Plastic pipe use in glycol and chiller circuits.

Background

ABS pipe chilled fluid circuits are often required to operate at temperatures close to, and well below freezing. To achieve this, a closed circuit containing a mix of glycol and water, along with a secondary heat exchanger is used. The ratio of glycol to water is dependent on the temperature required. The lower the temperature that the system will be operating at, the higher the ration of glycol to water is required. If too little glycol is added to the circuit, ice crystals can form, which will block and potentially damage the internal heat exchanger in the chiller unit. Too much glycol in the system will not do any harm, however it will increase the set up and operational cost of the unit (as glycol is more expensive than water!). The freezing points of different ratios are given below;

% Glycol by volume	Freeze protection point (degrees C)
18	-7
29	-12
36	-18
42	-23
46	-29
50	-34
54	-40

Table 1 – Freeze protection ratios for glycol

In order to ensure sufficient margin for error, it is generally recommended that a temperature at least 3 degrees lower than that expected is selected from Table 1. Different manufacturers may have different percentage of glycol in their product; Table 1 assumes 100% glycol.

Plastic pipe choice

The plastic of choice for glycol and non-glycol chiller circuits is ABS. ABS pipe and fittings are rated down to an operational temperature of -40 degrees C (-40 degrees F) and unlike PVC pipe, retain an excellent impact strength at these temperatures. The installation of ABS pipe is much quicker, and easier than steel or other metal pipe systems, as there is a wide range of fittings. The ABS fittings are assembled to the pipe using a solvent cement which is applied with a brush.

System design

The design of ABS pipe systems for glycol circuits primarily follows that of other plastic pipe systems. The number of elbows and tees used should be minimised and where possible, long radius bends like our product 12686, should be used to minimise friction loss and reduce pumping requirements. Although when the correct percentage of glycol in a circuit is used, there should be no formation of ice crystals, it is good practice to install a strainer in the ABS return line, just prior to the chiller. This can also serve to catch any plastic swarf that may be left in the ABS pipe system after installation. A mesh of 0.5mm is recommended. The strainer should be installed with ball valves either side that permit cleaning without draining the complete circuit.

Advice

Should you require further information on ABS pipe system, or any other plastic pipe systems, please feel free to contact one of our sales engineers.