By Grant Laidlaw

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in the industry is not in place.

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National TechnicalDiploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.
Kusaan asks: With regard to flammable refrigerant systems, it is obviously important to make sure that there are no leaks. What other aspects would be considered good practice when installing and maintaining these systems? Thank you.

Hi Kusaan. Yes, refrigerant containment should be a priority for everyone who installs and maintains refrigeration equipment utilising flammable refrigerants. In reality, this applies to all systems. I have dealt with the safety aspects around flammable refrigerants in previous RACA Journal editions.

Let us ask the question, why is it desirable to reduce/prevent leakage of refrigerant?

  • Impact on the environment is minimised
  • A system can only work properly if it is charged to its optimum Lower refrigerant levels lead to a drop in efficiency and an increased risk of the system being incapable of meeting the operational requirements
  • Leaking refrigerants can pose a risk to both health and safety when leaking into the environment
  • To prevent perishable stock losses
  • To maintain the cold chain
  • Operating/repair cost savings
  • To maintain a good client relationship

I am sure we can expand on this, but I think the point has been made.

How do we improve refrigerant containment and leakage reduction?

Servicing a system is of vital importance. Aside from the improved efficiency, reliability and longevity of the system, lack of maintenance can and will increase the frequency of leaks. As a condenser becomes dirty and blocked the heat transfer from the refrigerant is reduced.

This causes an increase in the high side operating pressures which in turn place additional strain on joints and components in the system creating a higher risk of refrigerant leakage. In addition this places particular focus on efficient leak detection.

‘Sealed systems’ should be the aim of design and installation. This means that, for example, all flared type connections should be avoided as far as possible, and one should utilise brazed or welded connections. This then means that in the selection of components, one should select components with brazed connections unless there is a specific technical reason why permanent joints are not appropriate. The selection of refrigerant circuit components should be made under the premise that their design reduces the potential for leakage.

On the design side it is desirable to keep refrigerant charge to a minimum. This can be achieved if the refrigerant suction lines and liquid lines are exactly sized and kept as short as possible.

In refrigeration applications, distances between plantroom (e.g. condensing unit) and evaporator position (refrigeration load) should be minimised as far as possible. In addition, heat exchangers and refrigerant receivers should be optimised to the system’s requirements.

With a unitary air conditioning system place the evaporator unit and the condensing unit as close together as practically possible. This not only removes the need for additional refrigerant but improves the efficiency of the system.

Note: I am aware that some manufacturers recommend rolling up excess tubing behind the condenser. However, should you have the correct tooling this is not necessary

Kusaan, the use of indirect refrigeration systems can also be used to reduce the amount of refrigerant required for a system. Indirect systems are frequently used in industrial refrigeration systems as well as commercial/industrial air conditioning systems. These systems utilise a chilled water or brine system on the evaporating side of the system and a condensing water system on the condenser side. This gives us the advantage of keeping all the refrigerant in the plantroom reducing refrigerant requirements substantially. Additional advantages can be found when considering that the refrigerant does not circulate into any retail, commercial or public space.

The most common application occurs when there is a large number of cooling positions distributed throughout a building. The supermarket industry is frequently using this type of system for food storage and display equipment. The refrigeration system is placed in the plant room and the heat transfer for system cooling and chilling is maintained with the use of intermediate heat exchangers and a so called ‘secondary refrigerant’ and a cooling water circuit. These systems are frequently found in the air conditioning segment of the industry applied for example to shopping centres, office buildings and hospitals.

Another aspect is vibration, as the effects of vibration include tubing and component failure and leakage. It then follows that vibration should be minimised. This is especially relevant for refrigeration circuit components generating pressure pulsations and liquid strokes, such as the compressor, solenoid valves and refrigerant circuit tubes that are not properly fixed. Heat exchanger fans, improperly installed or imbalanced, may also generate vibrations that can affect the structural integrity of tubing and cause vibration breaks.

Follow good installation practices for both refrigerant piping and electrical installations:

  • Fit vibration absorbers correctly
  • Components that are not part of the refrigerant circuit should not be attached to it or held by refrigerant pipes
  • Unintended contact of cables or other auxiliary materials to copper tubing should be avoided
  • Unprofessional installations may lead to fretting, chaffing and movement of wires against tubing and damage the copper tubes, leading to refrigerant leakages and consequently system failure
  • The arrangement of system components and valves within the refrigerant circuit should also be considered as they may be exposed to high pressures, temperatures and vibrations

As installation and assembly are among the main tasks, best practice procedures are necessary for efficient quality of installation, servicing and maintenance.

