Refrigerant compressor in propane refrigeration cycle

[chemicalENG]

If we have two stage refrigerant compressor in propane refrigeration cycle ( see the attach img ) that are driven by gas turbine . We know that the power produced by the gas turbine will decrease as the ambient temperature increases .Hence , the power of the refrigerant compressor wil decrease .

The propane refrigeration cycle is used to cool process feed gas ( in the evaporetor(CHILLER) ) .

My question is how reducting refrigerant compressor power will effect the temperature of the process feed gas ???

[RANGER1]

ChemicalENG,
We probably have very limited information.
But in a nut shell can only suggest that if turbine power decreases, possibly propane compressor speed changes, reducing capacity.
Low stage of system MAY not decrease in speed as much due to it's lighter load.
This might mean interstage pressure increases if second stage slows down as well as it's condensing pressure increases.
It also depends on what the system designer had in mind & how things were sized.

We do not know power of gas turbine compared to required power of propane compressor under the varying loads.
We also do not know the load on system running in lower ambient, is propane system running with reduced capacity to match load.


If either compressor slows down, it will lose some capacity possibly affecting process gas feed out of heat exchanger due to higher evaporating pressure/temperature, if process gas has same flow rate.

What the of propane compressor, screw or reciprocating?

[chemicalENG]

It is centrifugal compressor

[Josip]

Hi, chemicalENG :)

welcome to RE forums .... usually first post would be here ..
http://www.refrigeration-engineer.com/forums/forumdisplay.php?72-New-to-RE

to introduce yourself to our community ;)



If we have two stage refrigerant compressor in propane refrigeration cycle ( see the attach img ) that are driven by gas turbine . We know that the power produced by the gas turbine will decrease as the ambient temperature increases .Hence , the power of the refrigerant compressor wil decrease .

The propane refrigeration cycle is used to cool process feed gas ( in the evaporetor(CHILLER) ) .

My question is how reducting refrigerant compressor power will effect the temperature of the process feed gas ???

Not to much info, as RANGER said, but I believe that design engineer of your plant took in calculations all possible discrepancies .... i.e. there must be some control devices to reduce compressor capacity according to available input shaft power ... thus the plant will remain to work within envelope regarding pressure and temperature, but with reduced capacity ...

Best regards, Josip :)

[chemicalENG]

Hi, chemicalENG :)

welcome to RE forums .... usually first post would be here ..
http://www.refrigeration-engineer.com/forums/forumdisplay.php?72-New-to-RE

to introduce yourself to our community ;)




Not to much info, as RANGER said, but I believe that design engineer of your plant took in calculations all possible discrepancies .... i.e. there must be some control devices to reduce compressor capacity according to available input shaft power ... thus the plant will remain to work within envelope regarding pressure and temperature, but with reduced capacity ...

Best regards, Josip :)

What do you mean by compressor capacity ? You mean the compressor power ??!

[Josip]

Hi, chemicalENG :)


What do you mean by compressor capacity ? You mean the compressor power ??!

actually, yes ... at compressor inlet (I believe) you have a kind of regulating valve to regulate a mass of inlet gas into suction of compressor ... on another end you (probably) have a device to measure output of turbine torque-power which we need to run the compressor i.e. to do the work

(for electric driven motor we use to measure a maximum amperes allowed for that motor)

... so, when we are close to maximum of available turbine power, because we have a too big load at compressor inlet-suction we start to regulate i.e. reduce mass of gas at suction ... thus we have to push through compressor less mass i.e. less work to do and our turbine power would be enough to do that work ... that means ... you will run your compressor at less capacity and the work done will be less until you return to normal turbine output power needed to run compressor at full capacity ... i.e. suction valve full open ...

hope this will be of some help to you ..


Best regards, Josip :)

[chemicalENG]

Hi, chemicalENG :)



actually, yes ... at compressor inlet (I believe) you have a kind of regulating valve to regulate a mass of inlet gas into suction of compressor ... on another end you (probably) have a device to measure output of turbine torque-power which we need to run the compressor i.e. to do the work

(for electric driven motor we use to measure a maximum amperes allowed for that motor)

... so, when we are close to maximum of available turbine power, because we have a too big load at compressor inlet-suction we start to regulate i.e. reduce mass of gas at suction ... thus we have to push through compressor less mass i.e. less work to do and our turbine power would be enough to do that work ... that means ... you will run your compressor at less capacity and the work done will be less until you return to normal turbine output power needed to run compressor at full capacity ... i.e. suction valve full open ...

hope this will be of some help to you ..


Best regards, Josip :)

Ok Josip.. My purpose actually is to have more condensing of a certain component in the process feed gas mixture using refrigerant propane . Regarding the inlet mass flow rate of the compressor . I have a real data and I noticed that in the period between 10 am and 1 pm the inlet volumetric flow rate (KSCFH) always the highest but at other period ( at night for example ) , the flow rate is less Can you explain why ? Is that because the process feed gas temperature is higher at that period due to the ambient temperature so the propane refrigerant will vaporized more at the evaporator and that's why the amount of vapor returning to the compressor will be higher ???

[Josip]

Hi, chemicalENG :)


Ok Josip.. My purpose actually is to have more condensing of a certain component in the process feed gas mixture using refrigerant propane . Regarding the inlet mass flow rate of the compressor . I have a real data and I noticed that in the period between 10 am and 1 pm the inlet volumetric flow rate (KSCFH) always the highest but at other period ( at night for example ) , the flow rate is less Can you explain why ? Is that because the process feed gas temperature is higher at that period due to the ambient temperature so the propane refrigerant will vaporized more at the evaporator and that's why the amount of vapor returning to the compressor will be higher ???

