Page 24 - HVAC Pump Handbook

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Physical Data for HVAC System Design

Physical Data for HVAC System Design 21

As indicated in the table, the amount of air that can be dissolved in
water decreases with temperature and increases with system pres-
sure. This table demonstrates Henry’s law, which states that the
amount of air dissolved in water is proportional to the pressure of the
water system. This table should be used in place of similar charts for
open tanks and deaerators where the only pressure is atmospheric
pressure at 0 psig, and the amount of air dissolved in water approaches
zero at 212°F. It is evident from this table that makeup water that is
supplied by the domestic water system can contain a great amount of
air.
To demonstrate the release of air from water, assume that the re-
turn water has a temperature of 180°F and the system pressure is 40
psig. Makeup water entering the system at 50°F will have at least a
0.0836 ratio of air to water. It could have much more air than this,
since it may have been reduced from a higher city water pressure.
When the makeup water is heated to 180°F at 40 psig, the air content
will drop to a ratio of 0.0361, which is less than half that of the cold
makeup water.
An interesting and easy experiment to observe the release of air
when water is heated is as follows:
1. Take a frying pan and fill it with potable water from the kitchen
cold water faucet.
2. Place it on the stove, and heat the water to boiling.
3. Note that bubbles form as soon as the temperature begins to rise.
This is air coming out of solution with the water, since the water
cannot hold as much air with the higher temperature.
4. As the water approaches 212°F, the water begins to boil.
5. Allow the water to cool, and then reheat the water to boiling.
6. Note that this time bubbles do not appear until steam begins to
form. This demonstrates that the water has been deaerated during
the first boiling. It also provides a visual example of what happens
to cold water when it is heated in an HVAC water system.

As shown in Table 2.5, when water passes through pumps and the
pressure is increased, the water will increase its affinity for air. It is
therefore imperative that the air in the makeup water be removed
from the water as soon as it reaches system temperature by locating
the water makeup near the air-elimination equipment such as an
air separator. The optimal location for the air-elimination equipment
depends on the configuration of the water system. Generally, it may

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