Learn about the thermostatic radiator trap to understand how it works as well as where and why we use them in steam heating systems along with worked examples.
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| Radiator thermostatic siphon |
They are thermostatic traps and are installed in 2-pipe steam heating systems. But why do we need them and how do they work? That is what we are going to cover in this article that is kindly sponsored by State Supply. Visit www.statesupply.com to view the many types of steam traps available, purchase parts and accessories, or speak to a steam systems specialist about your specific needs. Just click HERE for more information.
Scroll down to see the YouTube tutorial.
What is a thermostatic steam trap?
Thermostatic radiator vent looks like this, although there are many variations.
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| Radiator thermostatic siphon |
These valves are attached to the underside of a steam radiator. It is a mechanical valve that allows air and water to pass through, but works automatically to prevent the passage of steam. We will see how this happens a little later in this article.
Steam heating systems can be found in residential, commercial, and even industrial sites. They are very common on large campuses and large old buildings.
These systems do not require pumps, they use the steam itself to distribute the heat throughout the building. Although we can find a condensate pump in the return line. So why do we need a thermostatic siphon in our radiators? To answer this, we must first understand how the steam system works.
Steam System Basics
If we add thermal energy, or heat, to water at standard atmospheric pressure, its temperature rises to 100°C or 212°F. At this point, it begins to boil and evaporate into steam. The thermal energy is carried by the steam.
If we capture and contain the steam by placing an ill-fitting lid on the container, we would see the lid lift off. If we were to stick the lid to the container well, we would see the internal pressure increase.
This is because the water molecules expand and take up more space. In cold water, the molecules are packed very closely together, but as heat energy is added, the water molecules become excited and vibrate rapidly, increasing their volume. It increases so much that a unit of water can expand into steam, about 1,600 times its original volume, at standard pressure
If the volume of the container is fixed and more heat energy is added, the water molecules will become excited and move faster. They will collide with the sides of the ship more often and with greater force. This increases the pressure inside the container.
The pressure pushes the steam, naturally it tries to reach a place of lower pressure. We can use this buoyant force to distribute heat energy through the pipes, to the radiators, and then to the boiler.
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| 2 pipe heating system |
In a typical 2-pipe steam heating system. The boiler adds thermal energy and heats the water which turns it into steam. The pressure pushes the steam along the pipe, towards a radiator.
The radiator heats the surrounding air in the room, so heat energy is transferred from the steam, through the wall of the radiator, and into the air in the room. As the air warms it rises and cooler air takes its place, this is repeated continuously.
The vapor gives up its thermal energy, in doing so it condenses into a liquid. The high pressure in the system will push this water back into the boiler, where it will be reheated and repeat the cycle.
We only want the condensed liquid to return to the boiler. We don't want steam getting into the return line. This would be a waste of energy, since it would heat the condensing liquid and also lose heat on the way back. We pay to create the steam, so we don't want to waste it.
The mixing of the steam with the condensate will also cause many problems for the system, such as binding and steam blow-off, which can be catastrophic for the system, so we must avoid this. One way to do this is to use a thermostatic radiator valve.
How Radiator Thermostatic Air Vents (Bellows) Work
There are different designs of thermostatic steam traps, but in this article we will focus on the type of bellows.
When we look at the valve we have the main body with an inlet and an outlet. At the top there is a hex bolt head that allows us to disassemble the unit for maintenance and repair. We will discuss this later in the article.
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| bellows design |
When we look inside, we find a set of bellows. These push a plug into the valve seat just before the outlet. The bellows are attached to the top of the trap.
The bellow is a sealed unit
that contains a liquid which vaporises at, or close to, the boiling point of
water. The liquid inside the bellow is typically water or a water and alcohol
mixture. We use this, because, as we know, water expands rapidly as it is
heated. So, we can use this to control the valve automatically.
The bellows is a sealed unit that contains a liquid that vaporizes at or near the boiling point of water. The liquid inside the bellows is usually water or a mixture of water and alcohol. We use it because, as we know, water expands rapidly when heated. So we can use it to control the valve automatically.
The bellows consists of a metal body with metal folds or undulations, which can expand or contract as the internal pressure of the expanding liquid increases or decreases.
Steam exists above 100°C or 212°F, which is above the boiling point of water. So when the steam comes in contact with the bellows, the water inside the bottom turns to steam almost instantly, causing the volume to expand rapidly. This causes the bellows to expand and we can use it to block the outlet of the valve.
When the system is started, the bellows is open and the siphon is full of condensate. When steam enters the trap, it heats up the bellows.
The water in the bellows heats up and evaporates. The pressure inside the bellows increases. Since the bellows is attached to the top of the trap body, it can only expand downwards, thus blocking the outlet and preventing steam from flowing into the condensate line.
This gives the steam in the radiator a chance to give off its heat into the room and cool down until it condenses into a liquid. Liquid condensate fills the body of the trap and draws heat away from the bellows. This allows the pressure within the bellows to drop and the bellows to retract to its original length and position, releasing the plug and opening the valve. The steam then drives out the condensate and the trap closes and repeats the cycle.
trap glitch
During operation, the bellows are constantly expanding and contracting, and like all things mechanical, they will eventually fail.
In a typical building, with, say, 1000 heating hours per season. The trap can open and close approximately three times per minute. Thus, in a single heating season, the trap can open and close approximately 180,000 times per year. After 5 or 6 years, the trap may have opened and closed more than a million times.
That's a lot of wear on the thin mechanical bellows. Simple metal fatigue will destroy the trap after a few years. It's like bending a piece of metal back and forth on the same axis, it will eventually break.
Also, the corrosive nature of the condensate will weaken the bellows. Typically, a steam trap only has a useful life of, say, three to five years.
Thermostatic radiator vents won't stop running slowly. When they fail, it's instantaneous and without warning. There is no slow deterioration of function, so the bellows should be replaced as part of routine maintenance according to the manufacturer's recommendations.
When the bellows ruptures, the water inside escapes and the bellows can no longer react to changes in temperature. When the bellows fail, the trap can fail in the open or closed position, depending on the type of construction.
open failure
With the rupture open, when the bellows break, it lets steam through. This happens because the default length of the bellows is not long enough to close the valve. The bellows is filled with water and alcohol, this expands and forces the bellows to expand beyond its predetermined position. So when the bellows ruptures, it returns to its predetermined length and the valve stays open, allowing steam to flow, wasting energy and causing system problems. This can be difficult to detect because the steam is still circulating so it will continue to heat up the radiator.
bug closed
The other type of failure is the closed failure. When the bellows ruptures, the bellows falls and jams the valve, effectively shutting it off. When the valve is built, the bellows hangs free and blocks the outlet. Water is placed inside and a vacuum is formed to draw the air out of the interior, causing the bellows to lift and compress. When steam enters, the bellows expand and block the outlet. If the bellows breaks, it falls and blocks the exit. The radiator will eventually stop producing heat, so you may notice that the valve has failed.
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