Are you experiencing pH and conductivity swings in your condensate? Does it sound like a rock band is playing in your steam lines? Are you feeding excessive amounts of steamline treatment to achieve your targeted pH? Have high iron readings in your condensate and steam line leaks? This might be caused by sub-cooled condensate, a common issue in condensate systems, especially when you have large load variations.
Detrimental Effects of Subcooling
If your system has sub-cooled condensate it will lead to the items listed below:
- Low pH condensate which increases corrosion
- Increased equipment failures
- Temperature hunting
- Lost energy
- High steamline treatment usage
To understand what is happening we need a quick thermodynamics lesson in saturation.
We define the saturation temperature as the temperature at which water boils at a given pressure. For each pressure, there exists one corresponding temperature. For example, a pan of water, at sea level, will boil at 212° F. The water cannot get any hotter than 212° F. The more heat the more steam, but the temperature does not raise about 212° F. If the water in the pan started out as 90° F, we would call the water “sub-cooled” as by definition the water’s temperature is below saturation temperature.
Heat Exchangers and Saturation
Now let’s look at saturation from the other end of the steam system–out in the plant where the steam is used. Below is a picture of a shell and tube heat exchanger. Assume that the shell side fluid is saturated steam and the tube side fluid is process water. Steam enters the top shell side connection, and the process water enters and leaves the tube side via the threaded connections on the left end of the exchanger. As the process water absorbs heat from the steam via heat transfer through the tubes, the steam cools in the shell and a portion of it changes state from steam to water. We call the process of changing state condensation resulting in water or condensate. This condensate then exits via the “Condensate Out” connection as a saturated liquid.
The temperature of the steam and the temperature of the condensate out, in theory, should show identical temperatures and it would correspond to the saturation temperature based on the pressure in the shell. Remember for each pressure there is corresponding temperature.
What happens when heat exchangers stall?
Steam heat exchanger systems can experience stall conditions which are normally caused by condensate draining problems. The drainage issues might be steam traps that are too small, too large, improperly installed steam traps, using steam to return condensate, failed traps, and many other reasons. When a heat exchanger stalls, condensate is in the space that normally is used by steam.
- This can cause heat control hunting which is when the heat transfer is reduced as the exchanger has water in it instead of steam, so the valve opens to increase temperature and pressure to remove the water. The steam space is cleared out and filled with steam again so the temperature increases and the process fluid overheats so the valve closes, and the cycle happens over again with uneven heating temperatures.
- This cycling can cause temperature shock to the heat exchanger with expansion and contraction. Plate and frame heat exchangers are very susceptible to this flexing of plates causing cracks.
- Steam collapse can also happen under low loads which forms a vacuum. The vacuum breaker opens allowing atmospheric air and the condensate to mix which sub-cools the condensate and increase the gasses in the condensate. More about this below.
- A back flooded heat exchanger will sub-cool the condensate as it has time to sit in the exchanger. As the condensate sits it will dissolve gasses that are found in the steam. Oxygen and carbon dioxide are some of the gasses that readily dissolve and are corrosive. Boiler feedwater contains carbonates and/or bi-carbonates which, when heated, volatilize with the steam. This CO2 is a non- condensable so it remains in the steam space as the steam condenses to condensate. If the condensate is allowed to cool below the saturation temperature the CO2 will dissolve rapidly to form Carbonic Acid. The Carbonic Acid causes the lower pH swings and the higher conductivity in the condensate.
Ways to Detect Subcooling
The easy way to detect is to test your condensate’s pH and conductivity regularly. Do you have large swings in the tests? Testing the temperature of the condensate at a condensate receiver can also help to track down locations that you are experiencing subcooled condensate. Trying to test at a steam trap can be difficult and may be dangerous with flashing steam. You may need to have a steam trap survey conducted to determine where you have subcooling. A survey will examine steam traps; function, size, installation, and type. If you are not able to do this on your own you can ask Watertech of America about performing a steam trap survey.
How to Reduce Condensate Subcooling
Eliminating all sub-cooling in the condensate is not realistic but you can reduce the issues that cause it.
- Make sure condensate receivers are sized properly and functioning properly.
- Using steam to push condensate back to the feedwater tank may cause stall.
- Install temperature gauges on the in and out of your heat exchangers.
- Confirm all vacuum breakers are functioning.
- Confirm all condensate receivers are vented properly.
Troubleshooting condensate issues can be very difficult especially when the steam loads in the plant are not operating at a steady state. Corrosive conditions can occur quickly with load variations reducing the service life of your equipment. Reviewing that your steam traps are properly designed as well as the heat exchanger surface area will greatly decrease the amount of sub-cooling. It is common to oversize steam traps as they are sized on the maximum steam load. Having a properly sized steam system will increase the safety, production, and reliability of your steam system.
Do you have questions about your steam system?
If you have questions about your system reach out to Watertech for a Site Survey and one of our technical engineers will be in touch.
Rob Pierick | Senior Territory Manager
Rob Pierick has worked for Watertech of America for 17 years and has over 20 years’ experience with industrial water treatment, including water testing and Legionella testing and remediation. Rob is a Certified Water Technologist (CWT) through the Association of Water Technologies. The CWT designation involves rigorous testing, peer review, and an ethics declaration. It must be renewed every five years to maintain this designation. Rob holds a BS from the University of Wisconsin Platteville.