Air in Hydronic Systems

Fundamentals to frontiers

Air in hydronic systems

By WILLIAM J . COAD excess of the the partial pressure of the various by Henry 's constituents of the air, the ratio be-

Problems resulting from air in hy- Law cann n solution; it tween maximum solubility and the dronic systems have plagued de- will leave and return to solubility state of the wateriair solu-

air binding of mains vices, to water-logged

condition. In a reservoir with a 60 F is the temperature a t s tandard atmo-

a term sphere, each cubic foot of water will nry's absorb approximately 0.02 cu ft of com- air from the standard atmosphere

r dif- (this could be called a 2 percent so-

been known an many years.

adjacent parts of the system with an - by volume at standard temperature air-water interface at some point; and pressure (STP) at various pres- the air can be in the gaseous phase sures on the abscissa and tempera- and "mixed" with the water in the tures on the ordinate. The state form of small or large bubbles; and point given above for the reservoir the air can exist in solution. In the condition is shown as Point A. last form (solution), there is a chem- Referring to Fig. 2, it can be seen ical limit as to how much air the

e is to decrease the temperature; other is to increase the pressure.

soluble gas and a liquid. the from approximately 2 10 percent. Thus, if an

be described as t he ma stability is a function of the time were available a t the amount of gas that the wa required for stability to be achieved. , each 100 gal of

On this page each month, sl7ure.s his engineering philosop ing a of topi ,fundarnenials io newj.konti t~ building enl~irotitnentul ~? .s t f . t? i~ . 1211. at the interface gas phase sta- C), ~ i h j ~ h would still be 3 percent Goad vice of Chrrrles J . '. bility is reached. For a given mass of above the initial condition in the ,Mc.Clurr & Associare.r und r$filiare profes-

of mechanical engi,,eei.ir7K at Waahhg- water, the time is a complex func- reservoir. If the temperature were ton Universitv, S t . Louis, WO. tion relating to Henry's constant, reduced to 50 F, the saturationpoint

HeatingIPipingiAir Conditioning. July 1980 53

would be approximately 1 1.5 per- cent (Point D), or 9.5 percent above the initial reservoir condition.

In a closed hydronic system, there are generally two areas of interface between the water and air. The known or design interface is the expansion tank. Other interfaces that could exist are those created by large or small air bubbles through- out the system.

The typical situation is where the initial water source is a municipal reservoir in which the water is at 60 F, at atmospheric pressure, and at equilibrium with the air (saturated). The water is pressurized by me- chanical means or fluid head to some pressure above 60 psig and is then introduced into a chilled water system where the "constant pres- sure" condition at the expansion tank is 60 psig and 50 F. This pro- cess is shown by the line A-B-D in Fig. 2. If there is an airlwater inter- face in the expansion tank, equilib- rium in the system will eventually be reached at Point D. Since there is fluid communication between the water in the expansion tank and that

in the rest of the system, and since the expansion tank by design is the known airlwater interface, the ex- pansion tank establishes the known solubility point of the system. The additional air, the difference be- tween 11.5 percent and the initial 2 percent, must come from the air volume in the tank. For every 100 gal of water in the system. the tank must provide 9.5 gal of air at STP, or approximately 2 gal at the 60 psig pressure.

This loss of air to the water was not taken into consideration in the expansion tank formulas presented in our May 1980 column since the dynamic nature of the mass transfer is a complex function of system de- sign. Some examples of system de- sign and mass transfer between water and air are:

In the initial air charge, approx- imately 21 percent of the air is oxy- gen and 79 percent is nitrogen. In its absorbed state, however, approxi- mately 35 percent is oxygen. In most systems, this oxygen will leave the water through a chemical process and combine with the met-

als of the system ( e .g . , Fe,rO,), creating a steady flow process be- tween the oxygen in the tank and the metal until all the oxygen has been removed from the tank.

If the pressure at any point in the system is lower than at the tank connection, a process represented by Line D-E in Fig. 2 reveals that some of the dissolved gases will re- turn to the gaseous phase. If this is a high point in the system (which it often-is) and the gas is purged, the effect is to continue to transfer the gas from the tank to the purge point until the tank is water-logged (re- gardless of the size of the tank).

The concepts presented here can be used in conjunction with Fig. 2 by the engineer to further his under- standing of how to design air-free hydronic systems. A future column will discuss specific examples and solutions. SZ

Author's note: Much of the information used to develop these materials is from pre- viously unpublished w,ork o f Professor Ferdinand Votru o f t he University of Rhode Island und Mr. Francesco Pornpei of Way- land, Mass.

54 HeatingIPipinglAir Conditioning, July 1980