Secondary cooling - helping to take the heat out of F-Gas phase downClimalife UK
With so much market attention on the switch to lower GWP refrigerants to meet the F-Gas phase down steps, many secondary refrigerants can also offer a quota and GWP free solution.
Secondary refrigerants have long been used in systems in conjunction with a primary circuit and can help to significantly reduce the amount of refrigerant used in a system and also provide the option to consider ultra-low GWP alternatives.
How and where is a secondary refrigerant used?
As its name suggests, a secondary refrigerant can be used alongside a primary circuit but unlike the refrigerant in the primary circuit, the fluid used in the secondary circuit does not undergo phase change and needs to stay as a free flowing liquid over a wide range of temperatures. The fluid normally performs the function of transferring thermal energy, hot or cold, from one location to another. Simply put, by using smaller quantities of refrigerant to cool the secondary refrigerant via a heat exchanger, the fluid can be pumped around the system rather than vast quantities of refrigerant.
These fluids are used across a range of applications, from small water chillers and heat pumps containing around 10-25 litres to larger refrigeration plant that transports cooling capacity from the plant room to various points in a system, and which may contain as much as 100,000 litres operating at temperatures well below the freezing point of water.
Secondary refrigerants are sometimes used in conjunction with ultra-low refrigerant options such as Ammonia which has a GWP of 0 or CO2 with a GWP of 1. For example, Ammonia would normally be too hazardous to pump in volume around an indoor facility, however by installing an outdoor chiller, smaller quantities of Ammonia are used to chill the secondary refrigerant which is then pumped around the facility. Similarly this process can be followed with refrigerant gases, whereby far less refrigerant charge is required as it simply chills the secondary refrigerant which is then pumped around the system.
With the impact of F-Gas being felt, hybrid VRF systems and water loop condensing units offer new options to lower GWP and reduce refrigerant charge. The use of A2L, mildly flammable low GWP refrigerants could be very viable in such applications.
Some high temperature applications, such as air conditioning, may only use chilled water as the secondary refrigerant, however its freezing point is a problem and if used without some form of anti-freezing agent or inhibitor package it runs the risk of freezing or corrosion which can cause costly damage to a system.
Types of secondary refrigerant on the market
- Mono Propylene Glycol (MPG) is probably the most commonly used secondary refrigerant as it is safe to use food manufacturing sites.
- Mono Ethylene Glycol (MEG), used in some industrial applications however is classified harmful where ingestion of as little as 90ml can be fatal.
- Bio glycol is growing in its popularity as a renewable option; made from a plant based resource – a safe sustainable alternative to MPG.
- Salts have a number of very advantageous thermo-physical properties, however if there is leakage onto metal components, corrosion can quickly occur.
- Specialist low temperature oils or silicones.
Good preparation & maintenance are essential when using secondary refrigerants
These fluids can be one of the least expensive components of a system if they are sourced and used correctly. There is also plenty of advice available in the market, and product data sheets and safety data sheets should also be consulted before use.
Always check that the system is correctly sized and material compatibility exists with the fluid to be used for all the components, such as pumps, pipework, material and joint seals. It is important to make sure a system is clean, free of dirt, grime, rust and other particles, and has no stagnant water in it. Test for leaks before filling, charge slowly to avoid air pockets and purge any entrapped air.
Secondary refrigerants come as concentrated or ready to use products. The correct dilution for freezing protection is essential and the density and viscosity of that concentration should be taken into account. When these fluids get close to their freezing point, their viscosity increases, making them harder to pump. Concentrated product should be diluted with demineralised or de-ionised water to protect the longevity of the system. This is because it will be free of Chlorides, Sulphates or Calcium which can cause problems such as sludge or scale to form. It is often simpler to use a ready to use product which has already been diluted to the required concentration.
Good quality secondary refrigerants will contain corrosion inhibitors which is a key feature for the longevity of any fluid. Check that these products comply with the ASTM or NFR standards. Some products now also use organic inhibitors and they are longer lasting and give better thermal transfer properties as they only form a protective film if there is a sign of corrosion. Traditional mineral based inhibitors will otherwise form a protective layer on the wall of the system.
Regularly check the PH levels to ensure there are no signs of corrosion and also check the concentration of the liquid to ensure there are no risks of freezing. Drain or top up the system with a good quality glycol and ensure the correct concentration levels are met. Topping up a system with water can dilute the protection levels and potentially cause costly damage to a system so don’t be tempted to cut corners.
Ensure your equipment is labelled with the product in use. This will facilitate future maintenance.
Working in synergy with an all-round maintenance plan
Secondary systems can have a big part to play as we move toward a lower GWP future. Good all round system maintenance and the use of high quality fluids can help achieve a sustainable option. Companies such as Climalife can offer advice on product suitability for your applications.
Read the article printed in ACR News June 2018