barometric leg

7/16/2019 Barometric Leg

http://slidepdf.com/reader/full/barometric-leg 1/3

Barometric Leg

A barometric leg is basically a condensate drain.

In a vacuum system that is used to condense steam and condensible vapors

through heat exchangers or condensers, the condensate is normally dropped into

a receiver tank that is often vented to atmosphere or a low pressure vent system.

This creates a situation where the condensate is under vacuum in the condenser

and it is trying to move toward a receiver tank that is under positive pressure. The

pressure difference is going the wrong way!

To overcome this pressure differential, the condenser must be located higher

than the receiver tank to allow enough static head pressure of the condensate to

exceed the pressure differential. The piping between the condenser and the

receiver tank is called the barometric leg.

Improper barometric leg design will reduce the performance of the condenser.

Since the condensate drains by gravity, the barometric leg must be high enough

to make sure the condensate does not enter the condenser and flood the lower

tubes. If the tubes flood, they will not be able to transfer heat effectively.

Be sure that the barometric leg extends into the receiver and is submerged

enough that atmospheric air/vent gases cannot be "pulled" into the piping. This

forms your very necessary seal. If you vent into a system under any pressure, the



pressure differential will increase requiring a taller barometric leg.

When you ask "how to calculate barometric leg", I assume you are asking how do you calculate the

height of the barometric leg. Consider a vacuum system where you are attempting to generate and

maintain nearly a complete vacuum (in Imperial Units, 1 atmosphere = 14.7 psia; complete vacuum = -

14.7 psig.) Typically, you would use steam jets or eductors or ejectors (I've heard them called all three)

to generate your vacuum. You'd be condensing the steam and condensible vapors in heat exchange

equipment using either direct contact (Barometric Condensers) or indirect contact (Surface Condensers).



With either, you produce liquid water, perhaps with other condensibles, at the pressure at your jet's

discharge nozzle. For simplicity's sake, let's just say it could be as low as the system pressure or -14.7

psig.

After the Condenser, you normally want to drop down into a Receiver (horizontal vessel) which is often

vented to atmosphere or to a low pressure vent system. So you have condensed water in the Condenser

at -14.7 psig and you have the destination (Receiver) at zero psig. The pressure difference goes the

WRONG way. How are you going to get water at lower pressure to flow into the Receiver at higher

pressure?

The answer is very simple. Just locate the Condenser a suitable distance above the Receiver, and the

water will flow downhill against the pressure difference. The calculation of what constitutes a "suitable

distance" is what you call "calculate barometric leg", since the piping between the Condenser and the

Receiver is called the barometric leg.

The difference in elevation between the Condenser and the Receiver must be such that the static head

of the water in the barometric leg exceeds the pressure difference. Since water at standard conditions

(60F or 20C) exerts a force of about 2.31 psi per foot, you would need an elevation difference of

14.7*2.31 = nearly 34 ft (10 m) to overcome the pressure differential. Divide this number by the specific

gravity of the condensate at your expected temperature to arrive at a more accurate estimate. In

designing your system, I would add a couple of feet to allow for some hydraulic loss in the barometric

leg and the vent pipe. Be sure that the barometric leg extends into the Receiver and is submerged

enough that atmospheric air/vent gases cannot be "pulled" in the piping. This forms your very necessary

seal. If you vent into a system under any pressure, the situation is more complicated as your pressure

differential will increase.

barometric leg

Documents