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Section 13: Air Separation and Removal

The vast majority of hydronic heating systems are designed to operate without air in the piping. Thus, it’s essen-

tial to provide the means of separating air from the water when the system is Àlled (e.g., purging) and preservingthis deareated state throughout the life of the system.

Air in hydronic systems can lead to the following problems:

• Noises in the piping and heat emitters• Inadequate circulator performance

• Inadequate heat output from the heat emitters• Accelerated corrosion due to oxygen in contact with ferrous metals

• Circulator noise or failure due to improper lubrication• Poor performance of balancing valves

• Complete loss of Áow and heat output due to large air pockets

Air exists in three forms within hydronic systems:

• Stationary air pockets

• Entrained air bubbles

• Gases dissolved within water

A well-planned air separation strategy must address all three forms.

Stationary Air Pockets:

Stationary air pockets are created from air that’s not expelled when the system is Àlled and purged. Since air islighter than water, it migrates to high points in the system. Such points are not necessarily just at the top of the

piping circuit. They can form at the top of heat emitters, even those located low in the building. Air pockets canalso form in horizontal piping that eventually turns downward or piping routed up, over, down and around obsta-

cles.

Stationary air pockets can also form as air bubbles not removed during system purging merge together and mi-

grate toward high points. This is especially likely in components with low Áow velocities, where slow-moving Áuidcannot push or drag the air along with it.

Proper purging at startup can usually dislodge stationary air pockets to the extent that the circulator can maintain

Áow in the system. Further removal of air pockets can be done through a combination of air vents at system highpoints and microbubble absorption.

Entrained Air:

A moving Áuid may be able to carry air bubbles along (entrain them) through a hydronic piping system. This canbe beneÀcial when the entrainment carries the air bubble from remote parts of the system back to a central air-

separating device, which can then capture and expel the air.

However, if the Áuid’s Áow velocity through the air-separating device is too high, the entrained air cannot be ef-Àciently separated and could end up recirculating through the system many times. The ability of a Áuid to entrainair is based largely on its velocity. A minimum Áow velocity of 2 feet per second is needed to entrain air bubbles 

within downward-Áowing pipes.

Microbubbles:

Water has the ability to absorb the gas molecules that make up air. These molecules are interspersed with water

molecules, and thus are said to be dissolved into the water. The cooler the water, and the greater the pressure onthe water, the more dissolved gases it can contain. Conversely, the hotter the water, and the lower its pressure,

the less dissolved gases it can contain.

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Pressure units

The psig (pound per square inch gauge) pressure scale is often used to measure pressure in pumpsystems. It is a relative measurement (relative to atmosphere) where 0 psig is equal to the pressure levelin the local environment.

The psia (pound per square inch absolute) allows the measurement of pressures that are lower thanatmospheric pressure. Zero psia is the lowest pressure possible and corresponds to a perfect vacuum.

The inch of mercury is often used as a unit to measure atmospheric pressure. A glass tube with the topend sealed is filled with mercury, the bottom end is resting in a bowl of mercury. Atmospheric pressure

acts on the fluid surface of the bowl changing the height of the mercury level in the tube as the pressurein the environment changes. The height of mercury corresponding to atmospheric pressure at sea level isapproximately 30 inches and this corresponds to 14.7 psia or 0 psig. The inch of mercury scale is alsoused to measure low pressures or pressure below atmospheric pressure.