2 replies to this topic

[Freester999]

Hi all,

The situation is blowdown release.
The gas is entering a 1"valve at 120barg and 19 Deg C and mass flow 10000kg/h.I have calulated the Joule thompson cooling for the above condition through Valve to be -60Deg C at 1 atm (using HYSYS).
But the pipeline upstream is designed for -40Deg C only.So I need to calculate what length of pipe is required to warm up the gas to a temp around -40Deg C.
i.e Inlet temp -60Deg C and Outlet temp is -40deg C.

Other physical ppts of gas at inlet condition are
mass heat capcity 3.775 kJ/kg C
mass density 175.7 kg/m3
Thermal cond 0.053 w/mK
viscosity 0.02014 cp

also the ambient temp is 5Deg C and no wind .assuming no heat transfer by radiation.
The Overall heat transfer coefficient (natural convection) calculated is U=15w/m2K

Other question is If I were to do a heat balance using Newton law of cooling Q=UA(Delta T)
What will be my A,is it Pi*ID of pipe*length or, surface area A=pi*ID of pipe^2 ?
and Delta T?

Please help me calculate the length of pipe?my brain is hurting!
pls ask questions if u have any also regarding above
Thanks

freester

[Kie]

If it were a real process issue, you wouldn't use a length of pipe to warm the gas up, you'd use some sort of plate fin heat exchanger. Just by looking at it, you'll need a long pipe. 10000kg/h through a 1" valve is quite a lot too!

Q=UAdT is mainly used for heat exchanger design. A is the heat transfer area. You usually have to calculate A after getting Q, U and dT in order to find out how big your heat exchanger will be (put simply).

In your case, looks like you need to find the heat loss per metre length of pipe. It's either external or free convection. I'd go for free convection because the air isn't moving.

[Freester999]

If it were a real process issue, you wouldn't use a length of pipe to warm the gas up, you'd use some sort of plate fin heat exchanger. Just by looking at it, you'll need a long pipe. 10000kg/h through a 1" valve is quite a lot too!

Q=UAdT is mainly used for heat exchanger design. A is the heat transfer area. You usually have to calculate A after getting Q, U and dT in order to find out how big your heat exchanger will be (put simply).

In your case, looks like you need to find the heat loss per metre length of pipe. It's either external or free convection. I'd go for free convection because the air isn't moving.

thanks kie,
The whole calc is done to see if the length of the pipe required is below or equal to 50-60m.if it is ,then go with the project otherwise some other alternative like HE would be reqd.
This is a real process and I know the flowrate is quite high(blowdown case).if the length of the pipe allows the gas to warm up from -60 to -40Deg C,then an expensive HE would not be a good idea.
I have to do this calc and see if it works or not.