air eliminationC1.pdf

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1/4C1-1 AMTROL, Inc. 1977, Reprinted October 2003

Definitions, analysis and corrective procedures regarding the problem of

System Air in closed hot water and chilled water HVAC systems

Chapter OneSYSTEMAIR ITS COMPOSITION, CHANGES OF STATE AND

TRADITIONAL METHODS OF CONTROLLING

BASIC DEFINITIONS OF SYSTEMAIR AND ITS PHYSICALCHARACTERISTICS

To properly approach the problem of System Air and its solution,we must first understand and agree to what it is and what it doesin the closed piping system.

Traditionally, the term System Air had been widely used todesignate the presence of gaseous fluids in the closed pipingsystem.

It should be more properly defined as a gas mixture which changes

its physical form as well as its chemical composition during systemoperation and life.

The chemical composition of air is mainly oxygen and nitrogen witha small portion of other gases which vary according to localconditions. For purposes of our discussion, here, we will assumethat air is composed of 21% oxygen (including a small mixture ofother gases) and 79% inert nitrogen by volume.

Changes in Chemical Composition

When air is introduced into a closed piping system filled with water,it undergoes a major change in chemical balance. As air is carriedthroughout the system in a dissolved state (in solution), its oxygencontent combines with metallic surfaces to form oxides. Inaddition to causing corrosion, this portion of the air mass is no

longer present as air in the system. The remaining air mass is theinert gas nitrogen. In other words, there is a reduction of the airmass in a gas state to 78% of its original volume. This change inchemistry begins fairly quickly after air is introduced. It shouldbe noted that when in a gaseous state, oxygen will not readilycombine with metallic surfaces.

Changes in Physical Form

System Air can exist in the water-filled piping in three physicalforms:

1. Free Air Bubbles: In this state, free air bubbles locate or collectat the top of vertical and horizontal pipes as well as at the top ofsystem components such as boilers, terminal units and theexpansion tank. This occurs when high temperatures and low

pressures are present to allow the formation of air bubbles andwhen there is no system flow or in large diameter componentswhere extremely low velocities allow vertical travel of the free airbubbles.

Section C

Oxygen (21%) Forms Ferrous Oxide or Rust

Inert Nitrogen (78%)or "System Air"

Free Air Bubbles

The Gas % of AirNitrogen . . . . . . . . . . . . . . . . . . . . . . . .78

Oxygen . . . . . . . . . . . . . . . . . . . . . . . . .21

(plus other gases)


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2. Entrained Air Bubbles: When system flow is present andvelocities are in excess of 1.5 feet per second, the free air bubblesare not allowed to rise and collect, but are carried through thepiping system (whenever temperature and pressure conditions wilpermit the formation of free air bubbles entrained in the flowingfluid).

3. Air in Solution:At lower temperatures and higher pressures, air

becomes more soluble in water. Under these conditions, the air isabsorbed or dissolved in the water and carried in the solution.There are no bubbles present. It is much the same as the changeoccurring when sugar or salt is dissolved in a fluid. It is in this statethat air is highly corrosive.

Inefficient Operation = Increased Maintenance and EnergyConsumption

The predominant problem that occurs in piping systems afflicted

with System Air is inefficient operation. Free gas bubblesentrained in system flow can cause harmful cavitation of pumps.Gas bubbles with system fluids not flowing tend to rise and collectin terminal units and high points in the piping system. Many timesa pocket of air will cause a blockage that is not dislodged whenflow occurs. This restricts and even stops circulation of hot ochilled water to areas demanding space conditioning. Thisinefficient operation increases maintenance costs and energyconsumption.

Corrosion

The oxygen content of System Air when carried in solutioncombines with all metallic components of the system including thepiping. Deposits of rust also reduce efficient operation, increaseenergy consumption, and will result in increased maintenance and

eventual replacement of inoperative components.The Sources of System Air

Designers have traditionally held that there are two main sourcesof System Air in the closed hot water heating or chilled watersystem.

1. Air Absorbed During the Initial Fill of the System

It has been contended that as water fills the piping, air is trappedin the system and becomes absorbed in the system water.

2. Air Carried in Solution in Fresh Make-Up Water

The other major source of System Air held by tradition is thatmake-up water which is added to the system during operationcarries with it considerable amounts of air in the solution.