It is not possible for me to explain why you have a bigger volumetric flow rate during some period of the day, what is a cause of that discrepancy, but can be a normal response to environment state ... your system is a closed system and for some reason volumetric flow rate arise to keep the system inside of working envelope ...

Volumetric flow rate is the unit of fluid which passes per unit of time ...

Q = v*A [m3/s]

where ...
Q = volumetric flow [m3/s]
v= flow velocity of the gas [m/s]
A= cross-section area/surface [m2]

where we don't know anything about mass .... for the moment ...


Mass flow rate is the mass of fluid which passes per unit of time

m = ρ * V(Q) =ρv * A [kg/s]

where ...
m = mass flow [kg/s]
V or Q = volume flow (m3/s)
ρ = mass density (kg/m3)
A = cross-section area/surface (m2)

to calculate refrigeration/cooling capacity we need a mass ...

Q = m * Cp * dT [kW]
where ...
Q = cooling capacity [kJ/s = kW] ... (J/s = W)
m = mass flow rate
Cp = specific heat capacity [kJ/kg K]
dT = temperature difference [K]

it is obvious that all variables are in correlation ... some of variables are given by the state of the system and some are possible to change ... to keep cooling capacity Q steady or to increase it or to decrease it for any reason requested by the system we can only change +/- compressor volumetric capacity-flow ...

Hope this is of some help to you ..

Best regards, Josip

[chemicalENG]

Hi, chemicalENG :)



It is not possible for me to explain why you have a bigger volumetric flow rate during some period of the day, what is a cause of that discrepancy, but can be a normal response to environment state ... your system is a closed system and for some reason volumetric flow rate arise to keep the system inside of working envelope ...

Volumetric flow rate is the unit of fluid which passes per unit of time ...

Q = v*A [m3/s]

where ...
Q = volumetric flow [m3/s]
v= flow velocity of the gas [m/s]
A= cross-section area/surface [m2]

where we don't know anything about mass .... for the moment ...


Mass flow rate is the mass of fluid which passes per unit of time

m = ρ * V(Q) =ρv * A [kg/s]

where ...
m = mass flow [kg/s]
V or Q = volume flow (m3/s)
ρ = mass density (kg/m3)
A = cross-section area/surface (m2)

to calculate refrigeration/cooling capacity we need a mass ...

Q = m * Cp * dT [kW]
where ...
Q = cooling capacity [kJ/s = kW] ... (J/s = W)
m = mass flow rate
Cp = specific heat capacity [kJ/kg K]
dT = temperature difference [K]

it is obvious that all variables are in correlation ... some of variables are given by the state of the system and some are possible to change ... to keep cooling capacity Q steady or to increase it or to decrease it for any reason requested by the system we can only change +/- compressor volumetric capacity-flow ...

Hope this is of some help to you ..

Best regards, Josip

I am really enjoying discussion with you...

For cooling capacity , I think if I am not wrong there are only change phase in the evaporator and the temperature will remain constant so we have only latent heat which is Q=m*(DHvaporization)

The total refrigerant mass circulation rate will be constant I guess . The change will be on the amount of vapor and liquid in the flash drum ( Economizer ) , right ???

As amount of liquid in flash drum increased , cooling capacity is increased I guess ( not sure ))!!!

[Josip]

Hi, chemicalENG :)




I am really enjoying discussion with you...

For cooling capacity , I think if I am not wrong there are only change phase in the evaporator and the temperature will remain constant so we have only latent heat which is Q=m*(DHvaporization)

The total refrigerant mass circulation rate will be constant I guess . The change will be on the amount of vapor and liquid in the flash drum ( Economizer ) , right ???

As amount of liquid in flash drum increased , cooling capacity is increased I guess ( not sure ))!!!

According to attached sketch of your plant in your first post refrigerant phase change you have in both vessels economizer and main HE (Chiller-Q) .... and both vessels are equipped with level control and liquid injection control - probably proportional flow control valve ...

Of course if we keep within HE the same evaporating pressure then evaporating temperature will remain the same there will be only change in amount of liquid which evaporate, but that again depend on the state and mass of cooled fluid within secondary circuit of HE in your case process gas for cooling ...

Your economizer vessel (I assume it is open flash type) is utilized to sub-cool liquid to feed the main HE (Chiller-Q) and that gas is going back to suction of the high/second stage compressor, but mixing within mixer with discharge gas from the first stage compressor .... for me it is a kind of normal HE more like an inter-cooler ... giving us a cold liquid with lower enthalpy which increases the amount of heat each kilogram of liquid refrigerant can absorb ... thus we gain some capacity only at evaporator i.e. chiller

Real economizer system is that one where we can have compressor with normal suction port with system suction pressure and another side-economizer port with higher (economizer) pressure thus increase in mass flow through compressor ... but very low increase in power consumption ....

in your plant both gases from the first stage compressor and from economizer are join together in mixer and then go to the suction of the second stage compressor and both at the same pressure thus we have only one pressure and no gain in capacity because we have only one suction pressure coming from mixer ...

Liquid level in both vessels is designed to be constant (usually at some max level allowed for vessel in question) with help of level control and liquid injection valve giving us a maximum capacity of that HE.

With lower then designed liquid level within HE less pipes are flooded with cold refrigerant thus HE will give less capacity, but only when we have shell&tube HE for open flash HE that is not the case.

Hope this will help you a little ...

Best regards, Josip :)