There are other aspects which we may consider, for example the simple Schrader valve cap. Schrader valves are commonly used for refrigerant circuit connections with service gauge manifolds or direct connections to control devices. For general system servicing they play an important role. A Schrader valve (without a sealing cap) is not leak-proof!

Several types of sealing caps are used in practice:

  • Knurled sealing cap with elastomer/rubber seal. Over time the elastomer/rubber seal of the knurled sealing cap ages and begins In addition, they become porous or get damaged by mechanical influences. If system components, compressors, condensers, evaporators, refrigerant transfer pipes have Schrader valves with knurled sealing caps these should be replaced by a hexagon cap nut or a flare nut with copper seal.
  • With regard to the hexagon cap with conic sealing surface and the flare nut with copper seal, when tightening the hexagon cap take care to ensure that the Schrader valve body is secured with a spanner. Tightening torque of 14 to 18 Nm applies; use of a torque wrench is considered best practice. The sealing face of the copper seal should not be coated with The smallest amounts of oil can cause copper seals to leak.

Kusaan, you may encounter a hexagon cap with rubber/elastomer seal. As with the knurled sealing cap it is not recommended as the tightening torque is not specified, damage to the seal poses an additional issue.

In the case of domestic refrigeration fridges, it is best practice to pinch the process tube and remove the Schrader valve.

Looking at the refrigerant tubing itself, as the world implements natural environmental refrigerants we should revisit refrigerant copper tubing. Copper is an excellent material for use with refrigeration and air conditioning systems. With decreasing temperature copper offers increasing stability and expansion.

Brittleness with low temperatures does not exist compared to other materials, such as steel. Copper pipes are resistant against reactions with all refrigerants except ammonia (NH3).

As we implement newer refrigerants there have been changes to copper tubing specifications and one should ensure that the correct grade of tubing is used for a specific refrigerant. Good examples of this are copper tubing used with CO2 and R410a.

Requirements and conditions specified for copper pipes are referring to:

  • Refrigerant tubing must be seamless
  • Quality of the inner surface
  • Closed (caps) pipe ends / dehydrated
  • Marking of the pipes
  • Stability against pressure

The term ‘temper’ describes the strength and hardness of the tube. In the pipe trades, drawn temper tube is often referred to as hard drawn tube and annealed as soft drawn tube.

Soft, annealed copper tubing is far more versatile than rigid copper tubing. It can be purchased in much longer lengths, is rolled up and requires fewer joints which reduces leak potential. Due to its fairly flexible nature, it can be positioned and shaped easily, which saves installation time. Soft drawn tube can be commonly joined by the use of flare-type joints. It is also possible to expand the end of one tube so that it can be joined to another by brazing, a procedure that can be efficient and economical in many installations (swaged joints).

Nevertheless, mechanical connections should be reduced in favour of brazed joints. Copper tube for RAC field service is designated by actual outside diameter. Soft drawn tubing typically ranges from 6.35mm to 22.2mm in size

Hard drawn copper pipes are rigid, and this type of piping makes a neater installation, but it is more time consuming and more difficult to install than soft tubing. It needs little mechanical support to keep it in position, compared to soft copper.

Rigid copper tube is generally joined by brazing, using capillary fittings or by welding and with the use of compression fittings. Hard drawn tubing is available in larger sizes than soft drawn tubing. Sizes range from 6.35mm to 104.78mm.

Lastly, if possible, formed pieces (hand bent fitting pieces)

that can be prepared on site using a mechanical pipe bender are preferred. If fittings cannot be avoided the tubing should be laid out to use a minimum number of fittings. Long radius elbows should generally be used with refrigeration tubing. A long radius elbow has a longer sweep and will have less pressure drop than a short radius elbow. Only use a short radius elbow if it is absolutely necessary for clearance considerations.

Kusaan, I hope that this helps with your understanding around ensuring system tightness and reducing, or even eliminating refrigerant leaks.

Grant Laidlaw

REFERENCES:

  1. ACRA
  2. 2.GIZ Proklima

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