. Since then other refrigerants have been introduced and marketed by ICI as Klea 134a.

Klea 32, the second in the Klea range, went into production in 1992. As these refrigerants were successful a number of blends were introduced to the range including Klea Blend 60 and Klea Blend 66, ozone benign refrigerants based on Klea 32 and Klea 134a. These are the alternatives to HCFCs. Each of these is non-toxic, non-flammable and has a good heat transfer property. All have undergone extensive trials, both in laboratory and systems in the field.

These refrigerants have a zero ODP and are currently not subject to regulation. European Union Regulation member states agreed to cease producing CFCs with effect from 1 January 1995. Regulations regarding the HCFC products meant that they would eventually be phased out although it was understood that interim products were necessary to assist the refrigeration industry during transition away from CFC refrigerants.

Tables 9 to 12 list the replacement and new refrigerants as long term CFC and HCFC replacements together with interim products.

There exists some confusion with regard to CFCs, HFCs and HCFCs; also refrigerant blends appear to come under various categories. In some quarters it is believed that CFCs are ‘banned’. This is not exactly correct; they are still obtainable for those willing to pay the price and are still widely used. The manufacture of CFCs has now ceased.

There are a great number of refrigeration systems charged with CFCs operating nationwide and users are unlikely to go to the expense of changing the refrigerant charge until a major breakdown occurs.

To summarize: by the year 2000 or possibly earlier R22 will be phased out, meanwhile the refrigerants listed in Table 13 may be used.

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imageZeotropic blends

These are unlike azeotropic blends which behave as single substance refrigerants having a constant maximum and minimum boiling point. The zeotropic blends do not behave as a single substance during the evaporation and condensing processes. The phase change occurs with zeotropic refrigerants in a ‘gliding’ form over a particular temperature range. This temperature glide can vary and is mainly dependent upon the percentage and boiling points of the individual components of its composition. When a blend has less than 5K glide it is sometimes referred to as ‘near azeotropic’ or ‘semi-azeotropic’, R404 being an example of this.

Refrigerant blends

Many refrigerant blends suffer from what is known as ‘glide’. The term boiling point is no longer applicable when dealing with these and instead the term ‘bubble point’ is used.

Briefly the bubble point can be regarded as the temperature at which vapour starts to form at a given pressure. If vaporization continues at the same pressure then the temperature at which the last drop of liquid refrigerant vaporizes is known as the ‘dew point’. The dew point therefore is also the temperature at which condensation starts as a superheated vapour is cooled at a constant pressure.

With a pure refrigerant and with a true azeotrope (such as R502) there will not be any temperature changes in an evaporator as the liquid refrig-erant vaporizes at constant pressure. With blends it becomes more complicated because the constituents have different volatilities. This means that they will vaporize at different temperatures. The blends do not boil at a constant temperature and pressure. Figures 116 to 118 give examples of the glide when plotted on a pH chart.

A plot for R12, R22 and R502 would be as shown in Figure 116.

A plot for the blends R404A and R407A would be as shown in Figure 117.

A plot for a refrigerant shows it to have a severe glide. An evaporator would be selected at a mean condition, as shown in Figure 118.

Figure 119 is a simple example of a plot on an enthalpy chart for a basic cycle with no liquid subcooling and no suction superheat. This illustrates bubble point, dew point, boiling range and glide. The cycle is based on 15 bar condensing pressure and 3 bar evaporator pressure.

Boiling range is the difference between dew point and bubble point at a given pressure. For 3 bar the boiling range = -12 - (-19) D 7 °C.

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It will be seen from a study of the chart for R404A and R407A that when evaporating at a constant pressure the evaporating temperature rises and this can be as much as a 5K rise. The reverse occurs when condensing.

At constant pressure condensation can commence at 40 °C and finish at 37 °C.

Expans on valve sett ng

If an expansion valve is set for 5K superheat, the temperature at the thermal bulb position would be (-7.5 C 5) D -2.5 °C.

Manufacturers of coolers, units and expansion valves will be pleased to supply capacity ratings for their products upon request.

Klea 407A is suitable for operation at temperatures between -35 °C up to 45 °C with discharge temperatures that compare favourably with R502.

Klea 407B, when used in existing R502 systems within a temperature range of -40 to 40 °C and where discharge temperatures are critical, also compares favourably.

Klea 66 (407C) can be compared generally to R22 with lower discharge temperatures.

