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Heated Nitrogen backfill

WaterFarmFan

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Hello all. So, I have been experimenting with heating Nitrogen backfill for a vacuum oven application, but I do not think that I have had much success with getting the gas heated. I created a basic heat exchanger using a 25' 3/8" wort chiller submerged in a large pot of oil on a hotplate controlled by 15a temp pid and accurate probe submerged in oil. I have a single regulator off of the tank and a series of ball valves before and after the chiller. Things seem to get weird with the regulator when I expose it to a vacuum. I had my regulator set at ~10psi and my oil temp at 190F.

So, my questions are:

I was previously isolating the nitrogen in the chiller with the ball valves and allowing temps to rise. If gas comes in chiller at 75F and 10 psi, how much pressure increase will there be as temps rise?

I understand that higher psi will allow more gas to heater faster, but how much time would be needed to fully bring gas up to temp?

Is there a better way to do this (heat and deliver a volume of nitrogen to a desired temp)?
 

Gray Wolf

A Posse ad Esse. From Possibility to realization.
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If the delta temperature between the hot oil and gas are fixed, then residence time is your remaining variable. You can increase heat by passing the gas through slower, or make the heat exchanger longer.

The pressure change is directly proportional to the change in temperatue as measured on the Kelvin scale.

http://chemistry.bd.psu.edu/jircitano/gases.html
 

WaterFarmFan

Active member
Veteran
If the delta temperature between the hot oil and gas are fixed, then residence time is your remaining variable. You can increase heat by passing the gas through slower, or make the heat exchanger longer.

The pressure change is directly proportional to the change in temperatue as measured on the Kelvin scale.

https://chemistry.bd.psu.edu/jircitano/gases.html

Thanks for the explanation. I have previously read up on the gas laws, but I will really try to plug some of my variables.

I think what is really screwing me up is the vacuum part of the equation. I manually open the backfill on the oven, sometimes a little and sometimes a lot, which causes significant drops in psi. I have a nitrogen regulator for beer that allows very fine adjustments in psi, and I have also tried setting the regulator to 2-3 psi, but it is very hard for me to tell what is working.

Would a best practice be to close the back ball valve (closest to oven), isolate the nitrogen at a static psi, close front ball valve (closest to regulator), open back ball valve, heat after calculating the residence time? I just calculated that my 25' 3/8" chiller is only ~.02 cubic feet. I am fairly certain that I using a far more gas than this each time that I backfill.

Perhaps, an always on (no ball valves) solution would be better, but I was worried about back steaming temps on regulator when static.

Appreciate the chat as always.
 
Thanks for the explanation. I have previously read up on the gas laws, but I will really try to plug some of my variables.

I think what is really screwing me up is the vacuum part of the equation. I manually open the backfill on the oven, sometimes a little and sometimes a lot, which causes significant drops in psi. I have a nitrogen regulator for beer that allows very fine adjustments in psi, and I have also tried setting the regulator to 2-3 psi, but it is very hard for me to tell what is working.

Would a best practice be to close the back ball valve (closest to oven), isolate the nitrogen at a static psi, close front ball valve (closest to regulator), open back ball valve, heat after calculating the residence time? I just calculated that my 25' 3/8" chiller is only ~.02 cubic feet. I am fairly certain that I using a far more gas than this each time that I backfill.

Perhaps, an always on (no ball valves) solution would be better, but I was worried about back steaming temps on regulator when static.

Appreciate the chat as always.

Try putting a needle valve in line between your nitrogen regulator and your oven. It will allow you to adjust your flow rate.
 

Pangea

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Hey WaterFarmFan, glad to see you still pursuing the terp work!

Trying to grasp your issue/setup a little more with some questions:

I think what is really screwing me up is the vacuum part of the equation. I manually open the backfill on the oven, sometimes a little and sometimes a lot, which causes significant drops in psi.
The pressure drops in the oven chamber or your heated nitrogen supply when you open the backfill valve? The oven is under vac at this point? [ I have a nitrogen regulator for beer that allows very fine adjustments in psi, and I have also tried setting the regulator to 2-3 psi, but it is very hard for me to tell what is working.

Would a best practice be to close the back ball valve (closest to oven), isolate the nitrogen at a static psi, close front ball valve (closest to regulator), open back ball valve, heat after calculating the residence time? I just calculated that my 25' 3/8" chiller is only ~.02 cubic feet. I am fairly certain that I using a far more gas than this each time that I backfill.
This sounds like what you described in the first post of isolating the nitrogen in the chiller/exchanger. I couldnt find a pressure/temp chart for nitrogen gas, but I'd recommend knowing what pressure your gas will be in your coil at 190F before isolating it, pressure can rise to extremely high levels and a failure anywhere in the hot oil bath could be a nightmare.

Perhaps, an always on (no ball valves) solution would be better, but I was worried about back steaming temps on regulator when static.
I think your right about back streaming, or at least pressure differential issues, if your bottle pressure is lower than the output in the system it will not flow and could backflow, i believe. Which would be the case if your trying to push it through heat exchange type situation.

