The principles governing the design of air separators are simple. One basic
principle is reducing the water velocity to 1 to 2 ft/sec or less. Water will not carry
along free air bubbles at that velocity. In boilers with large internal water passages,
water velocity is low.
Free air carried to the boiler by the system water, along with any
additional air released by heating, will readily rise to the top of the boiler. All of
this free air can then be piped from the collection point to the compression tank.
To prevent free air collected at the top of the boiler from being
circulated out into the system and into radiation, boiler dip tubes are used. Some boiler
manufacturers offer these dip tubes as standard equipment. B&G offers a wide range of
sizes of top outlet boiler fittings for boilers that are not so equipped. Use of this type
device is normally limited to Air Control Systems. Sizing depends upon the flow rate and
the size of the boiler tapping. 
However, a boiler is not always available, usable or desirable as the
point of air separation. In these cases, free air must be separated from system water by
other means. Other devices designed to operate on the basis of low velocity are available.
Two such devices are IAFs (Inline Airtrol Fittings) and IASs (Inline
Airtrol Separators). See Figures 1 and 2.
The IAF type separator is available with nozzle sizes of 2- , 3- or 4-inch
pipe, and the IAS type
is available with connections of 1- through 4-inch pipe. Because proper operation is a
function of velocity, sizing is determined by the flow rate, not the pipe size.
Another principle used in the design of the
air separators is centrifugal force. Tangential nozzles are used to create a vortex at the
center of a cylindrical vessel. Air, being lighter than water, collects in the whirlpool
on an air collector screen and is then directed upward. The air either returns to the compression tank in
an air control system or, in the case of the air elimination system, is vented to the
atmosphere. A big advantage of this concept is that the tank size required is much smaller
than that required for a low velocity type separator. B&G's version of this device is
called a Rolairtrol.
Standard pipe size nozzles range from 2 inches
through 24 inches and are provided with or without a strainer. Since efficiency of
separation is a function of velocity, here also, sizing depends upon the flow rate.
A third style of separator uses the principle
of coalescence. The coalescing medium in this case, stainless steel wires, attracts the
smallest (micro) bubbles on the wires' surface. These micro bubbles combine, making larger
bubbles which are much easier to separate from the water. This type of separator is
especially suitable for radiant panel and other applications where even small amounts of
air are troublesome. The range of sizes (1 through 2- inch pipe) limit their use to
smaller systems. Sizing is again a function of the flow rate. The flows associated with
pipe sizes in typical HVAC applications are normally not a problem for this type of
separator. 
Depending upon the type of air management
being used, the air separated by the coalescing medium can be directed to a standard type
tank or vented to the atmosphere by a replaceable vent. Bell & Gossett's version of
this type of separator is called an Enhanced Air Separator.
Any of these devices will do an adequate job
of separating free air from water. However, the overriding truth is whatever type is
chosen, air separators are a necessity for proper air management in a hydronic heating
system. |
Article
Morton Grove
Tel 847 966-3700
The Role of Air Separators in a Hydronic System - Part 2