Power boilers for HVAC applications

When does the situation call for a power boiler? Long-distance distribution, laundry or kitchen facilities, and absorption chillers requiring hot water service are just three examples where a power boiler may be preferred or even required. Read on for power boiler code info, and take a look at relevant tips pertaining to low NOx technologies, the Clean Air Act, permits, and current controls options.

The American Society of Mechanical Engineers (ASME) categorizes boilers as either heating boilers or power boilers. Many of us in the HVAC industry regularly deal with the former and may only occasionally work with the latter. The service restrictions for heating boilers are featured in Table 1.

Boilers that exceed these restrictions are termed power boilers and are widely used in industry and institutional applications such as hospitals, laboratories, college campuses, and prisons. These applications may involve long distance distribution that makes steam a practical heating medium or they may involve processes such as sterilization, laundry, or kitchen facilities that utilize steam. High temperature hot water is also used as an efficient means of transferring energy over long distances and can be used to drive the processes in equipment such as absorption chillers.

For whatever reason power boilers are employed, there are certain features that must be considered and incorporated into the design of these systems. These unique features of power boiler system design are a direct result of their higher operating temperatures and pressures. For instance, the pressure relief vent on a large, high-pressure steam boiler would not be terminated indoors as might be done on a smaller hot water heating boiler as the uncontrolled release of steam is a danger to operating personnel. Because of this danger, when two power boilers are connected to the same steam main, the steam connection from each boiler must be fitted with two stop valves with a free-blowing drain between them.

If one of the two boilers is down for its annual inspection, you cannot have the stop valve fail and bleed steam back into the open boiler. The first valve at the boiler would be a non-return valve installed at the outlet and the second valve would be an outside screw-and-yoke type. High temperature hot water boilers would also require two valves on the return connection to the boiler.