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Nitrogen Heater Design And Troubleshooting

nitrogen heater pressure drop explosion proof toubleshooting heat transfer vacuum distillation vacuum stripping

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#1 RickeyD09

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Posted 30 July 2014 - 09:47 AM

Hello Everyone,

 

Before I started working at my new job, the production facility installed an in-line nitrogen heater. The intended use is to heat the N2 to about 150-160 C in order to improve stripping efficiency in our process. The heating element can not be inside the reaction room because it is not explosion proof rated and the resulting pipe is about 100 ft long. By the time the N2 reaches the process vessel it is already cooled down to 35 C (temp controller set point is 250 C) On top of that, there is a huge pressure drop across the control valve as the N2 enters the vessel from roughly 120 psi to 3 psi. (Dropping the temp even more.) The piping is insulated with 1" fiberglass insulation. To top everything off, the temperature control thermocouple is located immediately after the heater. So I guess my question is can someone help me troubleshoot and try to fix this catastrophe of a project?

 

Here are a couple of the ideas I've had to potentially fix the problem.

 

1) Move the thermocouple farther down the piping to get the N2 to an acceptable temp closer to the vessel.

 

2) Regulate the pressure back before the heating element to about 10 psi to decrease the pressure drop across the control valve. (PV=nRT)

 

Any help with this would be greatly appreciated.

 

PS: Nitrogen is coming out of an outside microbulk storage tank from AirGas.

 

 

 

Much Thanks,

      A Frustrated Engineer



#2 PingPong

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Posted 30 July 2014 - 10:24 AM

.... the resulting pipe is about 100 ft long. By the time the N2 reaches the process vessel it is already cooled down to 35 C (temp controller set point is 250 C)
If the temperature drops from 250 to 35 oC over a pipe length of 100 ft then obviously the insulation is insufficient: not thick enough and/or poorly installed.

 

What is the nitrogen flowrate? What is the pipe diameter?



#3 RickeyD09

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Posted 31 July 2014 - 08:37 AM

 

.... the resulting pipe is about 100 ft long. By the time the N2 reaches the process vessel it is already cooled down to 35 C (temp controller set point is 250 C)
If the temperature drops from 250 to 35 oC over a pipe length of 100 ft then obviously the insulation is insufficient: not thick enough and/or poorly installed.

 

What is the nitrogen flowrate? What is the pipe diameter?

 

 

Skin temperature on the insulation around the element reaches a steady state temperature of 54.2 C so I really don't think the insulation is the biggest issue in this system. Nitrogen is flowing through 1/2" galvanized pipe wrapped with 1" thick fiberglass insulation. Flow rate is right around 3 scfm.



#4 PingPong

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Posted 31 July 2014 - 03:57 PM

Skin temperature on the insulation around the element reaches a steady state temperature of 54.2 C

Do you mean the element of the controller at 250 oC ?

What was the ambient air temperature when this skin temperature was measured?

 

... I really don't think the insulation is the biggest issue in this system.

The nitrogen temperature drops from 250 to 35 oC over only 100 ft of tubing, but you don't think the insulation is the problem?

What else could then cause this heat loss, in your opinion?

 

Flow rate is right around 3 scfm.

That is only 6 kg/h nitrogen. Are you sure the flow rate is that low?


Edited by PingPong, 31 July 2014 - 04:58 PM.


#5 Marc-Andre Leblanc

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Posted 31 July 2014 - 05:26 PM

Hello,

 

First of all , the temperature drop for nitrogen for 120 psig to 3 psig is only a few °F, this is not where you lose your heat. ( tested in HYSYS )

 

Considering your flow of nitrogen, only a heat loss of  around 1300 BTU/hr in your 100 ft of piping is required to reduce the temperature from 250°C (482°F) to 35°C

 

Your 3 SCFM is a mass Flow of 13.25 pounds / hrs

 

heat capacity of around 0.26 BTU/pounds/°F at 482°F and around 0.25 BTU/pounds/°F at 95°F will use average for simplicity

 

13.25*(482-95)*0.255 = 1300 BTU/hr

 

Using a simple heat loss from piping software ( I use 3e Plus, its available for no fee), you will see that your heat loss:

 

For 1" insulation is initially of 199 BTU/hr/ft of piping with fluid at 482°F for mineral fiber type 1 insulation

 

Since surface temperature estimated with the software is 138°F while your actual value is 130°F you can see the estimation of heat loss in the software is close to reality !!! 

 

With this value - you can see the temperature drop across the first 1 or 2 feets of piping with 1 inch of insultation would be of around 59°F 

 

(199 BTU/hr) / (13.25 Pound /hr) / (0.255 BTU/Pound/°F) = 59°F

 

Since the temperature dropped to 423°F , the heat loss in the second ft is around 156 BTU/hr ... for a temperature drop of around 46°F and so on until your reach temperature around 105 - 95°F(30-35°C) where heat loss per ft of piping is around 7 BTU/hr ... and at the end your nitrogen temperature is slighly higher or equal to atmospheric

 

As you can see, 1 inch of insulation isnt helping much and your actual situation is a demonstration of these calculations.

 

I doubt more insulation would be a solution either since the the temperature differential between the atmosphere and your nitrogen is very high and the heat capacity of nitrogen is very low.

 

You said a temperature required for your process would be around 150 to 160°C.

 

I dont know your plant, but if you have a saturated steam service close with a pressure around 50 psig or more, you could jacket the last 10 or more feet of your nitrogen line with a steam jacket, install trap and condensate return, insulation ... 

 

If you dont have a steam service or cost of modification would be too high, you will have to get the heater located as close as possible to the intake of your reactor.

 

Regards

 

Marc-Andre






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