Thursday, April 27, 2017

Condensate Drainage ... Why It’s Necessary in Industrial Steam Systems

condensate drain
Condensate drain
(Armstrong).
Abstracted with permission from Armstrong International

Condensate is the by-product of heat transfer in a steam system. It forms in the distribution system due to unavoidable radiation. It also forms in heating and process equipment as a result of desirable heat transfer from the steam to the substance heated. Once the steam has condensed and given up its valuable latent heat, the hot condensate must be removed immediately. Although the available heat in a pound of condensate is negligible as compared to a pound of steam, condensate is still valuable hot water and should be returned to the boiler.

The need to drain the distribution system.
Condensate Drainage
Figure 1: Condensate allowed to collect in pipes or tubes
is blown into waves by steam passing over it until it blocks
steam flow at point A. Condensate in area B causes a pressure
differential that allows steam pressure to push the slug
of condensate along like a battering ram.

Condensate lying in the bottom of steam lines can be the cause of one kind of water hammer. Steam traveling at up to 100 miles per hour makes “waves” as it passes over this condensate (Fig. 1). If enough condensate forms, high-speed steam pushes it along, creating a dangerous slug that grows larger and larger as it picks up liquid in front of it. Anything that changes the direction—pipe fittings, regulating valves, tees, elbows, blind flanges—can be destroyed. In addition to damage from this “battering ram,” high-velocity water may erode fittings by chipping away at metal surfaces.

The need to drain the heat transfer unit. 

Condensate Drainage
Figure 2: Coil half full of condensate can’t
work at full capacity.
When steam comes in contact with condensate cooled below the temperature of steam, it can produce another kind of water hammer known as thermal shock. Steam occupies a much greater volume than condensate, and when it collapses suddenly, it can send shock waves throughout the system. This form of water hammer can damage equipment, and it signals that condensate is not being drained from the system. Obviously, condensate in the heat transfer unit takes up space and reduces the physical size and capacity of the equipment. Removing it quickly keeps the unit full of steam (Fig. 2). As steam condenses, it forms a film of water on the inside of the heat exchanger. Non-condensable gases do not change into liquid and flow away by gravity. Instead, they accumulate as a thin film on the surface of the heat exchanger—along with dirt and scale. All are potential barriers to heat transfer (Fig. 3).

The need to remove air and CO2. 

Air is always present during equipment start-up and in the boiler feedwater. Feedwater may also contain dissolved carbonates, which release carbon dioxide gas. The steam velocity pushes the gases to the walls of the heat exchangers, where they may block heat transfer. This compounds the condensate drainage problem, because these gases must be removed along with the condensate.

Fig 3: Potential barriers to heat transfer: steam heat and temperature
 must penetrate these potential barriers to do their work.


For more information about any industrial steam or hot water system, contact Mead O'Brien by visiting www.meadobrien.com or call (800) 892-2769.