Hot Water Boilers - Avoiding Thermal Shock
I am often asked by engineering firms to help them on hot water system designs, especially regarding thermal shock. Unfortunately, often the help requested is after a problem has been realized. Thermal shock is a situation where the water in a boiler is suddenly displaced by water with substantially different temperatures. It causes rapid expansion or contraction of tubes, boiler plate, pipes, valves and fittings and is not good for the boiler or piping system. The end result will be leaking tubes, cracked pressure vessels, cracked cast iron sections, and a host of other problems.
To prevent problems I remind people that large industrial boilers are not household boilers that typically do start and stop the circulating pump. I suggest primary-secondary systems when possible. If the pump needs to be cycled, I recommend temperature sensors and control strategies to prevent it from sending water at drastically different temperature to the boiler. Slow start up can be done with small start up pumps, V.F.D. ramp up control or very slow control valves. I suggest starting the pump before the boiler, running the pump for some time after the boiler is shut off and always having flow when the unit is running. Use blend pumps to temper the return water when needed. Watch out for control or isolation valves, they need to be slow opening. You should hold the boiler in low fire for gradual warm up and use relays to force it to low fire before shutdown.

To prevent a surprise in your hot water system, contact a Rounds Company representative to discuss your control strategy. Often the best prevention is for us to sit down with the owner or engineer and the control contractor to work out a plan to make the boiler and the building work in harmony.

Glycol/Water Mixtures
Many systems now use glycol water solutions to protect coils and remote piping from potential freeze ups. Glycol reduces heat transfer so the design engineer should always check with the boiler supplier before sizing the boiler. Comfort boilers usually do not get down rated, but heavily loaded boilers for process heat often do. Cleaver-Brooks has specific suggestions such as minimum static pressure in the boiler, maximum operating temperatures and even approved glycol brands to use. Don't forget, glycol is expensive and often cannot be allowed down a municipal drain, so you may need a drain holding tank to allow for occasional boiler waterside inspection. The best advice is to contact us and we'll send you information on glycol water do's and don'ts.