6 replies to this topic

[nmh]

Dear experts,

 

I have a hydrogen compressor for 10000 Nl/h capacity. But if it compress the gas from 5 barg to 50 barg then its capacity (in terms of mass flow ) will be much higher under that condition (downstream of compressor). Because pressure will increase the gas density and hence the mass flow.

 

Please comment if I am thinking in right way.

 

 

regards,

nmh

Edited by nmh, 20 March 2013 - 03:26 AM.

[paulhorth]

nmh,

You are mistaken.

When you quote the capacity of the compressor as 10000 Nl/h, that means "NORMAL litres/h" which means that volume refers to the standard conditions of 1 atm and 0 deg C. That means it is equivalent to a mass flow which we can calculate.The machine has a capacity equal to this mass flow. Compressing the gas will not change this mass flow, You will still have the same mass fow and the same 10000 Nl/h at 50 bar, though the Actual volume flow will be much less.

We know from high school chemistry that 1kgmol of any gas has a volume of 22.41 Nm3 (metres cubed at 1 atm and 0 deg C). So your compressor capacity is (10000/1000)Nm3/h / 22.41  = 0.45 kmol/h of hydrogen. The mol wt of hydrogen is 2, so this equates to a mass flow of 0.45 x 2 = 0.9 kg/h . That flow is fixed regardless of the pressure.

 

It is a very small flow. Are you sure you have the units right - or did you mean 10000 Nm3/h which is a thousand times larger?

 

If it is in fact a larger flow and the compressor is centrifugal, then its capacity does vary with the discharge pressure. But your flow is far too small for a centrifugal machine.

 

 

Paul

[nmh]

Hello Paulhorth,

 

Thank you for the explanation.

 

Getting bit clear. Yes increasing pressure will increase the density of the gas but then volume will be less, hence total mass flow will remain constant regardless of higher pressure. Yes I have the right flow, I have a diphragm compressor and it can handle this small flow.

 

Regards,

nmh

[ankur2061]

nmh,

 

I am sure that your question is theoretical in nature because if you compress hydrogen gas from 5 barg to 50 barg without cooling and your inlet temperature of hydrogen to the compressor is say 30°C then theoretically your hydrogen temperature at the compressor outlet would be 375°C considering an adibatic efficiency of 75% which in real life is not possible to handle safely by any compressor

 

As a rule compressors flow rate is defined either in terms of mass flow (kg/h) or standard volume flow (Sm3/h or Nm3/h). Let us say that the flow rate is defined in Nm3/h which is at standard conditions of 0°C and 1.0135 bara. The density of hydrogen at these standard conditions is 0.09 kg/m3. Therefore the mass flow rate to the compressor inlet is volume flow rate in Nm3/h times the density. Thus in your case mass flow to the compressor becomes 10*0.09 = 0.9 kg/h. This mass flow remains unchanged based on principle of conservation of mass.

 

What changes is the actual volume flow rate also called as inlet volume flow rate and abbreviated as ACFM or ACMH due to the conditions of pressure and temperature at the compressor suction flange. You have already defined the suction and discharge pressure conditions as 5 barg and 50 barg respectively. For the purpose of doing a calculation let us assume a suction temperature of 30°C corresponding to your suction pressure of  5 barg. At 5 barg & 30°C, the density of pure hydrogen is 0.48 kg/m3. This means that the actual volume flow at the suction flange of your compressor is 0.9 / 0.48 = 1.875 m3/h.

 

Similarly at compressor discharge flange the pressure is 50 barg and temperature is 375°C (the temperature rise due to heat of compression). Density of pure hydrogen corresponding to this value is 1.9 kg/m3. Thus the actual volume flow at the compressor discharge flange is 0.9 / 1.9 = 0.47 m3/h.

 

Please note that the standard volume flow has remained unchanged (10 Nm3/h or 10,000 Nl/h as given by you) and so has the mass flow across the compressor. The only thing that has changed is the actual volume flow at the compressor inlet and the compressor outlet. 

 

The confusion about mass flow getting increased across the compressor is there for many process engineers and I hope I have been able to clarify this once for all.  

 

Regards,

Ankur

[shan]

To my understanding, the question is to switch a diaphragm compressor from 5 barg inlet pressure service to 50 barg inlet pressure service and to keep pressure differential the same.  Yes, the capacity (in term of standard gas flow and mass flow) will increase for 50 barg compressor service because diaphragm compressor is a positive displacement machine.  The same actual volume gas at 50 barg  will be about 8.5 (51/6 = 8.5) times mass at 5 parg if Z is identical.

[nmh]

Hello Ankur,

 

Thank you for your input. Now it is clear to me.

 

 

Regards,

nmh

[nmh]

Hello Shan,

 

But I understood that mass of gas (inlet) should be the same what is coming out, according to the comments from Ankur & Paulhorth. 

 

And it also applies to diaphragm compressor.

 

Regards,

nmh