or, It Depends …
This post is the last in a string of posts that started out as an e-mail answering a question from one of the folks taking the Existing Building Commissioning Workshop this year at the Pacific Energy Center. The question was about the energy content of a pound of steam, which seems like a simple question but it turned out not to be.
In the first post we explored different ways to address the question including using published conversion factors, rules of thumb, and steam charts and tables. In the second post, we took a closer look at how steam is procured, including on-site generation and district steam systems and how those approaches impact the amount of useful energy that is recovered from the steam. We also looked at ways to maximize the amount of energy that you extract from a pound of steam for use in your HVAC processes.
In this post, we will look at some common energy saving opportunities associated with steam systems. I should also mention that you will find a number of general resources about steam in this blog post.
Contents
I have included a table of contents that will allow you to jump to a topic of interest. The “Return to Contents” link at the end of each section will bring you back here.
Maintaining The Benefits
Even if set points and processes have been optimized, there are things that you should look for in order to maximize the benefits, no matter where your steam comes from and where the condensate goes. Typical issues (a.k.a EBCx and ongoing commissioning opportunities) include the following items.
Failed Condensate Return Pumps
Just because local boiler plants and campus district steam systems are set up to return their condensate and recycle it does not mean they are actually doing it. Condensate return pump failures are not unusual.
Typically, when this happens, the receiver drain valves are opened until repairs can be made. As a result, the condensate is dumped to the sewer, even though that would not be the case if the return pumps were operational. Unfortunately, the failed pumps and open drain valve are often forgotten.
A facilities director friend of mine at a large campus in the Midwest instituted a policy in his weekly meetings where each operator was required to report on the condition of the condensate return pumps in the facilities they were responsible for. “Not working” was the “wrong answer”, and the policy quickly resolved what had been an ongoing problem with failed condensate pumps, saving a lot of energy, water, and water treatment chemicals in at the boiler plant.
Failed Insulation
Condensate is hot and insulation will preserve the energy in the condensate. Repairing damaged insulation typically delivers a quick payback and can frequently be accomplished in house. All you need to do is measure the surface temperature with an infrared gun and look up the loss in a table or chart.
There are a number of resources at this link that will help you get started.
Steam Trap Failures
For a steam system to work properly, it is important to ensure that only condensate leaves the steam system. Steam traps accomplish this function but can fail if they are not properly monitored and maintained. If a trap fails, live steam enters the return system, wasting the energy it contains and potentially causing other issues on the return side.
The infrared thermometer shown above for checking out insulation savings will also help you find a failed stream trap. If there is a temperature drop across the trap with the leaving temperature being at or below the saturation temperature for the pressure in the return, then the trap is probably doing just fine, like this one.
But if the trap has failed, the temperature in the return line will be up near the saturation temperature of the steam, like this.
It is important to realize that the high temperature down stream of the trap means that a trap in the area has failed, not necessarily the trap you took the temperature across.
In other words, the steam leaking by from a failed trap will raise the temperature of all of the pipe in its vicinity. So to narrow things down, you may need to use an auto mechanic’s stethoscope to listen for the steam jetting through the outlet orifice in the trap.
There are resources at this link that can help you assess steam trap failures and the related savings.
Piping Failures Due to Corrosion
Condensate tends to be corrosive because the carbonate and bicarbonate ions that enter the boiler with the feedwater break down due to the heat and pressure in the boiler. One of the biproducts is carbon dioxide gas, which leaves the boiler with the steam and then reacts with the condensate to form carbonic acid.
There are water treatment strategies that can be used to control this as well as piping materials that can minimize the potential for failure. But my point here is that when a failure occurs, then the condensate is lost along with the benefits of returning it to the plant.
Long Pipe Runs to the Central Plant
As mentioned in the previous blog post under Paradoxes, long pipe runs to the central plant can result in parasitic losses, even if they are insulated. As a result, a number of campuses I have been involved with include a heat exchanger in the condensate return system that is used to recover energy from the condensate for local use, perhaps preheating domestic hot water or serving other loads that can be served by low temperature hot water.
Conclusion
Thus ends another string of somewhat long blog posts. Hopefully, they have given you some insights into how much energy is associated with a pound of condensed steam, techniques that can be used to evaluate it, and ways that you can maximize the potential and maintain the benefits of a system that uses steam as a source of heat.
David Sellers![PowerPoint-Generated-White_thumb2_th[2]](https://av8rdas.files.wordpress.com/2020/11/powerpoint-generated-white_thumb2_th2.jpg "PowerPoint-Generated-White_thumb2_th[2]")
Senior Engineer – Facility Dynamics Engineering Visit Our Commissioning Resources Website at http://www.av8rdas.com/
A Field Perspective on Engineering 
This was a wonderful series. Thank you.