Engineering Steam Systems -- Another Forgotten Skill
A look back at the days when steam heating systems were much more common.
This month, I will continue to discuss the forgotten skills of steam system design. Last month, I touched on several topics, including high- and low-pressure steam, boilers, deaerators, boiler feed receiver/pump sets, condensate receiver/pump sets, vacuum pumps, schedule 40 and schedule 80 piping, 100% outside air unit steam components, and steam humidification.
This month, I’ll focus on equipment that design engineers used to draw upon back when steam heating systems were more common, such as steam-to-hot water heat exchangers. These shell-in-tube heat exchangers transferred steam heat to hot water supplies that would serve baseboard finned tube radiation, convectors, and unit heaters as well as fan coil units and unit ventilators.
These steam-to-hot water heat exchangers would be used for domestic hot water storage tanks and air-handling systems that would often use hot water reheats in lieu of steam reheats. These reheats were used due to the water system’s flexibility to rise and drop the hot water supply and hot water return pipes. With steam supply lines, engineers needed to lay out the pipe distribution with the main line pitched in the direction of flow and then add an end-of-main condensate drip leg and steam trap to minimize the amount of steam condensate inside the steam pipe main.
An end-of-main drip leg dropped down 8-12 inches, and a steam trap was placed in the condensate return main approximately 8 inches below the steam main pitched, also in the direction of flow. To allow these two pipe mains in the ceiling cavity, a significant clearance was needed above the ceiling, which was something architects didn’t want to give up on the floor-to-ceiling height. Multiple steam traps above ceilings also created routine maintenance challenges.
Engineers in the past had to put a lot more thought into the steam system design because the pipe routing was more restrictive than hot water pipe distribution. Steam pipes, as a rule, were pitched in the direction of flow. That said, steam pipes could pitch up as well, allowing any steam condensate in the pipe to flow in the opposite direction, but the pipe sizing needed to be oversized to accommodate the counterflows. Another design criteria required steam supply and return branch runouts to be piped off at the top of the main, whereas the hot water supply and return branch runouts could exit from the top or bottom of the mains.
The design engineer would usually also specify a thicker pipe insulation for steam pipes at around 220°F steam and 200° condensate return temperature versus 180° hot water supply and 160° return temperatures. In addition, fiberglass insulation should not be used at temperatures above 212°, and, back in the 1960s, non-asbestos calcium silicate pipe insulation came along to replace the high-temperature, asbestos-laden insulation, but both were more costly to furnish and install when compared to fiberglass insulation.
Steam system engineering design had other challenges that included piping safety relief valves to the outdoors, placing them away from any one being burnt unexpectedly when a relief valve opened. Water relief valves often are piped to the floor in the vicinity of a floor drain.
Additionally, engineers didn’t have as many energy conservation opportunities in the past. With steam, the intent was to keep the temperature hot so as not to have condensation within the pipe. There was no compensated steam temperature or steam pressure, while hot water heating systems routinely provided a variable supply water temperature based on outside air temperature e.g., 180° supply at 0° outdoor air temperature and 120° when 60° outdoors.
At specific steam operating pressures, boiler plant operators had to have proper training and operator’s licenses, which was never an issue when working with conventional hot water boiler plants.
Today, steam systems still are being engineered but not to provide heat to buildings. More often than not, steam-engineered applications are now used for processes that bring the features and benefits of steam applications. As a result, many of the HVAC design engineers are not familiar and/or experienced with steam system applications, just like they are not familiar and/or experienced with refrigeration system applications (discussed in my November 2019 column).
While the challenges of refrigeration engineering have been rejuvenated in the HVAC industry through variable refrigerant flow (VRF) applications, there hasn’t been a resurgence in steam applications.