Steam is used in many industrial processes. However, you will typically produce steam in a different part of the plant from the processes where you use it. That means you need to move steam to benefit from its heat energy.
In this article, we will explore how you distribute steam, and the benefits of moving steam compared to low-temperature hot water (LTHW).
Why do you need to move steam?
For safety and maintenance reasons, steam is generated some distance from where it is needed. While steam1 is produced in a boiler house (A), the processes (B) that use steam’s heat energy could be hundreds of metres away.
If you have multiple processes that require heat energy, steam has to reach them wherever they are on site. Steam pipes (C) will distribute the steam from the boiler house to the processes.
For example, a food manufacturer’s processes that rely on steam for heat energy may include sterilising, pasteurising, cooking, drying and dehydration. Each process may take place in a different part of the plant.
How do you distribute steam?
Steam is a gas, so it moves under its own energy across a drop in pressure. As a result, when you produce steam at high pressure in the boiler house (A) and then lower the pressure at the processes (B), where heat transfer2 occurs, steam will naturally flow through the pipes from the boiler house to the processes. Unlike LTHW, you do not pump steam.
In addition to steam pipes, you also need separate pipes (D) to return liquid condensate from the process back to the boiler house. As steam transfers its useful heat energy to the process by condensing, the liquid condensate must be removed because it cannot give up any more heat energy.
The pressure / volume relationship of steam
As well as enabling the distribution of steam, pressure also impacts the specific volume of steam, because steam is a compressible gas.
Pressurising steam in the boiler reduces its volume. However, when you reduce the pressure at the process, the volume increases. By understanding the pressure/volume relationship, you can calculate the impact of changes in pressure on the volume of steam.
As the volume of steam reduces under pressure, this significantly impacts the sizing of the pipes required to carry steam.
How does pressure affect steam pipe sizing?
Choosing the correct size of steam pipes allows you to distribute the necessary mass of steam as efficiently as possible. By compressing steam to reduce its volume, you need smaller diameter pipes to move steam from the boiler house to the process.
For example, you would need pipes with a DN100 diameter to distribute 1,000kg/h of dry saturated steam at 2 bar g. However, increasing the pressure to 10 bar g compresses the steam into a lower volume, so you would only need DN50 pipes to distribute the 1,000kg/h of steam.
How do pipe sizes differ between steam and LTHW?
Both LTHW and steam require pipes for distribution. However, they use different sizes because steam is a gas and LTHW is a liquid.
LTHW cannot be compressed and will always occupy the same space. As steam is compressible, it takes up a smaller volume when produced under high pressure in the boiler house. Also, due to the higher heat content of steam, you need a larger mass of LTHW to produce the same heating effect as with steam.
These reasons mean the pipes taking steam from the boiler house to the process can be a smaller diameter than the equivalent pipes for LTHW.
In addition, water remains a liquid during the heat transfer process, so it needs the same diameter pipes from the boiler to the process and when it returns. With steam, the return pipes only need to carry condensate to the boiler house, so they will be smaller than for LTHW.
For example, to distribute 5MW of energy using LTHW would require 200mm pipes from the boiler house to the process and back again. However, distributing steam, which can provide the equivalent heat energy at 7 bar g, would only need 150mm pipes for the steam and 80mm for the condensate.
Why are smaller diameter pipes beneficial?
Moving steam in smaller diameter pipes has several advantages over liquid alternatives like LTHW.
More efficient use of space
Using smaller diameter pipes for steam and condensate allows you to use space more efficiently on your site. The pipes occupy less space and are easier to fit into plant room voids and other areas where the pipes are laid.
Lighter pipes
As smaller diameter pipes are lighter, they require fewer brackets and supports to secure them.
Lower installation costs
Installing smaller diameter steam pipes reduces the installation costs compared to larger diameter pipes. In addition to the pipes themselves and the supporting infrastructure, less insulation is necessary, and the labour cost for installation is reduced.
Less heat loss through radiation
As steam releases heat when it condenses on contact with a lower temperature surface, heat is lost through pipes due to radiation. While insulation reduces this loss, the size of pipes also affects heat loss.
Larger diameter pipework provides a larger surface area for steam to condense against and release its thermal energy. As a result, reducing the diameter of the pipes limits the surface area through which heat can be lost.
How much energy is needed to produce and distribute steam?
Generating steam requires more energy than generating LTHW, because you produce steam at higher temperatures and pressures. However, distributing steam consumes a lot less energy than LTHW.
LTHW needs energy to power the pumps that distribute it around the plant. Steam does not need pumps though, because it is a gas and flows naturally with a drop in pressure.
For example, heat losses from pipes to distribute 2MW of energy using LTHW at 70-90℃ would be 9.6kW. Heat losses to distribute the equivalent energy with steam would be 14.6kW. However, pumping the LTHW would use 6.5kW of electricity, while steam has a much lower energy requirement of 0.7kW to power a pump to bring water into the boiler house.
Across both generation and distribution, the total energy consumption for steam and LTHW is similar.
The advantages of distributing steam
Moving steam around a plant has several benefits over alternative sources of thermal energy. As a compressible gas, steam takes up less volume and is distributed through smaller diameter pipes, which is more space and energy-efficient. Unlike LTHW, steam is not pumped and so consumes less energy during distribution. Steam only needs a drop in pressure to move through the steam pipes.
Learn more about steam
1 What is steam? - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/what-is-steam
2 Heat transfer - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/heat-transfer