Appreciate the chat as always.

Check the blue, gw style.

I've been following a similar thread but with the use of jacketed spools and getting into using butane as the hot gas.

:dance013:
 

WaterFarmFan

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Veteran
Hey WaterFarmFan, glad to see you still pursuing the terp work!

Thanks Pangea! I have done multiple runs and am still refining my processes before following up again. I want to do another fresh flower run to really compare my numbers.

I think what is really screwing me up is the vacuum part of the equation. I manually open the backfill on the oven, sometimes a little and sometimes a lot, which causes significant drops in psi.

The pressure drops in the oven chamber or your heated nitrogen supply when you open the backfill valve? The oven is under vac at this point?

I have a nitrogen regulator for beer that allows very fine adjustments in psi, and I have also tried setting the regulator to 2-3 psi, but it is very hard for me to tell what is working.

The pressure drops on my line regulator (not tank). If I have the psi set to < 5, it can go to 0 psi when I open. The oven is under full vacuum and sometimes is isolated and sometimes has the vacuum valve open with pump running. This is the regulator:

https://www.kegworks.com/premium-double-gauge-nitrogen-regulator

Would a best practice be to close the back ball valve (closest to oven), isolate the nitrogen at a static psi, close front ball valve (closest to regulator), open back ball valve, heat after calculating the residence time? I just calculated that my 25' 3/8" chiller is only ~.02 cubic feet. I am fairly certain that I using a far more gas than this each time that I backfill.

This sounds like what you described in the first post of isolating the nitrogen in the chiller/exchanger. I couldnt find a pressure/temp chart for nitrogen gas, but I'd recommend knowing what pressure your gas will be in your coil at 190F before isolating it, pressure can rise to extremely high levels and a failure anywhere in the hot oil bath could be a nightmare.

You ain't bullshiting! I have the oil bath outside my lab space, as I did not want to be near it if it popped, and it is logistically the shortest path the rear of the oven. It is also the reason that I was setting a low (likely too low) psi on my regulator. I am going to order a longer 50' stainless coil and will do the calculations based on that.

Perhaps, an always on (no ball valves) solution would be better, but I was worried about back steaming temps on regulator when static.

I think your right about back streaming, or at least pressure differential issues, if your bottle pressure is lower than the output in the system it will not flow and could backflow, i believe. Which would be the case if your trying to push it through heat exchange type situation.

I was more concerned about off gassing on the regulator with the high temps, but would the tank have to be very low to go the other way through the regulator?
 

WaterFarmFan

Active member
Veteran
Try putting a needle valve in line between your nitrogen regulator and your oven. It will allow you to adjust your flow rate.

Thanks! Would it go between the regulator/tank and the heating coil or between the heating coil and vacuum oven? Could it help to prevent back steaming?
 

Pangea

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Veteran
Ah, so when you regulator drops to zero is when its being exposed to the negative pressures of the vacuum chamber, there will still be flow from the regulator/tank but the reg gauge will not show a increase in pressure or proper flow values until the negative pressure from the oven is closer to equilibrium. I think im interpreting this correctly?

If you want to be able to feed a constant supply and not worry about backstreaming put a check valve after the regulator supply valve, or maybe, as im not familiar with regulator valves if they already have a built in check valve.

Off gassing on the regulator should not be a concern as long as the regulator is not exposed to high vacuum levels which if its valved off when feeding a batch or if continuous supply of gas it will not be exposed to either high temps or high vacuum levels.

Putting a check valve after the regulator and a needle valve between the vac oven and heat exchanger would allow you to control the flow and not have to worry about the pressure back flow, though as the exchangers pressure increases the supply tanks ability to push in fresh nitrogen will be limited by the tanks pressure/regulated supply pressure vs the exchangers.

I'm not certain about any of this though! I'd also double recommend finding the pressure temp chart for nitrogen to make sure its within the ratings of your fittings, coil, etc.
 

WaterFarmFan

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Veteran
Ah, so when you regulator drops to zero is when its being exposed to the negative pressures of the vacuum chamber, there will still be flow from the regulator/tank but the reg gauge will not show a increase in pressure or proper flow values until the negative pressure from the oven is closer to equilibrium. I think im interpreting this correctly?

Bingo! I think that if the regulator is set higher than the flow rate, then the regulator will show the difference. So, if I set the regulator to 10 psi, and the negative pressure sucks the pressure down to 3 psi, then I have 7 psi of flow - I think?

The temp pressure data is in this pdf somewhere, but it seems to compare different methodology that appears to vary rather widely. Can you make sense of this? I want to know the pressure difference at roughly 300K and 360K.

A Reference Equation of State for the Thermodynamic Properties of Nitrogen for Temperatures from 63.151 to 1000 K and Pressures to 2200 MPa

https://www.nist.gov/sites/default/files/documents/srd/jpcrd593.pdf
 

Pangea

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Veteran
The regulator logic sounds good, when you did your previous runs you just couldnt temper the psi drop with your valve setup because your regulated pressure was to low to finesse it from bottoming out? If you had a flow meter you could see the volumes too.

