Handbook of air conditioning and refrigeration / Shan K

7.24 CHAPTER SEVEN
Penalties due to Presence of Air and Gas
The presence of air and gas in a water system causes the following penalties for a closed water system
with a plain closed expansion tank:
● Presence of air in the terminal and heat exchanger, which reduces the heat-transfer surface
● Corrosion due to the oxygen reacting with the pipes
● Waterlogging in plain closed expansion tanks
● Unstable system pressure
● Poor pump performance due to gas bubbles
● Noise problems
There are two sources of air and gas in a water system. One is the air-water interface in a plain
closed expansion tank or in an open expansion tank, and the other is the dissolved air in a city water
supply.
Oxidation and Waterlogging
Consider a chilled water system that uses a plain closed expansion tank without a diaphragm, as
shown in Fig. 7.8. This expansion tank is located in a basement, with a water pressure of 90 psig
(620 kPa�g) and a temperature of 60°F (15.6°C) at point A. At such a temperature and pressure, the
solubility of air in water is about 14.2 percent. The chilled water flows through the water pump, the
chiller, and the riser and is supplied to the upper-level terminals. During this transport process, part
of the oxygen dissolved in the water reacts with the steel pipes to form oxides and corrosion. At
upper-level point B, the water pressure is only 10 psig (69 kPa�g) at a chilled water temperature of
about 60°F (15.6°C). At this point, the solubility of air in water is only about 3.3 percent. The
difference in solubility between point A and B is 14.2 � 3.3� 10.9 percent. This portion of air,
containing a higher percentage of nitrogen because of the formation of oxides, is no longer
dissolved in the chilled water, but is released from the water and forms free air, gas bubbles, or
pockets. Some of the air pockets are vented through air vents at the terminals, or high points of the
water system. The chilled water returns to point A again and absorbs air from the air-water interface
in the plain closed expansion tank, creating an air solubility in water of about 14.2 percent. Of
course, the actual process is more complicated because of the formation of oxides and the presence
of water vapor.
Such a chilled water recirculating process causes the following problems:
● Oxidation occurs because of the reaction between dissolved oxygen and steel pipes, causing corrosion
during the chilled water transport and recirculating process.
● The air pockets vented at high levels originally come from the filled air in the plain closed expansion
tank; after a period of recirculation of the chilled water, part of the air charge is removed to
the upper levels and vented. The tank finally waterlogs and must be charged with compressed air
again. Waterlogging also results in an unstable system pressure because the amount of filled air in
the plain closed expansion tank does not remain constant. Oxidation and water logging also exist
in hot water systems, but the problems are not as pronounced as in a chilled water system.
Oxidation and waterlogging can be prevented or reduced by installing a diaphragm expansion
tank instead of a plain closed expansion tank. Air vents, either manual or automatic, should be
installed at the highest point of the water system and on coils and terminals at higher levels if a
water velocity of not less than 2 ft/s (0.61 m/s) is maintained in the pipes, in order to transport the
entrained air bubbles to these air vents.
In a closed chilled water system using a diaphragm expansion tank, there is no air-water interface
in the tank. The 3.3 percent of dissolved air, or about 2.6 percent of dissolved nitrogen, in