41 replies to this topic

[Anup Paul]

Dear Gentlemen,

I would like to reduce our compressor inlet temperature to reduce Compressor power

Stage-1
Ratio of CP/Cv=1.386
Suction Pressure=18.1 bara
Discgarge pressure=37.12 bar a
Suction Temperature= 37 C
Molecular Wt= 9.8

Stage-2
Ratio of CP/Cv=1.388
Suction Pressure=36.5 bara
Discgarge pressure=76.5 bar a
Suction Temperature= 26 C
Molecular Wt= 10.04

Current power consumption is 54 MW
Turbine driven
Now by reducing 1st stage inlet temperature to 25 C, then how much power can be saved

Your valuable comments are highly welcomed.

Thanks
Anup Elias

Edited by aelias, 02 May 2012 - 06:31 AM.

[jrtailor09]

Dear aelias,

Please refer below link posted by Ankur2061 for compressor power calculation.

http://www.cheresour...h__1#entry58777

I hope this will helpful to you for power estimation.
Regards,

Jatin Tailor

[Art Montemayor]

Anup:

When you reduce the suction temperature of the gas to any compressor, the following takes place:

• the density of the gas increases (mass/unit volume);
• the compressor continues to displace the rated volumetric capacity (compressors recognize volume and not mass);
• since the density has increased, the rated volumetric capacity is now related to a higher mass throughput;
• compressors - like any fluid transport equipment - do work on fluids by transporting mass/unit time;
• if more mass is being transported per unit of time, MORE horsepower (kW) are required to carry out the increased load - not less.

Therefore, by reducing the temperature of the suction gas, you will require more power fed to the compressor.

[ankur2061]

Anup,

You have missed out an important data, the flow rate either as mass flow or standard volumetric flow rate through the compressor.

Simplified equaton for power consumed by a centrifugal compressor considering polytropic compression path is:

P = M*H_poly / 3.6*10⁶*η_poly

where:

P = gas power required, kW

M = mass flow rate of gas, kg/h

H_poly = Polytropic Head, N.m / kg

η_poly = polytropic efficiency expressed as a fraction

H_poly is a function of suction or inlet temperature. Lower the suction or inlet temperature lower will be the polytropic head.

Hope this helps.

Regards,
Ankur.

[ankur2061]

Art,

The OP has failed to mention the flow conditions at the suction of the compressor. Most gas flow for compressors is reported in standard volumetric flow conditions such as SCFM or Sm3/h and the composition or molecular weight provided.

For calculation purposes for compressor head and power the head and power equations are more conveniently represented in terms of mass flow than in terms of volume flow.

On the basis of what I have mentioned above, a decrease in the inlet temperature with the same inlet pressure and same molecular weight or composition of the gas will cause the power consumption to reduce contrary to what you have mentioned.

Would love to have your comments.

Regards,
Ankur.

[ankur2061]

Anup,

You also need to provide the gas composition in order to calculate the suction and discharge compressibility factor and hence the average compressibility factor.

Do you have access to a process simulator such as HYSYS where you can model your compressor? If yes, model it and change the inlet temperature to see the effect on the power consumed by the compressor.

If you don't have a process simulator and are required to do compressor calculations on a regular basis then buy a compressor excel program from the online store of "Cheresources" for a nominal price of USD 40. Link is provided:

http://www.cheresour...wer-calculator/

Regards,
Ankur.

[Anup Paul]

Dear Ankur,

Composition:
Methane: 3 mol%
CO2 : 7.9 mol %
CO: 15.18 mol%
N2: 0.55 mol%
H2: 73 mol%
Flow rate = 492500 Nm3/hr

Can you pl. provide me how to caluclate polytropic head?
As Mr. Art said, there is increase in density which increases mass?
How does it impact on the power, I was not clear with your above discussion.

