Engineering Steam Systems -- Another Forgotten Skill
Once an industry standard, steam system engineering has evolved into a lost art.
Last month, I wrote about refrigeration design engineering being a lost skill. This month, I’m going to discuss another lost engineering skill: steam systems. When I started out at an HVAC consulting engineering firm back in the 1960s, our services were primarily focused on hospital design engineering. Back then, steam system engineering was standard for those building applications.
In the 1960s, fuel oil was inexpensive, and, quite often, No. 6 oil required a steam heating coil in the underground fuel oil storage tanks to keep it fluid, as it would solidify without this heat. Hospital boiler operators often required a license to operate the high-pressure (125 psig) steam boilers that were also a standard application. The purpose of the high-pressure steam was to serve the steam sterilizers and central sterile equipment washers that preferred 80 psig, while the building’s heating systems were low-pressure (15 psig) steam and, for the most part, used steam to hot water shell-in-tube heat exchangers.
These requirements for high- and low-pressure steam demand were accommodated with the use of pressure-reducing valve (PRV) station(s), so the high-pressure steam could be reduced to low pressure, usually via a 2-PRV station. A rule of thumb was to avoid reducing the steam at an 8:1 turndown because a single PRV dropping the pressure from 120 psig to 15 psig would create a high-velocity noise, which was a disadvantage of trying to stay with a single PRV station. A better selection would be a PRV reducing to 60 psig and a second PRV would occur to reduce the pressure from 60 to 15 psig.
With all boilers, especially high-pressure boilers, air control from the city water makeup and air imbedded within the steam condensate return was a critical design engineering issue that needed to be addressed. Minimizing air within the boiler, as well as the steam supply pipe, was essential to extending the useful service life of the equipment. Deaerator units, rather than boiler feed tanks, were usually provided to remove air from the water/condensate makeup to these boilers. Steam system condensate return from terminal equipment, e.g., steam-to-hot water heat exchanger, steam coils, etc., often required condensate receivers and pump sets to return steam condensate back to the boiler feed tank when condensate return could not make its way back by pitching the return pipe(s) down with gravity doing the work. Condensate could also be returned to the boiler room using a vacuum return pump in lieu of gravity return.
The pipe material specified by the design engineer was usually schedule 40 black iron steel for the steam supply and schedule 80 steel with a thicker pipe wall because condensate return combined condensed steam with air that found its way into the return system, making for a more corrosive environment on the inner pipe wall.
With steam systems, it was common that air-handling units in a hospital were 100% outdoor air systems back in the 1950s and 1960s. To heat this outdoor air, the designer would sometimes start with a one-row series of vertical pipe manifolds supplied at the top with steam supply flowing down in this snow-melting coil, draining out via a steam trap at the bottom. This single-row coil would be located upstream of the air filters to melt snow and thus prevent the filters from getting covered with snow, clogging the filter media and restricting supply airflow.
Downstream of the filters would be a second steam coil with a face and bypass damper. This allowed the steam to fill the coil, prevented the coil from overheating the supply air, and allowed the damper to modulate to a bypass position to bend the cold air with the heated air, resulting in a mixed-air temperature around 55°F. Within the air-handling unit was a cooling coil. In the summer, when outdoor air was humid, the cooling coil would dehumidify the supply air temperature, requiring a steam reheating coil(s) to raise this temperature to satisfy the space thermostat(s), signaling for warmer air to the area served.
Steam humidifiers were also an integral feature of central air-handling systems in hospitals. Humidification has become an even more important and broader-based topic today. Refer to Dr. Stephanie Taylor’s columns found at the beginning of each issue of Engineered Systems for more on IAQ and human comfort. More on steam next month.