I couldnt make much of that doc, I did read a sentence on page 1373 that said data shows pressures up to 30 MPa for temps from 250 to 350K.

Is your chamber also at the same temp as the gas your sparging in?
 

WaterFarmFan

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Veteran
For my previous runs, I would just attempt to isolate gas with the ball valves and then open to release when I wanted flow, but I never went over 10 psi and often lower. Bottom line is too little gas with too little exposure. I set my oil PID to about 10C cooler than oven temps for my tests, but this was out of an overabundance of caution of pressure concerns. If I can get temps dialed in, I would like them to be the same.

I think I need a higher psi in the heat exchanger with a valve setup like you described above to limit flow to the oven and that is constantly recharged by the tank. Perhaps something like this (with madeup numbers for example):

Tank -> Regulator (10 psi setting) -> Check Valve -> Heat Exchanger (20 psi max) -> Needle Valve (5 psi release) -> Vacuum Oven
 

Gray Wolf

A Posse ad Esse. From Possibility to realization.
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Any recommendations on those fore and aft valves? Thanks.

PTFE valve seats and seals works at those temperatures. Either a stainless PTFE ball valve from Paramount or Swagelok should work.
 

WaterFarmFan

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Veteran
PTFE valve seats and seals works at those temperatures. Either a stainless PTFE ball valve from Paramount or Swagelok should work.

Thanks. Is Paramount a manufacturer or distributor - paramountsupply.com? I am looking at valves now. What kind of hose would you recommend for this application?
 

WaterFarmFan

Active member
Veteran
If the delta temperature between the hot oil and gas are fixed, then residence time is your remaining variable. You can increase heat by passing the gas through slower, or make the heat exchanger longer.

The pressure change is directly proportional to the change in temperature as measured on the Kelvin scale.

https://chemistry.bd.psu.edu/jircitano/gases.html

Gay-Lussac's Law states that the pressure of a gas is directly proportional to the Kelvin temperature if the volume remains constant. P₁÷T₁ = P₂÷T₂

So, 68947.57 Pa (10 psi) / 294.26 K (70 F) = X / 366.48 K (200 F) gives X (New Pressure after 130 F increase) as 85869.31 Pa (12.45 PSI) or roughly a 25% increase in pressure. Does this sound about right?

Now to try and figure out gas volumes for heat transfer calculations...
 

Gray Wolf

A Posse ad Esse. From Possibility to realization.
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Thanks. Is Paramount a manufacturer or distributor - paramountsupply.com? I am looking at valves now. What kind of hose would you recommend for this application?

I use http://www.paramountsupply.com/branchinfo/mainpage.php?branch_number_select=1

Stainless overbraided PTFE should work.
 

Gray Wolf

A Posse ad Esse. From Possibility to realization.
Mentor
ICMag Donor
Veteran
Gay-Lussac's Law states that the pressure of a gas is directly proportional to the Kelvin temperature if the volume remains constant. P₁÷T₁ = P₂÷T₂

So, 68947.57 Pa (10 psi) / 294.26 K (70 F) = X / 366.48 K (200 F) gives X (New Pressure after 130 F increase) as 85869.31 Pa (12.45 PSI) or roughly a 25% increase in pressure. Does this sound about right?

Now to try and figure out gas volumes for heat transfer calculations...

Yup, about what I get.

Check out formula for flow through orifice.

https://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html

or through a pipe:

https://www.tlv.com/global/TI/calculator/air-flow-rate-through-piping.html
 

WaterFarmFan

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Since Nitrogen is such a large part of air, do you think that I could get fairly accurate results by testing residence time using fresh air at atmospheric pressure. I would of course need to use a check valve and eventually set my regulator to ~14.7 psi (perhaps a bit more?) to match, but I think this seems like a good and safe compromise. I could then place a thermocouple inside the port mouth of the vacuum oven and tweak the flow with needle valve. I am not super-interested in knowing the exact thermodynamics of the coil (other than of course making sure it is safe), and really want to just know and standardize the temps of gas entering oven.
 

WaterFarmFan

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As for safety, I currently have a 25' copper coil, and from my reading these have a psi rating of >1000? Does this sound correct? If so, even if I raised my oil to temps, the psi would be in ~20 range and way below tolerances. If this is not enough heating surface area for my desired flow rate, I could then upgrade to a 50' stainless.
 

Gray Wolf

A Posse ad Esse. From Possibility to realization.
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ICMag Donor
Veteran
Since Nitrogen is such a large part of air, do you think that I could get fairly accurate results by testing residence time using fresh air at atmospheric pressure. I would of course need to use a check valve and eventually set my regulator to ~14.7 psi (perhaps a bit more?) to match, but I think this seems like a good and safe compromise. I could then place a thermocouple inside the port mouth of the vacuum oven and tweak the flow with needle valve. I am not super-interested in knowing the exact thermodynamics of the coil (other than of course making sure it is safe), and really want to just know and standardize the temps of gas entering oven.

You should be able to use compressed air to tweak your system. I picked up a flow meter at Grainger when we were testing pump flows.
 

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