Industrial refrigeration: propylene glycol is not a luxury…
Food industry has been experiencing a bull market during last years. It is no wonder that food technologies evolve by leaps and bounds and the achievements of yesterday become just a history today. It is impossible to imagine how our great grandmas and grandpas managed to live without fridges, after all it is the integral part of any enterprise.
Systems with an intermediate cooling medium (sometimes it’s called ‘intermediate heat medium’) are used at large-scale enterprises with vast amount of remote refrigeration users (meat factories, milk factories, breweries and etc.) and hypermarkets with centralized cold supply. An intermediate cooling medium participates in the process of heat exchanging by extraction of heat from a cooled object and transporting this heat to the refrigeration unit that directly produces artificial cold using a cooling medium (for example, ammonia or freon).
Usage of an intermediate cooling medium gives the following possibilities:
- to reduce substantially the amount of a cooling medium in a system;
- to avoid possible contact of a cooling medium with food products;
- to localize possible accidents with leakage of a cooling medium into a machine room;
- to simplify regulation of temperature at cooled objects;
- to simplify operation of a refrigerating system;
- to put the system into commission and increase the amount of refrigeration users on a step-by-step basis.
Here is the list of substances most commonly used as an intermediate cooling medium
Water is cheap, accessible, non-toxic and has good thermal-physical properties. It allows to come into contact with the generality of food products. The main disadvantage of water as an intermediate cooling medium is the restricted usage range — not less than +1оС. At lower temperatures water crystallizes into ice that creates inner pressure and can break equipment and communication lines.
Ethanol is not so extensively used because of high evaporability, inflammability and it is also hard to get at the Ukrainian market in last years. It is well-known that sometimes ethanol is of the great interest for personnel of enterprises. Unpleased with this fact management stuff in some cases ends up with rejection to use this substance in order to avoid such problems.
Brines — solutions of inorganic salts (NaCl, CaCl2, MgCl2) are commonly used at food enterprises, are very cost effective taking into account direct expenses. However they have high corrosion activity, which may lead to significantly higher indirect expenses because of equipment failures exceeding the direct ones. CaCl2 is a nutritional supplement (Е509), so it is quite safe. Although, introduction of such solution into a food product will make it bitter by the flavour. Generally, it leads to spoiling of cooled products.
Glycerin — a colourless sweetish liquid without odour. Environmentally safe, it is a nutritional supplement (Е422). Because of its viscosity it is used in cosmetic, medical and food industries, it is also used in other fields as a viscosifier. Using glycerin solutions as cooling media, it is necessary to take into account that their viscosity indexes increase greatly at medium and especially low temperatures. This leads to growth of load on equipment and reduces its life. Installation of an additional pump for circulation of a cooling medium may be required in some cases.
Water glycol solutions — solutions of ethylene glycol and propylene glycol — can ensure higher operational reliability at moderate low temperatures during long periods of time. Ethylene glycol is commonly used as the base of antifreezing agents for cars (a cooling fluid). A lot of manufacturers of heat exchanging equipment (HVAC systems) recommend to use it. But ethylene glycol is toxic and poisonous, that is why it is hard to use it in food processing industry. Since the end of the last century, USA, Germany, France and other developed countries started to refuse from usage of ethylene glycol under living conditions. Even vehicle industry began transition to so called environmentally friendly antifreezes. And now this is time to set our eyes on the other substance of the same class — propylene glycol, which combines all advantages of ethylene glycol and has none of its disadvantages. Nowadays propylene glycol is commonly used in pharmaceutical, food, tobacco industries, in production of cosmetic products, plastics, polyester resins... In the former USSR propylene glycol was produced scarcely, which explains why so few of the vast amount of manufacturers are familiar with it.
So, propylene glycol is a colourless thick liquid, almost without odor. Good for mixing with water and spirit, has hygroscopic properties. Its density at 200С is 1.035-1.037 g/cm3, boiling point under atmospheric pressure — 184-1890С. There are a lot of data in science literature on the account of the propylene glycol freezing temperature, for example −600С but it is practically proven that pure propylene glycol becomes very thick and stagnant under relatively low negative temperatures. That’s why its water solutions, freezing temperature of which depends on propylene glycol concentration, used as cooling media.
Cooling media on the base of propylene glycol have a set of indisputable advantages, which became the reason of high rates of usage of this cooling media at food productions for last 10-15 years, abroad at first and now in Ukraine. Based on the experience of usage and characteristics of propylene glycol heat media optimal temperature range for usage is up to 330С.
Propylene glycol is a nutritional supplement (Е1520), which is important in food production. So the question on environmental safety is solved automatically. Propylene glycol has almost no taste, colour and odour, which should be taken into account when evaluating the level of safety of a cooling medium of a secondary circuit for manufactured products, in case of their accidental contact. Food enterprise technologists should consider this factor, because the cases of selling of salty ice-cream and bitter mineral water are known.
Fire safety question is resolved in quite a positive way too: solutions of propylene glycol with concentrations that ensure specified temperatures are non-inflammable.
Now let us look at corrosion activity of a cooling media on the basis of propylene glycol
The substance itself is slightly aggressive to metals. It is possible to reduce corrosion effect almost to zero using specially treated water (distilled or demineralized) and substances that are corrosion inhibitors for different metals. Propylene glycol is a surface active substance and it facilitates to dissolve and wash out deposits from a system.
In addition, it is worth to notice the pretty long operational life of propylene glycol cooling media. Only periodical inspections may be needed to conduct after passing of expiration date, not immediate change of a cooling medium.
Considering tendencies of the market of cooling media for food productions, «Stron» enterprise started to use propylene glycol and its solutions since 2002. In 2005 the enterprise developed and put into production the cooling media on the basis of propylene glycol under TEPRO (TEPRO-P since 2009) trademark, which are recommended for usage in food industry.
TEPRO-P cooling media are environmentally safe, non-toxic, non-inflammable and have minimal corrosion activity!
Physical and chemical characteristics of TEPRO-P product line are specified at here.
TEPRO-P cooling media are tested in certified laboratories and provided with the Safety and Health Certificate of the Ministry of Health of Ukraine that allows to use them in food industry.
The enterprise has flexible approach to every client, which allows to make selection and production by individual orders with account of specific details of each application (freezing temperature, colour, equipment configuration, production dates, delivery, selection of instruments, additional laboratory researches and others).