Refrigerants in postharvest refrigerating systems
Refrigerants in postharvest refrigerating systems are used to create the temperature difference needed to transport heat from the cold storage room to the warm outside. The choice for the most optimal refrigerant is mostly based on safety (product, environment and people), refrigerating capacity and temperature level(s) of the refrigerating system for its application in the fresh chain. Every refrigerant has its specific characteristic in efficiency in range of temperature / pressure, ozone depleting factor, global warmth potential, safety (explosion, fire, pressure, health) and many more.
Primary and secondary refrigerants
Refrigerants can be classified in many different ways e.g. primary and secondary refrigerant. A primary refrigerant is used in the active cycle where the temperature is lifted by pressure change. A primary refrigerant always makes use of phase change (liquid -> gas -> liquid) to uptake/reject heat. A refrigerant only transporting heat over a distance is a secondary refrigerant (or brines). Secondary refrigerants, are mostly in liquid phase only and do not change phases. Other used classifications are natural/synthetic, boiling point/boiling traject, toxic /non-toxic refrigerants.
Natural refrigerants
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Different options natural refrigerants. Image by WUR Different options
The main natural refrigerants are ammonia, CO2 and propane, all suitable as primary refrigerant. The refrigerating systems based on natural refrigerants can be a direct system, where the evaporating (primary) refrigerant directly cools the air using an evaporator. It can also be an indirect refrigerating system, where the primary refrigerant cools the secondary refrigerant with an heat exchanger. The secondary refrigerant cools the air using an air cooler and transports the heat back to the heat exchanger. -
Photo by g0d4ather/Shutterstock.com Environment
The GWP (Global Warmth Potential) of all natural refrigerants are limited and for this reason they are introduced as refrigerants with minimal CO2 emission. -
Ammonia installation. Piping in stainless steel reduces leakage. Photo by WUR. Ammonia
Ammonia is one of the oldest refrigerants with most high efficiency. Unfortunately ammonia is poisonous for human and most fresh products. A minimal leakage of ammonia will alert because of its penetrant smell. With installing and servicing all components of ammonia refrigerating plants according to the specific (national) safety demands, the application of refrigerating equipment with ammonia is widely accepted. -
CO2 direct pump installation. Photo by WUR. CO2
CO2 as refrigerant is relatively new. Using CO2 in postharvest refrigerating technology, the pressure in phase change is relatively high and because of this, electrical use is more critical than for other refrigerants. CO2 is life dangerous in case of leakages in air closed areas. -
Propane (R290)-glycol system. Photo by WUR. Propane
Propane and other carbons are functional refrigerants. The fire and explosion sensitivity of this coolant is limiting the system content (kg) of propane in the refrigeration unit. Installing and servicing systems with this propane requires trained maintenance people. -
Visualization of an indirect pump system. Image by WUR. Indirect refrigerating systems
A practical way of preventing safety issues, is using an indirect refrigerating system based on an outdoor chiller and using a food grade brine as secondary refrigerant. This outdoor chiller is the active refrigeration cycle which cools the secondary refrigerant.
Synthetic refrigerants
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Controlling leakage of refrigerants and regulation of use of high impact refrigerants helps to minimize the impact on emission of ozone depleting substances.Photo by Alexandr Shevchenko/Shutterstock.com Regulation
Since the Kyoto agreement, most countries agreed to minimize the impact of refrigerants on emission of ozone depleting substances and emission of CO2 by controlling leakage and regulation of use of high impact refrigerants. This regulation starts a development of implementing more natural refrigerants in the postharvest industry. -
Photo by Calin-H/Shutterstock.com CFK, HCFK, HFK
The first synthetic refrigerants like CFC-12 (CCL2F2) contain chlorine responsible for the breakdown of ozone. Refrigerants with less or no ozone emission were introduced, by replacing the chlorine atom (CFC-22, CHClF2), followed by a generation without Clorine (HFKs). Refrigerants with chlorine are not allowed to be used in newly build or refilling existing refrigerating equipment in Europe. -
Photo by g0d4ather/Shutterstock.com GWP
In next phase of limiting the use of synthetic refrigerants, the Global Warmth Potential (value in CO2 emission equivalent) is used as reference described in the Montreal Protocol (EU F-Gas regulation, 2015). In this protocol, refrigerants with a high greenhouse gas emissions (GWP) > 2500 kg CO2 equivalent (e.g. R404a, R507), are restricted in use since January 2020. -
Sight glass with bubbles. Photo by WUR. Detection
Leakage of refrigerants are costly in material and service costs. Operational small leakages will be followed by increased malfunctioning of the equipment. Detection will be functional in early signal of leakage, next to frequently checks of leakages for the total refrigerating system. -
Liquid vessel containing freon. Photo by WUR. System content
Minimizing the refrigerant content in a system is one of the ways to limit operational costs and negatively environmental impact. A critical look on the principle (direct/indirect) and sizing of liquid lines and liquid receivers are helpful to reduce the content of coolant, while designing a refrigeration system.