C1-2 AMTROL, Inc. 1977, Reprinted October 2003

Oxygen Forms Ferrous Oxide or Rust

Entrained Air Bubbles

Air in Solution

Blockage Caused by "Air Pockets" Can Restrict

or Stop Circulation of Water

1. Initial Fill of System and

Compression Tank

3. Trapped Air Absorbed as

Static Pressure Increases

2. Fresh

Water

Make-Up

The Symptoms of System Air in Hot Water Heating

and Chilled Water Piping Systems

Boiler

AirSeparator


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Plain Steel Expansion Tank Also Considereda Source of System Air.

Designers, in recent years, contend that the air cushion in the plainsteel tank also contributes air to the system.

This has been refuted by advocates of the plain steel tank as theyclaim air control procedures properly employed will not onlyresult in the escape of air to the system, but also will allow System

Air from initial fill and make-up water to be separated, collectedand directed to the air cushion in the tank removing it from systempiping.

Air Control vs. Air Elimination

Before a detailed examination of the characteristics of system aircan be properly made, we must understand the principles orphilosophies behind the two completely opposite practicescurrently used in combatting System Air.

Air Control / Plain Steel Tank

The use of the plain steel tank as the pressurizing agent for hotwater heating or chilled water systems dictates the necessity ofair control.

Since it is acknowledged that air is absorbed by system waterand carried in solution from the tank to other parts of the system,the advocates of the plain steel tank strongly recommendseparating and collecting free air bubbles at a point convenientto the expansion tank. Then, using special devices, direct thecollected air bubbles back to the air cushion of the tank tomaintain the proper air volume needed for system pressurization.Complete air elimination would remove the effective air volumefrom the system, hastening the loss of pressurization throughwaterlogging.

Air Control Based on Temperature Increase

The underlying philosophy in air control is that air, carried insolution, is brought out of solution as temperature increases. Asthe temperature increases from ambient to design, water loses its

capability to hold air in solution. The often quoted example of aglass of tap water forming air bubbles as it warms from cold taptemperature to room temperature is an example of this philosophy.

The familiar term, boiling out the air, is based on this philosophyof air being brought out of solution by temperature increase.

Air Control producers, then, dictate locating the separation andcollection point in the system where the hottest water is found.This is the supply side, or top, of the boiler. This is also, remember,at the one system point where it is most convenient to direct freeair from the separator back to the plain steel expansion tank whichis installed directly above the boiler.


Air Cushion at

MinimumOperatingPressure

Air Cushion atMaximum

OperatingPressure

Air CushionContributes

Air to SystemCausing Loss ofSystem Pressur

Special Device Used to

Direct Collected AirBubbles Back to Air Cushion

220 F

240 F

200 F

Location ofSeparation andCollection Point

in System WhereHottest Wateris Found

Boiler

Compression Tank

Air

Separator


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Air Elimination Requires a Captive Air Cushion

The basic difference between air control and air elimination isthat with a plain steel tank air cannot be eliminated from thesystem as this will hasten the waterlogging problem andsubsequent loss of vital system pressurization.

However, on the other hand, if the necessary air volume requiredto maintain system pressurization is sealed-in and not allowed

contact with system water during temperature and pressurechanges (as in a pre-pressurized diaphragm-type or bladder-typecompression tank), then all air in the system piping can beeliminated from the system without adversely affecting pressur-ization and accommodation of expanded water.

Furthermore, since it is not necessary, or advisable, to separateand collect free air at a point convenient to the tank (there is noneed to add air to the pressure cushion), then the separation andelimination point can be located in the one place where systemwater has the lowest capability to hold air in the solution. Thispoint is usually notat the boiler.

Pressure Decrease is Key to Air Release

A look at the solubility tables 12 and 13 (pages REF-13 and REF-14 in the Reference Supplement) for System Air or nitrogen

indicates that two changes in system operating conditions willower the capability of water to hold air in solution. Temperatureincrease....and pressure decrease.

The traditional glass of warm tap water illustrates the loweredcapability of water as its temperature rises.

If one could also visualize a bottle of soda water, also brought toroom temperature, one would see very few bubbles, until the capwas removed. Then the instant pressure decrease would cause amultitude of gas bubbles to instantly come out of solution.

This dramatically illustrates the greater affect pressure decreasehas in lowering the capability of water to hold air or gas insolution.

In fact, as we shall see in Chapter Two of this section, pressuredecrease and increase determine the rate and amount of air thatcomes out of solutionwhere it comes outand where it goesback in. While temperature does have an effectthe greaterimpact of pressure change often counteracts the effect oftemperature change and becomes the determining factor for thechange from air in solution to free air bubbles and back.


Point of Lowest Solubility

of Air in Water is at Top

of System

Boiler

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