Thanks
Anup Elias

[ankur2061]

Anup,

When I use the data provided by you the following results are obtained for Stage-1 for the two cases:

Case-1
Suction Temperature: 37 deg C
Polytropic Head: 197,494 N-m / kg
Gas Power Required: 151. 45 MW

Case-2
Suction Temperature: 25 deg C
Polytropic Head: 189,799 N-m / kg
Gas Power Required: 145. 55 MW

The difference comes out to be approx. 6 MW i.e less power by a value of 6 MW is required with the lower gas temperature.

Regards,
Ankur.

Edited by ankur2061, 02 May 2012 - 10:28 AM.

[Anup Paul]

Dear Ankur,

Thanks a lot for your valuable comments. I used Aspen plus with Polytropic compressor using ASME method.
I got some what same results. Indeed it was great learning Experience.

Thanks for all your support
Anup Elias

Edited by aelias, 02 May 2012 - 02:43 PM.

[Padmakar Katre]

Anup,

When I use the data provided by you the following results are obtained for Stage-1 for the two cases:

Case-1
Suction Temperature: 37 deg C
Polytropic Head: 197,494 N-m / kg
Gas Power Required: 151. 45 MW

Case-2
Suction Temperature: 25 deg C
Polytropic Head: 189,799 N-m / kg
Gas Power Required: 145. 55 MW

The difference comes out to be approx. 6 MW i.e less power by a value of 6 MW is required with the lower gas temperature.

Regards,
Ankur.

Hello Ankur,
I hope you are fine. Further to comments made by Art and your formulas above, i.e. compressor power is function of mass flow, so reduced suction temperature will have higher gas density and for the same volumetric flow rate, your mass flow rate will go up and hence the power consumption. So how you get 6 MW less power with suction temperature of ~25-26 C instead of 37 C? For new designs I understand as those are custom/tailor made and can be sized for the rated volumetric throughput. But in existing if you make this change i,e, changing suction temperature will it bring down the compressor power? Please correct me in my understanding.

Edited by Padmakar S Katre, 08 May 2012 - 02:51 AM.

[ankur2061]

Padmakar,

Yes, reducing the suction temperature will reduce the power consumption. For the gas with same molecular weight and based on a defined mass flow rate @ inlet pressure / temperature a decrease in the suction temperature will reduce the power consumption because the polytropic head will reduce. The equation for polytopic head is as follows:

H_poly = (8314 / MW)*T₁*Z_avg*(n / n-1)*((P₂/P₁)^n-1/n-1)

where:
H_poly = Polytropic head, N.m/kg
MW = molecular weight of the gas, kg / kg-mol
T₁ = absolute temperature at inlet conditions, K
Z_avg = average compressibility factor
P₁ = inlet pressure, kPa (abs)
P₂ = discharge pressure, kPa (abs)
n = polytropic exponent

Note that in this equation T₁ has a role to play. If T₁ decreases H_poly will decrease with other conditions remaining unchanged. If H_poly decreases, the gas power required will also decrease.

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 08 May 2012 - 08:50 AM.

[Shivshankar]

Hello all,

This is just to share an example from The five-minute chemical engineer by Bill korchinski and Lee Turpin, Hydrocarbon processing Jan 1996. What Ankur has said, i agree with it. I don't have full article. If anybody have it, please share.

See Example 4

Regards
Shivshankar

Attached Files

•  Polytropic compression.zip   1.26MB   564 downloads

Edited by Shivshankar, 08 May 2012 - 01:02 PM.

[sheiko]

Hi,
I believe the required horsepower can either increase or decrease while the suction temperature decreases. It depends which effect (mass flowrate or polytropic head) overrides the other.
There might be a point on a plot of horsepower versus suction temperature at which the required fluid horsepower is at a minimum or maximum, so depending upon which side of the "peak" the compressor is operating, the horsepower could go up or down.
Maybe a compressor manufacturer could confirm or infirm that.

Edited by sheiko, 08 May 2012 - 10:35 PM.

