Steam based heating systems are similar to hot water heating, but they’re not quite the same. In a hot water system, liquid water enters, circulates, and returns to the furnace. This is not the case for a steam system. In steam, hot steam leaves the furnace, and water returns. This may seem like a minor difference, but it has massive implications to system efficiency.
The Science of States
When steam cools down, it becomes liquid water and falls down to the bottom of the heating system. On the one hand, this means less plumbing needed to capture and reheat the water. On the other hand, it means we have mixed-temperature fluids in the same space. The condensed water will absorb heat and hinder the operation of the heating system. It’ll cool down the radiators and the steam itself.
The big problem is that water takes a lot of energy to move from being liquid to being steam. It’s not a linear graph. When you heat the water, it’ll eventually rise to the boiling point, about 100 C at sea level, and then the temperature won’t actually increase. The molecules in the water will start to absorb the energy until it’s enough to breaks the hydrogen bonds between them and form steam.
As you can see in the chart, we continue to add energy into the system, and between states, the temperature rises, then we hit a limit. When its time to actually change state, all the energy that would cause the water to increase in temperature is used to break the hydrogen bonds holding the molecules together as a liquid. It’s hard to make an analog at a non-molecular scale, but maybe think of it as all the energy it takes to get your fingers unstuck when you accidentally glue them together. You will remain in that state (glued) until you put a lot of work into being un-stuck.
Preserving the Energy
As long as water and steam exist in the same system, we’re effectively driving down the highway, with one foot firmly slamming down on the brakes, while the other is firmly slammed on the gas. The medium we use to accelerate is a natural brake. In order to be effective, we need to get that liquid water out. This is where the steam trap enters the picture.
There are a number of ways to actually design a steam trap, but the basic principle is just about always the same: water falls and steam rises. All the condensation in our system will drip down. The solution the, is to have a bowl that collects all the water and immediately gets rid of it. We’re building a valve, a very specific valve.
One of the most basic designs is essentially a bowl, a float, and a valve. When the water level has risen to the point that no steam can reach the drain pipe, the float is able to rise and push a linkage which opens the valve in the drain pipe to let water flow out. The trap just has to get rid of the water, without wasting steam. Some steam loss is acceptable, just so long as we don’t lose more steam and energy than the liquid water would’ve taken away.
Just kidding. It’s more complicated than that. Liquid water isn’t our only enemy. We’re also fighting against other gasses. Gasses in the air, like oxygen and nitrogen don’t have the same thermal properties as water, and they don’t have better properties either. Every bit of air and other gasses in the system is going to cause us more trouble. In a large-scale system, we need to ensure we have as much steam and as little of anything else as possible.
More advanced installations and traps can remove air from the system without venting significant steam. Think of this like bleeding the brakes in a car or bleeding a radiator. We remove all the unwanted material that interferes in the system.
In heating applications, these gasses may pose a smaller issue, but consider steam’s bigger areas of operation: power generation and industrial equipment. Steam turbines are still one of the world’s biggest systems for power generation. Every bit of the system that isn’t steam, is going to have exponential impacts on power generation and efficiency. Ideally, you want your turbine to receive nothing but high pressure steam, with enough heat and force to crush a car like a tin can. If any other gasses get in, you’ll lose pressure because the air doesn’t behave like steam.
Keep It Efficient
If you’re using steam, make sure your system has the proper types and installations of traps. When you do your maintenance inspections, be sure to have the traps inspected to ensure they’re operating correctly. Like valves and all other pieces of actuating equipment, traps wear out over time or can become blocked by debris. Luckily, traps are also refurbishable and fixable, cutting down on your maintenance costs.