Chang ng the refr gerant charge

The actual task of replacing the refrigerant in an existing plant is not a simple one like replenishing a charge. Various service organizations no doubt have their own procedures. A few manufacturers have adopted the method known as retrofit procedure but this procedure is not common to all replacement refrigerants and it is imperative that the following points should be noted.

1 Check the existing system design and operating pressures to ascertain that they are compatible with the new refrigerant. The manufacturer of the equipment should be consulted if in doubt.

2 Change the compressor lubricating oil to that recommended according to the refrigerant used.

3 Flush the system through with the original refrigerant to reduce the percentage of oil contamination.

4 If necessary change shaft seals, filter driers, components containing elastomers and expansion valves to required types.

5 Adjust expansion valve superheat settings for optimum performance.

6 Reset pressure operated safety switches if necessary. If fusible plugs are installed check that the rating is satisfactory for operation with the replacement refrigerant.

A refrigeration system charged with a CFC or HCFC will probably use a mineral oil for lubrication. This type of oil may not be suitable for use with the replacement refrigerant. Before commencing any procedure it is advisable

to consult the equipment manufacturer to determine suitability of the system components with the replacement refrigerant. It is also most important to determine the performance of the evaporator with a proposed replacement refrigerant.

A polyol-ester (PE) lubricating oil has been introduced which is generally a preferred choice. The PE lubricating oil may be used with some of the new refrigerants but not all of them.

For example, the stability of a system charged with HCFC 134a will

possibly be reduced if the system contains chloride ions. HCFC 134a is an acceptable replacement for CFC 12 in a new installation which has not previously operated with a CFC. Before the system is commissioned the type of lubricating oil in the compressor should be checked. HCFC 134a should not be used in a system that has previously operated with CFC 12 until the system has been flushed extensively. Mineral oils normally used with CFC 12 are insoluble in HCFC 134a and a suitable oil must be used.

Replacement refrigerants may act as a solvent to elastomers or other materials used in the construction of shaft seals, filter driers and various types of valves. Gaskets between planed surfaces may also be suspect and once again it must be stressed that advice regarding system components should be sought to avoid unnecessary malfunctions.

Reclaiming the refrigerant

When the refrigerant in a system is to be removed and the system compressor is operational it is possible to use the compressor to recover the charge. Obviously, the arrangement of the service valves on the system will affect the exact procedure.

It is possible to pump down the system in the usual manner and decant the refrigerant into a cooled recovery cylinder. It may also be possible to use the recovery cylinder installed at the compressor discharge to act as both a condenser and receiver.

In cases where the compressor has failed and the system is to be charged a recovery unit could be used to reclaim the refrigerant. There are many types of portable recovery units available which can be connected to any system service valves. A recovery unit can handle refrigerants in either liquid or vapour form. It will remove a system charge to any suitable pre-set pressure. Most recovery unit capacities range from 50 to 100 kg/h and operate on single phase 220/240 V, 50 Hz electrical supply.

It is possible to recover refrigerant from a hermetically sealed system which has no service valves by using line tap valves following the recovery unit manufacturer’s instructions.

Changing the refrigerant charge

As previously stated, exact procedures adopted by service agents may vary and the one given here is typical for replacing a refrigerant charge of CFC 12 with R134a.

The refrigerant in a system that requires replacement may be identified by the following methods:

(a) Refrigerant type stamped on the compressor nameplate.

(b) Refrigerant charge indicated on the expansion valve.

(c) By the system standing pressure.

Procedure

1 Isolate the compressor after evacuating the vapour in the compressor crankcase.

2 Drain the old mineral oil from the crankcase.

3 Replace that oil with a suitable PE oil.

4 Open compressor service valves and operate the plant.

5 Repeat step 1.

6 Drain the PE oil from the crankcase.

7 Replace with fresh PE oil.

8 Open compressor valves and operate the plant.

9 Repeat step 1 again.

10 Drain the oil and carry out a contamination check. There must be less than 1% contamination by the original mineral oil. (This test requires a special oil test kit and is easily applied.)

11 If less than 1% residual mineral oil, recover the CFC 12 refrigerant from the system.

12 Change filter drier, expansion valve and any other component necessary.

13 Evacuate the system and replace the PE oil once more.

14 Charge the system with R134a refrigerant.

Note: After step 10, if the oil contamination is still in excess of 1%, steps 1, 2, 3 and 4 must be repeated, after which another oil test must be carried out.