[ankur2061]

Sheiko,

Can you please provide a calculation or an example where the horsepower increases due to reduction in suction temperature. My impresssion all along has been that with other conditions (mass flow rate, molecular weight, pressure ratio) remaining constant or unchanged a decrease in the suction temperature will reduce the absorbed power. At least, that is what the thermodynamic equations for gas compression predict theoretically.

I again want to emphasize that the assumption in this is that other conditions as mentioned above remain unchanged. For me it would be very enlightening to learn something new on gas compression since this is an area of great interest to me.

Regards,
Ankur.

[sheiko]

Dear Ankur,
My understanding of compressors is that mass flowrate isn't constant when the gas density varies (as the suction temperature does).
Maybe I'm wrong. But if so, could you please clarify?

Edited by sheiko, 10 May 2012 - 10:42 PM.

[ankur2061]

Sheiko,

As I mentioned earlier gas flow rates are either mentioned as standard volumetric flow (Sm³/h or SCFM) or mass flow (kg/h or lb/hr) for convenience in engineering calculations. While performing calculations for head and power for centrifugal compressors it is usual to convert the standard voumetric flow rate to mass flow rate by multiplying with standard density (kg/Sm³ or lb/ft³). This mass flow rate is then used to find out the inlet volumetric flow rate at the inlet pressure and temperature. The use of this inlet volumetric flow rate is to determine the polytropic efficiency (remember my blog on polytropic efficiency as a function of inlet volume flow). However for small varaitions in inlet volume flow due to change in inlet temperature the effect on polytropic efficiency is negligible. But the inlet temperature does have a rather more pronounced effect on the polytropic head as evident from the equation I have presented in my post #12. A decrease in the inlet temperature has a greater impact in reducing the polytropic head and vice versa. The absorbed power being the mass flow rate times the polytropic head means that a decrease in the polytropic head is going to reduce the absorbed power for the same compression ratio.

I have done the calculation using my own spreadsheet for centrifugal compressors as well as a simulation software and it indicates the phenomena that a decrease in the inlet temperature causes the absorbed power to be reduced.

I have no better explanation to offer other than this, or there is something that I have missed which would explain this phenomena.

Regards,
Ankur.

[bhatt_ms]

Hi aelias,

i tried to simulate the compression process for 1st stage using unisim(steady state). I found the power required reduces by 3 %. Please check the dew point of gas before reducing the inlet temperature.

[S.AHMAD]

Dear all
1. In terms of polytropic head, it is obvious that lower temperature reduces polytropic head.
2. However, this does not mean that the power required is also reduced since the overall efficiency of a compressor is not constant, depending on the actual inlet volume to the compressor. If I am not mistaken, from the efficiency curve supplied by vendor, there is a point where the efficiency is maximum. If the flowrate is lower or higher than this maximum point the efficiency will be lower.
3. Therefore, whether power is lower or not depends on the overall efficiency.

[ankur2061]

I have already mentioned that the change in polytropic efficiency due to change in inlet volume flow is not drastic, unless there is a drastic change in volume flow.

I would like to conclude that based on calculations done by me and others who have used process simulation software to perform the calcualtions, it is unanimous that a decrease in suction temperature with other conditions such as gas mass flow, molecular weight and pressure ratio remaining unchanged will reduce the absorbed power.

Regards,
Ankur.

[narendrasony]

Based on calculations and with the assumption that all other process parameters (e.g. MW, pressure ratio, mass flow) are fixed , it is apparent that power consumption will reduce with temperature reduction.

However the assumption, that all of other process parameters namely suction pressure (Ps), discharge pressure (Pd) and mass flow (or molar flow rate, N) will remain fixed with change in suction density - is not true in my opinion.
Consider for example, suction temperature reduction for following two of the possible scenarios for a fixed RPM machine ( Assumption : Fixed composition & MW, nearly constant polytropic efficiency) :

Scenario-1 : Fixed suction and discharge pressure -> Lower Polytropic head -> higher ACFM (actual cubic feet per minute, as per compressor curve) i.e. higher mass flow

Scenario-2 : Fixed mass / molar flow and suction pressure -> Lower ACFM at higher density -> higher polytropic head (as per compressor curve) -> higher discharge pressure

Power consumption is prooprtional to ( mass flow rate X Polytropic head)

So, in scenario-2, power should increase due to higher head at fixed mass flow.

