Page 3 of 4 FirstFirst 1234 LastLast
Results 101 to 150 of 152
  1. #101
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems



    Hi corineramer,

    Use this link to review vessel information.
    http://www.evapco.com/media/pdf/asme-pressure.pdf

    ...the cycle that we have studied is the vapor compression cycle
    Nothing has changed. This is still a vapor compression cycle. It's just not a direct expansion system.

    If you try to add other coils which are operating at higher evaporting temperatures, you would need back-pressure valves on those coils operating at the higher temperatures (greater than -30C).

    This really complicates a liquid overfeed system and the explanation of how it operates.

    If this is supposed to be a school project your professor has actually given you one of the hardest problems to work on.



  2. #102
    Join Date
    Aug 2006
    Location
    manila
    Age
    39
    Posts
    6
    Rep Power
    0

    Re: Liquid Overfeed Systems

    Hi US Iceman,

    I'm a groupmate of corineramer. We are really confused about the liquid overfeed refrigeration system. This is what we understand about it.

    You will supply more liquid refrigerant flowrate in the evaporator than required to enchance heat transfer by providing a higher velocity in the coils.


    In our subject, we need to design a refrigeration system(liquid overfeed) for ice cream manufacturing plant.

    We have used a separate refrigeration system for each process, freezing, hardening, storage.

    Our professor said that we can tap the discharge line in the compressor because it has the same discharge pressure.


    Is it possible if we only have 1 to 2 condenser? is it possible if we only have one condenser and 3 expansion valve?


    Back to liquid overfeed. for example, in our freezing process, we have a -10 degree C ammonia. The ammonia pump will pump the refrigerant to the evaporator and increase the pressure of the refrigerant thus increaseing the boiling point. So we need the hand expansion valve to bring back the saturation pressure at -10 degree C? In liquid overfeed, the refrigerant that comes out from the evaporator is a mixture of liquid refrigerant and vapor right? then it will go to the separator to separate liquid and vapor. Then the compressor will suck the vapor at saturation pressure of the -10 degree C. The discharge pressure will depend on the ambient temperature in the condenser. We have a 35 degree C. Then after passing in the condenser, it will pass to the expansion valve to bring down the pressure to the saturation pressure at -10 degree C then stored in the liquid receiver for use when the level of refrigerant in the separator is low. is this correct?

    Sorry if we ask so many question, we are really confused and thank you so much for spending time and helping us. Thanks

  3. #103
    rbartlett's Avatar
    rbartlett Guest

    Re: Liquid Overfeed Systems

    Quote Originally Posted by ipelatak
    Hi US Iceman,

    I'm a groupmate of corineramer. We are really confused about the liquid overfeed refrigeration system. This is what we understand about it.

    You will supply more liquid refrigerant flowrate in the evaporator than required to enchance heat transfer by providing a higher velocity in the coils.


    In our subject, we need to design a refrigeration system(liquid overfeed) for ice cream manufacturing plant.

    We have used a separate refrigeration system for each process, freezing, hardening, storage.

    Our professor said that we can tap the discharge line in the compressor because it has the same discharge pressure.


    Is it possible if we only have 1 to 2 condenser? is it possible if we only have one condenser and 3 expansion valve?


    Back to liquid overfeed. for example, in our freezing process, we have a -10 degree C ammonia. The ammonia pump will pump the refrigerant to the evaporator and increase the pressure of the refrigerant thus increaseing the boiling point. So we need the hand expansion valve to bring back the saturation pressure at -10 degree C? In liquid overfeed, the refrigerant that comes out from the evaporator is a mixture of liquid refrigerant and vapor right? then it will go to the separator to separate liquid and vapor. Then the compressor will suck the vapor at saturation pressure of the -10 degree C. The discharge pressure will depend on the ambient temperature in the condenser. We have a 35 degree C. Then after passing in the condenser, it will pass to the expansion valve to bring down the pressure to the saturation pressure at -10 degree C then stored in the liquid receiver for use when the level of refrigerant in the separator is low. is this correct?