In reality R134a should not be charged until the oil contamination is less than 1%.

The chlorine free refrigerant R134a was selected as an example because it is available in sufficient quantities to replace R12. Another reason for its selection was because there are probably a greater number of small R12 systems in

the commercial field than the larger systems using other refrigerants. It can be used as an alternative for most R12 applications.

Compressor lubricating oils

The traditional mineral and synthetic oils are not miscible (soluble) with R134a. Oils which are not miscible can become entrained in heat exchangers in undesirable amounts to prevent adequate heat transfer, thus reducing the system performance.

The new lubricants developed, polyol-ester (PE) and polyalkene-glycol are miscible. These oils have similar characteristics to the traditional oils. They are more hygroscopic, dependent upon the solubility of the refrigerant. This means that the oils readily absorb moisture from the atmosphere.

Special care must be exercised during service, storage, charging, during dehydration and evacuation to avoid chemical reaction in systems such as copper plating. PAG oils tend to be more critical and are mainly used where high solubility is required. Ester oils are preferred by the industry in the main and information to date regarding their usage has proved satisfactory when the moisture content in the oil does not exceed 100 ppm. Experience has determined that systems should be well dehydrated and evacuated and relatively large drier capacities should be provided.

Other oils available and supplied by Castrol are alkylene benzene 2283 and 2284 which supplement the polyol-ester Icematic SW range.

Klea recommend EMKARATRL for use with Klea 407A, 407B and 407C application. This oil is fully compatible with these refrigerants.

Refr gerant recla m

This refers to the process of removing refrigerant from a system to be passed through a reclaim unit using large capacity filter driers which are capable of retaining moisture and acid content. The reclaimed refrigerant is then discharged by the reclaim unit into refrigerant cylinders for re-use. Alternatively the reclaimed refrigerant can be despatched for processing by specialists.

Refr gerant recovery

By way of an example assume that a condenser was found to have developed a slight refrigerant leakage. The leak being on the high pressure side of the system rules out the possibility of any moisture contamination. Under these circumstances the refrigerant could be recovered by using a recovery unit,

discharged into a service cylinder or cylinders if the charge is large. When repairs to the condenser have been completed the condenser should be pressure tested and evacuated. The recovered refrigerant can then be recharged into the system and the operating charge ‘topped up’ with the same refrigerant if necessary.

Due to the boiling points of various substances in refrigerant blends the actual recharging or initial charging of systems should be in liquid form. If a system is vapour charged there is a danger of drawing off from the refrigerant cylinder a greater quantity of one of the substances constituting the blend. This would undoubtedly alter the percentage of the mixture.

When reclaiming or recovering refrigerant from a commercial plant, whether the system compressor is operational or not, most of the refrigerant will be in the receiver and condenser in liquid form. The refrigerant is best removed from a system in liquid form using a reclaim unit. If it is possible to ‘pump down’ a system this task is much easier.

Obviously the process will be carried more rapidly and more efficiently if the refrigerant is drawn from both the high and low pressure sides of the system. The valve plate assembly of the compressor, expansion valve or the capillary will resist refrigerant passing through them. Inevitably some refrigerant will remain entrained in the oil which will settle in various parts of the system, for example the evaporator and compressor. If a compressor is fitted with a crankcase heater it can be actuated for a short period to raise the temperature of the crankcase oil which will then release the refrigerant from the oil. Defrost heaters on an evaporator can be employed in a similar manner.

Caution: Do not overheat the compressor or evaporator. The use of flame producing devices is not recommended for this purpose.

During a reclaim/recovery operation it is essential that sufficient cylinder capacity is available and double valve cylinders are desirable. Where possible cylinders should be cooled. Cylinders should never be overfilled.

It is an advantage to install an oil container between the recovery/reclaim

unit and the liquid receiver of the system.

Refr gerant hoses

Most manufacturers of pressure hoses supply them complete with Shraeder connectors. These can be removed to further reduce any resistance to the flow of refrigerant during the reclaim/recovery operation.

imageDomestic and commercial sealed hermetic systems

Domestic refrigerators and freezers do not normally have liquid receivers but access to both high and low pressure sides of the system can be achieved by using line tap valves. Some small commercial sealed systems may well fall into this category.

Other commercial sealed systems will have liquid shut-off valves at the receiver and suction service valves at the compressor.

Reference to the reclaim/recovery unit arrangement will show how hose

connections can be made.

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