In Scenario-1, power may reduce or increase depending on the operating point on compressor curve. In flatter part (near surge point - which is the normal operating range), power will increase with reducing temperature where as in verticle part (near choke flow) power will reduce.

However, with variable RPM machine, power will reduce since for given mass flow polytropic head can be reduced.


Please correct me if I'm wrong.

Regards
Narendra

Edited by narendrasony, 11 May 2012 - 11:19 AM.

[ankur2061]

Narendra,


The OPs question is that if I reduce the inlet temperature from a given higher temperature and do not change the pressure ratio (discharge pressure / suction pressure) with the same gas composition (read molecular weight) what will happen to the absorbed power. So the question of changing the pressure ratio or in other words increasing the discharge pressure as proposed in scenario 2 cannot be considered. This maybe a hypothetical scenario that the pressure ratio remains unchanged, but the correct answer would be that the absorbed power decreases.

This has been demonstrated by calculations that I have done and by the OP himself as well as others. I would like somebody to demonstrate that as per the original post made, the absorbed power increases considering that all other conditions except the suction temperature remain unchanged. Otherwise it would be just postulating theory after theory without any substance.

Regards,
Ankur.

[sheiko]

Ankur,
It is true that, at design stage, one can perform a sensitivity analysis on a single variable (all other variables being fixed) with a simulation tool. But, I believe that in "real life" we cannot neglect the fact that changing the suction temperature may also change the gas density (PV= nRT) among other process variables.
"My theory" is the following:
Assuming that the suction pressure and gas composition remain the same althought the suction temperature decreases:
- If the machine is a positive-displacement type, mass flowrate will increase due to the higher density.
- If the machine is a centrifugal one on the other hand, the actual suction volumetric flowrate will increase because the polytropic head is going down, thus increasing even more the mass flowrate.
Now the questions are:
- which effect (mass flowrate or polytropic head) is predominant?
- is it possible that the change in suction temperature also leads to a change in gas composition?

Edited by sheiko, 12 May 2012 - 12:14 AM.

[breizh]

Hi ,

To support the discussion.

Hope this helps
Breizh

[narendrasony]

Dear Ankur,
My assertion is that keeping all the parameters fixed (pressure ratio as well as mass flow rate) is impossible with change in suction temperature. If pressure ratio is fixed, ACFM and hence mass flow will change.

As Sheiko has mentioned, for fixed pressure ratio, power may reduce or increase depending on the relative change of polytropic head (reduced) and mass flow (increased) and it depends on which part of curve are we operating.

Regards
Narendra

[ankur2061]

The calculations on the basis of the data provided by the OP for the mass and volume flow rate are as follows:

Case-1
Suction Pressure: 18.1 bara
Discharge Pressure: 37.12 bara
Gas Molecular weight: 9.8 kg/kg-mol
Mass Flow Rate: 215,334 kg/h (volume flow at std conditions: 492,500 Nm³/h as given)
Suction temperature: 37 deg C
Inlet Volume Flow Rate: 31,400 m³/h (at inlet conditions of pressure and temperature)
Gas Power: 153.5 MW

Case-2
Suction Pressure: 18.1 bara
Discharge Pressure: 37.12 bara
Gas Molecular weight: 9.8 kg/kg-mol
Mass Flow Rate: 215,334 kg/h (volume flow at std conditions: 492,500 Nm³/h as given)
Suction Temperature: 25 deg C
Inlet Volume flow Rate: 30,177 m³/h
Gas Power: 147.5 MW

% change (decrease) in volume flow rate from Case1 to Case 2: 4 %

% change (decrease) in absorbed power from Case 1 to Case 2: 4%

Regards,
Ankur.