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ENERGY SAVING Cooling systems


By Jan Gielen Manager / Climate & Energy Specialist DLV Plant MUSHROOMS j.gielen@dlvplant.nl


and energy saving


There are numerous technologies and options available in the mushroom growing industry desig- ned to save energy. This article examines the differences between the cooling systems used in the sector and their influence on energy consumption, and pays particular attention to cooling based on a direct pump system.


Direct and indirect cooling systems The best-known system is indirect cooling, also called a chiller, whereby a working medium such as water or glycol is used as a heat transfer fluid (see diagram 1). This cooling system is characterised by its use of an often-standard chiller, supplemented by a water/gyclol-based component. There are also di- rect cooling systems whereby the refrigerant (freon, ammonia or CO2), itself is used as the heat transfer fluid. As direct cooling does not use a working me- dium, the energy consumption of this system is lower than an indirect cooling system. The most important criteria to consider with cooling systems for ap- plications in mushroom growing is not only energy consumption, but also the controllability of the temperature after the heat sink. A stable temperature after the heat sink will also result in stable evapora- tion and temperature control in the growing room. Direct cooling systems can be subdivided into on-off controlled systems (unsuitable due to the large temperature fluctuations) or electronically control- led systems (see diagram 2) and pump systems (see diagram 3). Compared with an electronically control- led system, a pump system (in addition to slightly preciser control) offers the possibility of free cooling (cooling with deactivated compressors) at external temperatures of 8°C and lower. The combination of economical energy use and good controllability have prompted this article’s focus on the workings of direct pump systems, illustrated by some practical examples.


Direct pump system (working principle) In a pump system (diagram 3), the liquid freon is pumped from the freon tank/separator to the heat sink in the climate unit where part of the freon vapo- rises. This mixture of fluid and vapour is then trans- ported back to the freon tank/separator. From here, the compressor sucks in the freon gas and compres-


ses it to a higher pressure. This processes heats the freon gas which is then transported to the condenser. The condenser is usually located outdoors and is cooled using outside air, which causes the freon gas to cool and condense to form a liquid. Incidentally, water-cooled condensers, or condensers that use a combination of air and water cooling, are also avai- lable - which enables heat recovery. The liquid freon flows to the expansion valve. At this stage, the pres- sure is lowered causing part of the liquid to vaporise and cooling the rest of the liquid. The liquid runs into the tank/separator before it re-drawn into the com- pressor where the cooling process is repeated. Pump systems are extremely energy efficient, but require a relatively large quantity of refrigerant, which is pro- blematic in the event of any leakages. The aim is limit the refrigerant charge used in such installations as far as possible. This can be achieved by using com- pact heat exchangers and a well-engineered design/ central location with a minimum piping length.


Indirect cooling system (working principle) An indirect cooling system or chiller (diagram 1) uses a heat exchanger, possibly with a storage tank, where freon is vaporised on one side causing glycol to be cooled on the other side. The freon part works in a similar way to a direct pump system, except in this case a metered quantity of freon is comple- tely vaporised by the expansion valve and used to indirectly cool the glycol. The cold glycol with a temperature of around 6°C is pumped from the heat exchanger to the heat sink in the climate unit. Here the glycol absorbs heat up to +/- 12°C and is then transported back to the heat exchanger where it is indirectly cooled again. An indirect cooling system contains a considerably smaller quantity of refrige- rant than a pump system as the secondary compo- nent of the system uses glycol. Water can also be used, but only in situations where the outside tempe-


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