Controlling Heat Input Into a Gas Fired Boiler

[JCraigR]

I would like to open a discussion about controlling heat input into a gas fired boiler. My reflux has 4’ X 2 ½ “ column using ss pot scrubbers as packing media. I can generally distill to a maximum of 93% to 94% purity. From the literature that I have read that diameter column should be able to handle a maximum of 5000 watts (I generally run at just over 2000 watts for a spirit run and 3000 watts for a stripping run). The problem is in determination of the heat input to my 15 gal keg from a propane burner. I welcome any critique or suggestion in my method in determining this heat input.

I control the heat input by measuring the discharge temperature of the condenser cooling water to get an estimate of the heat input to the boiler. The scheme that I use is as follows:

◦ As the boiler is heating up and producing no vapor/steam, I establish a water flow rate through the condenser which is measure by timing the time it takes to fill a 500 ml graduated cylinder. I then convert it to L/min (example 500 ml in 35 sec = .5L/35 sec * 60 sec/min = .875 L/min) and plug that number into the “Flow” variable in my HP 48SX Scientific calculator. Also at this time, I measure the outlet water temperature of the condenser and because there is no heat input into the condenser the inlet temperature (Ti) is equal to the outlet temperature. I plug this temperature into the “Ti” variable in my calculator.

◦ When boiling starts, I cut back on the boiler heat input and from experience I shoot for a condenser outlet temperature say around 55 C. After the outlet temperature has stabilized, I enter this temperature into my calculator and run the program to determine the heat input to my condenser which should closely approximate the heat input to the boiler. While on total reflux, I adjust the boiler heat input to the desired Watts.

◦ I have an Omega digital thermometer with dual inputs. With one of the thermocouples I am able to monitor the head temperature; the other I use to monitor the condenser outlet temperature. Because the thermocouples are of a fine gage, I was able to make pin hole, about chest high, in the clear vinyl tubing (5/16” ID) and insert the thermocouple directly into the condenser outlet water flow stream. This arrangement helps me set and monitor the cooling water flow rate. I adjust the flow to a point where the thermocouple pin hole just quits sucking air into the cooling water flow stream. If flow increases the pin hole leaks water; if it decreases it sucks air.

The calculator is programmed with the following equation:

Q(watts)=Mr×Cp×(Ti-To)

Ti = Condenser Inlet Temperature °C
To = Condenser Outlet Temperature °C
Mr = Condenser H2O Mass Flow Rate Kg/sec (see Note)
Cp = 4200 (Watts · sec)/(Kg · °C) – Specific Heat of H2O @ approximately 50 °C

The program for the HP 48SX calculator is:

Set up the variables “Watt”, “Ti”, and “Flow”
Key in < < Ti-Flow*4200*60 / > > and store it in “Watt”
Store the inlet temperature in “Ti”
Store the flow rate in “Flow”
Enter the outlet temperature into the display
Press the Watt’s key to run the program
The watts should display

The program for the HP 35S calculator is:
STO T = Ti , STO F = Mr

W001 LBL W
W002 RCL T
W003 -
W004 RCL F
W005 x
W006 4200
W007 x
W008 60
W009 ÷
W010 STOP
W011 GTO W001

With Ti = 21, Mr = .875 and the Outlet Temperature entered as 55
the Program should calculate the Watts to be 2082.5

Note: The flow, even though measured as a volumetric rate, closely equates to a mass flow rate at the temperature ranges involved (1 Liter H2O ˜ 1 Kg H2O)

[Saltbush Bill]

From the literature that I have read that diameter column should be able to handle a maximum of 5000 watts (I generally run at just over 2000 watts for a spirit run and 3000 watts for a stripping run).

Im not a numbers freak like some, mostly go by experience of what works what doesn't.
From past experience I'm going to say 5000W through a 2.5 inch reflux is way to much.....at a guess Id say 3500W max, even that might be stretching it.

I control the heat input by measuring the discharge temperature of the condenser cooling water to get an estimate of the heat input to the boiler. The scheme that I use is as follows:

◦ As the boiler is heating up and producing no vapor/steam, I establish a water flow rate through the condenser which is measure by timing the time it takes to fill a 500 ml graduated cylinder. I then convert it to L/min (example 500 ml in 35 sec = .5L/35 sec * 60 sec/min = .875 L/min) and plug that number into the “Flow” variable in my HP 48SX Scientific calculator. Also at this time, I measure the outlet water temperature of the condenser and because there is no heat input into the condenser the inlet temperature (Ti) is equal to the outlet temperature. I plug this temperature into the “Ti” variable in my calculator.

◦ When boiling starts, I cut back on the boiler heat input and from experience I shoot for a condenser outlet temperature say around 55 C. After the outlet temperature has stabilized, I enter this temperature into my calculator and run the program to determine the heat input to my condenser which should closely approximate the heat input to the boiler. While on total reflux, I adjust the boiler heat input to the desired Watts.

I see no mention of what type of reflux still you are using , LM ,VM, CM, CCVM, that information will help......a photo would be even better.
When running a reflux .....unless its a CM its best to run the reflux coil water as hot as you can and still have the reflux condencer knocking down all of the vapour. Cooling the refluxed spirt more than necessary is just making the still work harder to do its job.
Most people that I know stick a finger under the the water outlet to see how hot? cold it is.
You sure your not over thinking this a real lot ?

[JCraigR]

Sure I am over thinking this - that is my nature and part of the fun. However, I can get back to a repeatable heat input. I agree 5000 way to much and closer to 3500 watts. One of my first runs I overpowered the column, (what a disaster - lots of cleaning) thus my desire to determine the heat input. My still is liquid management system - one output valve with a reservoir overflow back to the column. Good input on running it hotter - I'll try it.

Thanks

[JCraigR]

Note: The flow, even though measured as a volumetric rate, closely equates to a mass flow rate at the temperature ranges involved (1 Liter H2O ˜ 1 Kg H2O)

Alternatively, to using volumetric flow rate to determine mass flow rate for Mr, a true mass flow rate can be determined by timing the time it takes to measure 1 Kg on kitchen scales. The program requires this rate to be stored in the Mr flow variable Kg per Min so 60 should be divided by the measured time (1 Kg x 60 ÷ Time)