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Everything I've read on the Rankine cycle indicates that the feedwater pump is necessary for the cycle to work. However the steam cycle used in traditional household boiler / radiator systems seems to challenge this.

The cycle for the household boiler seems to be a closed loop cycle with just a heat source (boiler) and cold source (radiators). But those two pieces are sufficient to have steam flow through the radiators, condense, and then return to the boiler.

  • Is my understanding of the boiler / radiator cycle correct?
  • If so, and if the system was fitted with a turbine, could it produce any amount of power?
  • If it produced any amount of power, how inefficient would it be compared to a Rankine cycle?
  • Is there a name for the type of cycle in a boiler / radiator system?
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  • $\begingroup$ The water temperature through radiators is usually below or close to 60 deg C, so why do you think household radiators have steam in them? $\endgroup$
    – Solar Mike
    Commented Jul 16, 2019 at 8:49
  • $\begingroup$ There are hot water systems but there are also steam systems. They are not as common for residential use any more but are still found in some old homes and in commercial buildings. From what I've read they operate below 15psi and circulate steam throughout a building without feedwater pumps. There are a bunch of references online but here's a wiki page: en.m.wikipedia.org/wiki/Radiator_(heating)#Steam $\endgroup$
    – remitnotpaucity
    Commented Jul 16, 2019 at 12:30
  • $\begingroup$ I understood "household" as 1 or two storeys, not a skyscraper... $\endgroup$
    – Solar Mike
    Commented Jul 16, 2019 at 12:33
  • $\begingroup$ As I said, some older homes do have them. Cast iron radiators are a good sign a house is heated with steam $\endgroup$
    – remitnotpaucity
    Commented Jul 16, 2019 at 12:35
  • $\begingroup$ My "goto" book is Engineering Thermodynamics Work & Heat Transfer by Rogers & Mayhew (3rd ed) and steam starts on page 204... $\endgroup$
    – Solar Mike
    Commented Jul 16, 2019 at 12:40

3 Answers 3

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Is my understanding of the boiler / radiator cycle correct?

Boiler radiator cycles that are driven by free convection certainly exist, but most heating systems use one or more heating pumps to circulate the water (not steam). But the fundamental difference is that a rankine cycle extracts mechanical work from the steam, a heating system doesnt.

If so, and if the system was fitted with a turbine, could it produce any amount of power?

No. The boiler in a steam engine is the part of the system at the highest pressure. Feed water will not flow into it without a pump.

Is there a name for the type of cycle in a boiler / radiator system?

Thermodynamic cycles refer to cycles that extract work. A boiler/radiator system is no thermodynamic cycle in this sense.

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From my understanding, feedwater lines normally refer to a secondary fluid that is being heated by "extra" thermal energy in a cycle (see modified Rankine cycles). The boiler heats the water (normally keeping it in a liquid state in most modern residential applications) and either stores it in a tank or directly circulates it. This is the key difference between indirect and direct fired boiler systems (https://www.energy.gov/energysaver/heat-and-cool/water-heating/tankless-coil-and-indirect-water-heaters). Turbines come in several different forms. A Pelton wheel is a type of turbine that operates with liquids only based on the movement of the liquid. Turbines in power gen applications operate with a superheated gas, which is probably not found even in steam heating systems. This fluid must be of high pressure and temperature to keep the steam quality above 1 for most of the expansion process. Thus, a turbine on a conventional hydronic heating system would not produce a lot of power, if any. It would probably damage a conventional turbine via liquid water ingestion. There are some micro-combined heat and power systems that use the Rankine cycle to produce heat and electricity from the same heat source (boiler), but they are not common. Even then, the circulating fluid (the domestic hot water) is not the working fluid in the turbine.

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Your heating system is not a Rankine cycle. In fact it's not considered a thermodynamic cycle at all because all it does is transfer heat. If you add a steam turbine you could possibly produce a tiny amount of mechanical power, but it would be very limited because the temperature and pressure are so low.

But if you do that you'll quickly discover the problem - steam will be leaving the system in order to power the turbine. That means you'll run out of water, so you have to condense the steam and pump the water back in, or vent it to the atmosphere and pump new water in. In either case, you'll need the feedwater pump.

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