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38 FEBRUARY 2014 | FUEL OIL NEWS | www.fueloilnews.com T he following discussion is based on information collected from chemical companies, equipment manufacturers, hydronic system design manu- als and research performed by the Hydronics Institute, Inc. It represents a synthesis of the best infor- mation available at the time of publication. The intent of the author is to provide guidelines to help licensed contractors and engineers in designing, servicing and maintaining hydronic systems that use glycol based anti-freeze. EthylEnE Glycol or ProPylEnE Glycol Both ethylene and propylene glycol possess many characteristics that make them ideal for use in heat trans- fer systems where protection from freezing is required. Desirable properties include high boiling points, low freezing points, stability over a wide range of tempera- tures and high specific heats and thermal conductivities. Furthermore, used with an appropriate inhibitor, glycols demonstrate a non-corrosivity that may substantially pro- long system life. Ethylene glycol based solutions work well in most anti- freeze applications because of their excellent heat transfer efficiency. The low viscosity of ethylene glycol allows sys- tems to operate at lower minimum temperatures and is more energy efficient due to its reduced pumping require- ments. The primary drawback to ethylene glycol is that it is listed as a "toxic chemical" under SARA, Title III, Section 313, due to its acute oral toxicity. Inhibited propylene glycol should be used for freeze pro- tection where direct contact with foodstuffs or incidental contact with drinking water may occur. Although propylene glycol is generally recognized as safe by the FDA, it is not intended for human consumption. FrEEzE ProtEction The glycol mixture design concentration must be deter- mined with due regard to the minimum temperature that the system is expected to encounter. It behooves the designer to thoroughly evaluate the application environ- ment in order to both guarantee adequate freeze protection, and to avoid using overly concentrated solutions; which add expense and reduce system efficiency. Generally, the design concentration should be targeted for the range of 20% to 50% glycol by volume. Normally, with proper expansion volume available, a concentration of 15% to 20% will provide bursting protection. Solution concentrations much over 50% become proportionally less thermally efficient and less cost effective. Table 1 shows the expected freezing point as a function of concentration. Glycol ProPErtiEs A comparison of propylene glycol and ethylene glycol is shown in Table 2 below: HVAC/ HYDRONICS By Larry ashton, P.E., rayPak Concentration by volume Ethylene Glycol Propylene Glycol 55% -50F -40F 50% -37F -28F 40% -14F -13F 30% +2F +4F 20% +15F +17F Table 1. Freezing Point anti-Freeze in hydronic systems A comprehensive guide to hydronics with glycol-based anti-freeze Ethylene Glycol Propylene Glycol Heat transfer @180F with no increase in flow rate 20% solution .96 .97 50% solution .87 .90 Flow Rate Correction Required (with no change in pump curve) 100F 116% 140F 115% 180F 114% 110% Pump Head Correction Required (with increase in flow) 100F 149% 140F 132% 180F 123% 123% Specific Gravity @ STP 1.125 -1.135 1.045 -1.055 Pounds/Gallon @ 60 9.42 8.77 pH (of glycol concentrate) 9.3 9.5 Note: Except as indicated, comparisons are of 50% glycol solution to water. Table 2. Glycol Properties