    Sorry if we ask so many question, we are really confused and thank you so much for spending time and helping us. Thanks
    For a brief description of liquid ampliciation go here-

    http://www.hysave.com/

    and click on 'live demo'

    which is a small vid of the process..however each case needs careful examination and application which I understand is where they badly went wrong in the early days..

    cheers

    richard

  4. #104
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    BTW ipelatak is my groupmate... his message is the one after your latest post pls read his message, tnx... actually our professor told us to pick an industrial plant and design one of its utilities so that it would be easier for us because if he would choose what we're gonna design then it will be very difficult but unfortunately we picked a hard one not the easy ones... we have problems in selecting the compressor because our reference is the suction pressure, is this correct? for example our suction is 42.3psig and discharge press is 190psig... our prof told us to base our compressor to this given data but when im looking for the compressor they are giving the refrigeration capacity not the pressure... is this correct? or convert the TOR to refrigerating capcity and use it in selecting the compressor.. we're really confused... sorry if we'er disturbing you..

  5. #105
    Join Date
    Mar 2006
    Location
    Philippines
    Age
    53
    Posts
    124
    Rep Power
    19

    Re: Liquid Overfeed Systems

    Hi kurimaw & latek,

    Expansion valve before liquid reciever is WRONG!

    I think you should go back from the very basic of refrigeration before jumping out for the design. You shoul know the function of every components of refrigeration.

    Are you a first year college student? what school?

    Don't rely too much in your prof. you should do your own researh & study.

    Regards,
    Guapo

  6. #106
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Hi ipelatak,

    Quote Originally Posted by ipelatak
    You will supply more liquid refrigerant flowrate in the evaporator than required to enchance heat transfer by providing a higher velocity in the coils.
    You are supplying more liquid into the evaporator than the amount that boils off. The purpose of using a liquid overfeed system is that the excess liquid provides a complete wetting of the heat transfer surface of the evaporator coil.

    In a DX system, you loose some of the capacity of the evaporator due to the need to provide superheat. If you look at the evaporator ratings you will see a DX coil has about 15% less capacity than the same coil for liquid overfeed.

    Quote Originally Posted by ipelatak
    We have used a separate refrigeration system for each process, freezing, hardening, storage.
    OK, that makes the design process much easier then.

    Quote Originally Posted by ipelatak
    Is it possible if we only have 1 to 2 condenser? is it possible if we only have one condenser and 3 expansion valve?
    You can have one condenser or multiple condensers. All of your compressors can be connected to a common discharge line, which is piped to a single large condenser, or smaller multiple condensers. The condenser(s) would drain down to a common high pressure receiver. From this receiver, you can feed liquid to your separator (the one with the refrigerant pumps).

    Quote Originally Posted by ipelatak
    The ammonia pump will pump the refrigerant to the evaporator and increase the pressure of the refrigerant thus increaseing the boiling point. So we need the hand expansion valve to bring back the saturation pressure at -10 degree C?
    The pump provides the pressure to transport the liquid to the hand expansion valves. The pressure supplied by the pump overcomes the friction and other losses to ensure the liquid is getting to the hand expansion valves.

    You are correct about the increase in boiling temperature. The added pressure generated by the pump actually seems similar to subcooling of the liquid. Therefore, the liquid will not boil until the pressure is reduced. The hand expansion valve is really a balancing valve. You are using it to adjust the flow rate into all of the evaporators.

    Quote Originally Posted by ipelatak
    In liquid overfeed, the refrigerant that comes out from the evaporator is a mixture of liquid refrigerant and vapor right? then it will go to the separator to separate liquid and vapor. Then the compressor will suck the vapor at saturation pressure of the -10 degree C.
    OK, so far.

    The discharge pressure will depend on the ambient conditions and the capacity of the condenser. If you are using an evaporative condenser, you are concerned with the condensing temperature and entering wet bulb temperature of the air.

    Quote Originally Posted by ipelatak
    Then after passing in the condenser, it will pass to the expansion valve to bring down the pressure to the saturation pressure at -10 degree C then stored in the liquid receiver for use when the level of refrigerant in the separator is low. is this correct?
    In a typical system, the liquid will flow into a high-pressure receiver after it drains out of the condenser. When the level in the separator begins to decrease, a control system will allow liquid from the high-pressure receiver to flow into the separator to maintain the liquid level. The liquid in the high-pressure receiver would be at 35C.

    I'm not sure whay you have an expansion valve after the condenser as you described. (Note: I have done something like this before, but quite different than what you are describing.

