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Patrick Lach, Maxi-Therm

When deciding on the distribution source for heating buildings in a campus or healthcare setting, many engineers will design a primary /secondary loop the buildings and mechanical rooms. The question is what to use as a primary heating source–hot water hydronic or steam?

Assuming the need for 35 MMBTU primary heating system, with a hydronic loop you will probably have a design on high temperature (HTHW) with a differential temperature around 75 deg F in order to reduce the flow and on sizing for the circulation pump.

If the system uses steam at 125 psig, there is no need for a primary circulation pump. Supply pipe diameter will be roughly equal to the hydronic system but the condensate return line will be much smaller.

There are two ways to design a steam network. One option is to design for high pressure steam distribution and low pressure steam supply in the building. The boiler will operate at high pressure in order to have smaller pipes to feed the buildings and mechanical rooms.  In the buildings, you will need a pressure reducing valve station and other equipment including steam safety relief valves, condensate return pumps with vents to the roof, vacuum breakers that add constant air and oxygen into the condensate piping that can lead to corrosion, requiring addition of amines as corrosion prevention. The list goes on and on!

Vertical Flooded Steam Heat Exchangers

Another approach involves the use of vertical flooded steam heat exchangers that allows the use of high pressure steam directly into the heat exchanger. This technology avoids the need for steam pressure reducing valves (1/3-2/3 station) and steam safety relief valves with vents to the roof. It does not require condensate pumping stations (including no flash tank, so no vents) nor a vacuum breaker in operation. This method is much simpler and can be more cost effective. Having no more vents and no more vacuum breaker in operation, the system does not assimilate oxygen anymore, which makes the condensate piping, up to 6 times less corrosive. An architect can also design a brand new building with no chimney and no vents by using a steam district energy loop in a city or a campus or any major existing boiler room nearby.

Yet there is more to it… When using vertical flooded steam heat exchangers, it will be 100% closed loop which also eliminates the need for a deaerator and outside vents, reducing make-up water and water treatment costs as well as very low surface blowdowns on the steam boilers. Plus, if you have a minimum average load of 8 MMBTU, you can consider adding a saturated steam engine directly connected to your main electric panel and generate a minimum of 100 kW. In southeast regions of the country, building heat may be required year-round to reheat the cool down air to dehumidify. In northern climates, you might consider adding steam absorption chillers in order to have year-round steam demand and enabling year-round electricity generation.

What about the boilers?

In my experience, steam boilers tend to be more robust and often industrial grade construction. In all cases, water treatment and monitoring chemistry are very important to maintain system life and longevity. Many operation professionals believe that hydronic boilers are more efficient on a combustion and annual fuel use efficiency basis. Even considering cold return temperatures to the boiler, at least with hydronic systems there are no blowdowns, no deaerators and limited steam vents and secondary condensate pump stations. A vertical flooded steam heat exchanger system addresses all those components at one time. Plus, if you bring colder condensate (feedwater) to a condensing stack economizer before entering the steam boilers, you will increase the total steam boiler efficiency.

Steam is safe and reliable

Well maintained steam systems with vertical flooded steam heat exchangers can provide multiple benefits in comparison to hydronic hot water loops. With 75%+ fewer moving parts and fewer installed components, there are operational, economic and maintenance considerations in favor of steam distribution systems. 

Article Originally Published on DistrictEnergy.org