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The Department of Agriculture, Water and the Environment has released a new report by the Expert Group which identifies opportunities to reduce emissions by tackling refrigerant leaks.

As most technicians know, the source of leaks can be broadly defined by the location/component at which the leak occurs.

There are many potential leak locations, especially on larger systems that have numerous joints, valves and compressors.

Two problems raised in the report are liquid line issues and refrigerant health.

Liquid line issues mean restrictions in the refrigerant pipeline and associated components (expansion valves, filters) that increase resistance and starve the evaporator of the correct amount of refrigerant.

The report referred to several issues covered by this fault including incorrect pipe sizing, poor commissioning such as not setting up superheats on the thermal expansion value (TEV), and issues with liquid line components, e.g. when particles accumulate within and block the refrigerant filter in the refrigerant circuit

Liquid line restrictions or the accumulation of particles in filters can increase the flow resistance of the refrigerant circuit and the pressure difference across the compressor. It also reduces the evaporating temperature and leads to lower cooling capacity and efficiency.

“Not setting up the TEV correctly can result in efficiency losses for the entire lifetime of the equipment,” the report said.

“It is important to note that TEVs are refrigerant-specific valves and if a system is retrofitted to a new refrigerant the TEV should be either replaced or in some instances the same TEV can be used with the superheat readjusted according to manufacturer’s instructions.

“This is a widespread problem which requires training in liquid line sizing and setting up TEV.”

The report recommends maintenance schedules which include the replacement of refrigerant filters, inspection of the system for restrictions, checking by diagnosing system performance and an automated fault detection device (AFDD) for liquid-line restrictions.

Refrigerant health

When a RAC unit is not properly evacuated prior to being charged with refrigerant, the unit runs with a mixture of air and refrigerant.

The air is non-condensable and is typically trapped in the high-pressure vapour downstream of the compressor.

The emergence of blends, makes it more complex for technicians going forward. Both incorrect set-up during retrofit or leakage can cause a change in composition consuming energy unnecessarily.

During leak events, higher pressure components of the zeotropic blend may leak first, changing refrigerant composition and resulting in an energy penalty. With incorrect expansion valve adjustment and composition change two things can happen.

First, the liquid may not vaporise before reaching the compressor, which can cause inefficiency and lead to compressor damage. Second, the blend may completely boil part way through the evaporator, leading to a loss of efficiency.

Blended HFC and hydrofluoroolefins (HFO) refrigerants, made by mixing several refrigerants together in specific proportions to create the desired operating characteristics, can behave differently to single-component refrigerants, the report warns.

For zeotropic blends, the temperature varies between dew (saturated vapour) temperature and bubble (saturated liquid) points in a constant pressure process. ASHRAE classifies blends as azeotropic (R500 series) and zeotropic (R400 series).

“Some zeotropic blends can fractionate when leaking from systems while in a superheated or subcooled condition, which will alter the refrigerant’s overall composition and energy performance,” according to the report.

“Where a leak of fractionated refrigerant has occurred, the entire refrigerant charge may need to be recovered and replaced with virgin blend refrigerant to ensure the correct blend ratio (and hence system performance) is maintained.

“This can be an expensive maintenance practice, especially for central plants such as chillers or supermarket systems. The fractionation effect is dependent on the extent of temperature glide of the refrigerant blend.”

 

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