    Quote Originally Posted by ipelatak
    Sorry if we ask so many question, we are really confused and thank you so much for spending time and helping us.
    My pleasure. I hope I have not added to the confusion.

  7. #107
    Join Date
    Aug 2006
    Location
    manila
    Age
    39
    Posts
    6
    Rep Power
    0

    Re: Liquid Overfeed Systems

    Hi US iceman,
    Quote Originally Posted by US Iceman

    I'm not sure whay you have an expansion valve after the condenser as you described. (Note: I have done something like this before, but quite different than what you are describing.
    Uhm we thought we should need to use a expansion valve after the condenser to reduce the pressure back to the refrigerants saturation pressure at -10 degree C.

    So after condenser, we should put a high pressure liquid reciever. The temp of the refrigerant is 35 degree C = to the ambient temperature of the condenser. When the level in the separator begins to decrease, a control system will allow liquid from the high-pressure receiver to flow into the separator to maintain the liquid level. The temp in the separator is -10 degree C and the temp in the high pressure reciever is 35 degree C. We thought we need a expansion valve to bring back the pressure and temperature of the refrigerant in high pressure reciever equal to the refrigerant's temp and pressure in the separator.

    Do we still need a pump after the high pressure reciever or the pressure will transport the refrigerant back to the separator and bring back the temperature and pressure because of the pressure drop in the pipe?

    thanks a lot masters, were really thankfull that you guys are helping us. thanks a lot

  8. #108
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Hi corineramer,

    I had a long explanation of the compressor rating procedures written out and when I tried to post it, something happened and I lost the whole thing.

    The short version is:

    You have to include the suction and discharge pressure losses to correct for the actual compressor performance.


    Discharge pressure = condensing pressure + piping pressure lossses

    Suction pressure = evaporating pressure - piping pressure losses

    Convert the suction and discharge pressure to their equivalent saturation temperatures. These are the temperatures you want to use for selecting the compressors.

  9. #109
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by ipelatak
    The temp of the refrigerant is 35 degree C = to the ambient temperature of the condenser.
    No. The condensing temperature is not the ambient temperature. If you want to condense at 35C, then you have to select the condenser for a condensing temperature of 35C with whatever your local wet bulb temperature is for. In the Phillipines it is probably quite high.

    We thought we need a expansion valve to bring back the pressure and temperature of the refrigerant in high pressure reciever equal to the refrigerant's temp and pressure in the separator.
    You need an expansion device to feed the liquid into the separator to maintain the liquid level. The liquid comes from the reciever at 35C, goes through the expansion valve, into the separator.

    Do we still need a pump after the high pressure reciever...
    Ahh, crap... Know I see. You guys are designing a Hysave system. That is not a liquid overfeed system, it's a DX system.

    The refrigerant pump in an overfeed system is under the separator and operates at suction pressure.

  10. #110
    Join Date
    Jan 2006
    Location
    Croatia
    Age
    68
    Posts
    2,299
    Rep Power
    31

    Re: Liquid Overfeed Systems

    Hi

    Quote Originally Posted by ipelatak

    Do we still need a pump after the high pressure reciever or the pressure will transport the refrigerant back to the separator and bring back the temperature and pressure because of the pressure drop in the pipe?
    thanks a lot masters, were really thankfull that you guys are helping us. thanks a lot
    No, from receiver to surge drum (-10C) you have only a pipe with liquid ammonia under condensing pressure and little subcooled. On this pipe you can have (because there are some other solutions) installed stop valve, solenoid valve, expansion valve and again stop valve just before entering into separator (surge drum -10C). At this expansion valve our high pressure and high temp ammonia expand giving us a mixture of liquid and vaporized ammonia with temperature what we have in our surge drum (-10C), liquid remain in separator and vapour we remove from surge drum with compressor. This liquid with temperature of -10C we can transport (pump) to our cooling units (homogenizers).


