Solar monotube boiler design
Solar monotube boiler design
(OP)
I am constructing a hybrid solar/ solid fuel generator system. An insulated water tank will evaporate R134a, in a coil(s) configuration,always below the boiling point of water open to the atmosphere for safetys sake. The vapor will drive one or more rotary vane a/c compressors modified as expanders then be condensed and returned to the tank. My questions are:
1) How do I control the feedpump to maintain the proper amount of fluid in the monotube coil(s) according to variable demands for power?
2) Is there a particular design feature of the coil needed to prevent slugging and facilitate evacuation of accumulated refrigerant oil in the coil(s)
Thanks
1) How do I control the feedpump to maintain the proper amount of fluid in the monotube coil(s) according to variable demands for power?
2) Is there a particular design feature of the coil needed to prevent slugging and facilitate evacuation of accumulated refrigerant oil in the coil(s)
Thanks





RE: Solar monotube boiler design
In steam power plant applications, a once thru steam generator is controlled to maintain a fixed outlet steam temperature, and the ratio of ( feedwater flow/ fuel firing rate) is perturbed to maintain a fixed outlet steam temperature. As a change in power output signal is recieved, both the firing rate FR and feedwater flow FW are adjusted proportionally. A +10% increase in power is addressed by a + 10% change in both firing rate and feedwater flow, and if the steam temperature departs from setpoint then the ratio of ( FW/FR) is perturbed to return the setpoint.
In the refrigerant case, it is not likely that final temperature is useful, so it is likely that one will need to use some other measurement to monitor the final property setpoint ( enthalpy). Perhaps a measurement of outlet conductivity, or indication of outlet density via gamma ray densitometer, or ratio of outlet flow element DP vs inlet feed liquid FE DP can yield a reliable indication of outlet dryness .
The indication of power input would likely be the temperature difference or LMTD between the bulk water bath minus the temperature of teh refrigerant at the midpoint of the coil. Thus FR= LMTD in this application. One would adjust the feedwater flow in direct proportion to the current LMTD, and adjust the ratio of ( FW/FR) if the outlet conductivity departed from setpoint. Also note the outlet setpoint property ( conductivy or density) will vary with outlet pressure.
RE: Solar monotube boiler design
RE: Solar monotube boiler design
If the unit is designed as a "once thru " unit using a coil and no downcomer or drum , then the prior message would hold true. The design of the coil would normally use a smaller diameter for the inlet half of the coil ( liquid pre-evaporator section") and a progressively larger diameter as the fluid boils to vapor. This use of a progressive diameter will reduce the geysering and chug flow instabilities.
RE: Solar monotube boiler design
Bob
PS: The boiler assembly interior is now together and was hydrostatic tested ysterday to 900psi. No leaks, yea.
RE: Solar monotube boiler design
The prior remarks about progressively increasing the diameter primarily applies to the 3 heated risers operating in parrellel ( parrellel channel instability)
RE: Solar monotube boiler design
As far as FW level, I am considering a cylindrical float in a tube (w/ opposing sight glasses) with light source/photodetector switch. That will minimize vias to the outside world and decrease leakage opportunities.
Thanks for your guidance
RE: Solar monotube boiler design
If the pressure increase for the fluid flowing down the downcomer ( at a CR of 1:1) does not exceed the pressure loss of the fluid flowing up the heated risers, then the system might dryout and not generate the vapor at teh expected rate. For your system, a good CR ratio might be 2:1 . If the system pressure drop calculations do not indicate a 2:1 CR , then you may need to modify the heated risers by increasing the outlet secttion diameter ( upper half of tubes).
RE: Solar monotube boiler design