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Modulating Valve for Low Pressure LPG/NG
allenb
After many hours of research over the last few years I?ve found only one manufacturer (Maxitrol) of compact and at the same time, reasonably priced modulating gas control valves for use with low pressure propane or natural gas (1? ? 11? WC).

One of their valves that have been used extensively in numerous commercial sample roasters and small to medium sized coffee roasters is the Maxitrol Selectra M420RH and M420H and comes in 3/8" or 1/2" pipe sizes.

I?ll give a quick description of how the valve operates.
The valve requires a variable input between 0-20 volts DC to modulate it from minimum to maximum output. From 0 to 5 vdc the valve operates in "low fire" mode which is a minimum firing rate user adjustable bypass port that only stays engaged up to 5 vdc. This is for making sure your burner never gets less gas flow than needed to keep it running at a stable burn. From 5 vdc to 15 vdc the valve leaves low fire mode and modulates like a normal proportional valve. As long as the input stays between 5 and 15 V the valve will stay in ?modulation? mode. If the input signal increases beyond 15 V then the valve jumps into "high fire" mode and will stay there as long as the input is between 15 and 20 volts. High fire mode is accomplished by the valve opening fully so the pressure you?ve got entering the valve is what?s leaving it. So the ?high fire? rate of flow is based on what pressure you?ve got your low pressure regulator set at which is typically between 11?-15? WC.

The typical commercial setup includes a Maxitrol SC11b control interface board which will accept either a 0-10 volt dc or 4-20 mA signal from a PID controller and outputs a corresponding 0-20 vdc to the M420 valve. So, if going the route of the typical commercial roaster setup, you?ll need a twin stage low pressure propane regulator, preferably with an adjustable output of between 11"-15? WC, a normally closed solenoid valve rated for propane/nat. gas (this is the safety shutoff), an interface control board (Maxitrol SC11b) and the modulating valve (Maxitrol Selectra M420H or M420RH) plus whatever burner ignition/safety control and PID controller you?ll be using to output to the SC11b.

If you?ll only be roasting via manual control (which I highly suggest for drum roasters), you have the option of coming up with your own source of a 0-20 vdc output to the M420 and will negate needing the SC11b interface board. From what I?ve seen, most people using this valve with a drum roaster are using it in manual mode with a panel mounted pot with knob to be able to manually control rate of rise. For fluidbed roasters, using a PID ramp/soak controller with the SC11b allows profile following automation.

I'll post photos of the components in the next post.

Edit Edit Edit:
7/8/2016
I was recently informed by Maxitrol that the Selectra series valves including the above mentioned M420 has a minimum flow rate of 5 cubic feet/hour which equates to 12,500 btu/hr or an equivalent power of 3600 watts. Due to this finding, there is a limit as to how small of a drum roaster this valve would be a fit for. Be sure to research what your requirement will be in relation to minimum btu/hr before considering the Maxitrol Selectra valves.

Allen
Edited by allenb on 07/08/2016 5:54 PM
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
allenb
M420
allenb attached the following image:
maxitrol_m420.jpg

1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
allenb
SC11b interface board
allenb attached the following image:
maxitrol_sc11b.jpg

1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
allenb
Twin stage low pressure propane regulator
allenb attached the following image:
two_stage_reg.jpg

1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
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