Lesson 8 - Refrigeration Technology

Secondary refrigerants are used to transfer the refrigeration from the primary refrigeration to the point of use in the brewery. They are chosen for their ability to effectively transfer heat (or should we instead say cold?!).

Unlike with primary refrigerants, we are not working with phase changes. When it comes to heat transfer capability, we require the secondary refrigerant to have a high thermal conductivity and a high specific heat capacity. As it is pumped around the brewery, a low viscosity will minimise the energy required to do this. Furthermore, secondary refrigerants should have a low freezing point.

Other considerations for choosing a secondary refrigerant are:

• Toxicity – as secondary refrigerants are often just one thin heat exchanger plate away from your prized product, it is important that they are non-toxic (just in case leaks occur).

• It should be non-corrosive – heat exchanger plates and gaskets should not be affected.

The most commonly used secondary refrigerant in brewing is a mixture of propylene glycol and water often just called 'glycol'. (Be careful not to confuse this with ethylene glycol, which is toxic.)

Water is a near perfect secondary refrigerant, but with a freezing point of 0°C, this doesn’t cater for many of the cooling temperatures required in a brewery. However, if we add another substance to water to depress its freezing point, we then have the basis of most secondary refrigerants. These brine (salt and water) refrigerants use inexpensive calcium, sodium, and potassium chlorides to depress the freezing point – but they also have the disadvantage of being corrosive to metal pipework and plant.

Now that we have reviewed the refrigerants, let’s look at refrigeration circuits.