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Working With It - Selecting

Loop Fluids
Fluid Types

Water
Glycols & Alcohol
Salts

Water

Water is the least expensive circulating loop fluid. Its main disadvantages are its relatively high freezing point of 32 degrees, and that it expands upon freezing, rupturing pipes.

It has a density of 8.334 pounds per gallon at 60 degrees and a specific heat, for practical purposes, of 1.000 Btu per pound per degree. At 40 degrees its viscosity is 1.550 centipoise.

The refrigerant temperature in the evaporator runs about 10 degrees lower than the exit water temperature. Antifreeze protection is required when there is a possibility that the minimum operating evaporator refrigerant temperature will drop to or below 32 degrees.

The coldest expected evaporator exit water temperature during heating operation determines the required fluid freezing point. The earth coil fluid freezing point should be about 10F below that exit temperature. This freezing point then determines the solute concentration required to protect the heat pump evaporator from freezing.

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Glycols & Alcohols

Glycols and alcohols are available as concentrated liquids, salts as granular or powdered solids, and potassium acetate as a 50 percent solution. The brine is mixed with water, on- or off-site, prior to being charged into the piping system.

The solutions of alcohols in water - methanol and ethanol waters - have both good and bad properties for circulating fluid use. Methanol water has a low frictional pressure drop and a relatively high heat transfer coefficient. However, it is highly volatile, highly flammable, and highly toxic.

Ethanol waters are less toxic and less flammable than methanol waters, but are still highly flammable. They also are more viscous, and have a larger pressure drop and lower heat transfer. Corrosion inhibitors can be added to make them non-corrosive. Ethanol is more expensive than methanol but less expensive than the ethylene glycol solutions.

Ethylene glycol in water is a common and often favorable alternative. It offers low corrosivity, volatility and flammability. Properly prepared and inhibited, it is relatively non-corrosive. It is the most common antifreeze for automotive use.

The main disadvantages of ethylene glycol are:

  • They are somewhat toxic and must be stored and handled with care.
  • The viscosity becomes high at low temperatures.
  • They have a definite life and the pH should be checked yearly; and spent solution replaced.

Propylene glycol solution is an alternative with the only true advantage of being less toxic than ethylene glycol. Below about a 20F freezing point, the viscosity gets excessively high, resulting in high pumping costs.

Typical Properties of Several Antifreeze Solutions - 15 F Freezing Point

Antifreeze Solution

Weight % of Solution

Lbs per

Gal per

Pounds per gallon

lbm-F
(pounds mass)

Viscosity, centipoise

100 gal pure water

Ethylene Glycol

20.0

209.0

24.20

8.63

0.92

2.7

Propylene Glycol

23.5

256.0

29.90

8.55

0.96

6.3

Methanol

13.6

131.6

19.41

8.18

1.01

3.3

Calcium Chloride

14.3

139.2

NA

9.47

0.795

3.0

Sodium Chloride

13.3

127.9

NA

9.21

0.851

2.6

The quality of the water used with the glycols is important. It must be "soft" and have a low concentration of chloride and sulfate ions. Factory-inhibited ethylene and propylene glycol solutions are sold by manufacturers.

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Salt

Salt solutions of calcium and sodium are commonly used as antifreeze solutions, typically in industrial applications. Sodium chloride (table salt) in water has a low toxicity and is not flammable.

Both salt brines are very corrosive and require inhibiting and periodic checking and maintenance, even in closed loops such as earth coils.

Corrosion is best controlled by minimizing contact with air. Strangely, dilute solutions tend to be more corrosive than concentrated solutions. The ph must be maintained between 7.5 and 8.0 to minimize corrosion.

Sodium dichromate inhibitor is generally considered the most economic and effective means of combating corrosion with salt brines. It comes as orange crystals than are readily dissolved in warm water. It dissolves very slowly in cold water and should never be put in a brine tank in crystal form.

The dichromate quantities recommended by ASHRAE for long-term protection are:

  • 1.25 pounds of sodium dichromate per 75 gallons of calcium chloride brine
  • 2.0 pounds of sodium dichromate per 75 gallons of sodium chloride brine.

Addition of sodium dichromate makes the resultant brine acidic, whereas a slightly basic solution is desired. Before adding this inhibitor, determine the pH. If the untreated brine is acidic or neutral, add caustic soda to raise the pH to 8.0.

If the pH is 7.5 to 8.0, add 0.27 pounds of 76% commercial caustic soda for each pound of sodium dichromate added. The soda must be thoroughly dissolved in warm water before adding to the brine.

If a salt brine is used, several considerations should be followed:

  • During system startup, be sure to purge or remove all air.
  • All air traps must be designed out. Do not use air vents as they eventually leak.
  • Do not use any yellow brass components in pumps, fittings or valves. Red brass is satisfactory.
  • Avoid any alloys with a high (over 15%) zinc content. Substitute 300 or 400 series stainless steel.
  • Copper-nickel heat exchangers are more likely to survive the salt brine environment.
  • Dissimilar metal connections should be avoided, such as cast iron to copper. An electrical insulator such as plastic should be used.

If the system is drained, flush thoroughly with clean water at once. Corrosion pits can develop in a few hours.

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