    Quote Originally Posted by corineramer
    hi us iceman,

    BTW ipelatak is my groupmate... his message is the one after your latest post pls read his message, tnx... actually our professor told us to pick an industrial plant and design one of its utilities so that it would be easier for us because if he would choose what we're gonna design then it will be very difficult but unfortunately we picked a hard one not the easy ones... we have problems in selecting the compressor because our reference is the suction pressure, is this correct? for example our suction is 42.3psig and discharge press is 190psig... our prof told us to base our compressor to this given data but when im looking for the compressor they are giving the refrigeration capacity not the pressure... is this correct? or convert the TOR to refrigerating capcity and use it in selecting the compressor.. we're really confused... sorry if we'er disturbing you..
    Coming to compressor sizing first you have to calculate your heat load=compressor capacity (not exactly, but good for example) for defined evaporating temperature/pressure (-10C, -15C, -35C or -40C) and defined condensing temperature/pressure i.e. one compressor can run at each suction temperature/pressure from -15C to + 5C with the same (or different) condensing temperature/pressure but allways with different capacity.

    To obtain needed capacity, compressor capacity is very important but power of motor also.

    Hope this will help a little

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

  11. #111
    Join Date
    Feb 2001
    Location
    Tampa Florida
    Posts
    1,086
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Ahh, crap... Know I see. You guys are designing a Hysave system. That is not a liquid overfeed system, it's a DX system.
    Are you sure, Iceman? Seemed to me they are trying to sort out an overfeed system. I was actually puzzled why Richard assumed they were looking at using a pump to increase liquid pressure. The mention of hand expansion valves had me puzzled until you sorted them out as balancing valves. But the original premise is returning liquid and vapor to a separation drum. This is not DX to my thinking.

  12. #112
    Join Date
    Mar 2006
    Location
    Philippines
    Age
    53
    Posts
    124
    Rep Power
    19

    Re: Liquid Overfeed Systems

    Hi,

    I based my condenser designed to 45C, you will have a very huge condenser if you based on 35C.
    Ambient is already 28-34C.

  13. #113
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Hi Dan,

    Here is why I thought the discussion took a different turn of events:

    Quote Originally Posted by ipelatak
    Do we still need a pump after the high pressure receiver...
    That to me says a Hysave system or something very similar. The only reason to have a pump after the high-pressure receiver is to boost the liquid pressure.

    The post that Mr. Bartlett made did not really have anything to do with a liquid overfeed system, other than it uses a refrigerant pump. Different system, different reason.

    Quote Originally Posted by Dan
    But the original premise is returning liquid and vapor to a separation drum. This is not DX to my thinking.
    I agree Dan. They did state they had a separator, and it certainly sounded like an overfeed system, but when they mentioned the pump after the high-pressure receiver, that is something completely different.

    I guess I made some assumptions too. I should have asked what type of condenser they are using. I assumed they were evaporative condensers, since it was an ammonia system.

    I just hope the students are beginning to understand this type of system. An overfeed system is not hard to understand, but it sounds like their professor has told them some conflicting information.

    I think we can get them to understand this, but I'm not sure if they have enough time before their school project is due.

  14. #114
    Join Date
    Aug 2006
    Location
    manila
    Age
    39
    Posts
    6
    Rep Power
    0

    Re: Liquid Overfeed Systems

    Hi to all,

    Sorry if we make some confusion in what we said in the previous posts.

    This is what we understand so far, pls correct us if we're wrong.

    New example:
    Ambient temp = 34C
    Condensing temp = 45C
    Evaporating temp = -10C

    The liquid refrigerant in the separator is at -10C. The refrigerant will be pumped by a ammonia centrifugal pump. The pump will supply more liquid refrigerant into the evaporator than the amount that boils off.

    The pump provides the pressure to transport the liquid refrigerant. The pump will increase the pressure of the refrigerant thus increasing the boiling point so
    we need a hand expansion valve to bring back the saturation pressure of the refrigerant.

    The liquid refrigerant will pass through the evaporator and leaves at the same temperature of -10C but a mixture of liquid and vapor. The mixture of the refrigerant will go to the Separator to separate the liquid refrigerant and the vapor refrigerant.

    The vapor refrigerant at -10C will be sucked by the ammonia compressor. The discharge pressure of the ammonia compressor will be the pressure of ammonia at the ambient temperature in the condenser.

    The refrigerant will be condensed at 45C by a water-cooled condenser. The refrigerant still under high pressure leaves the condenser and passes to a high-pressure receiver where it is stored for later use for supply in the separator.

    We expansion device after the high-pressure receiver to feed the liquid into the separator to maintain the liquid level. The liquid comes from the reciever at 45C, goes through the expansion valve, into the separator.

    what are the valves we need and where to place them? sorry out professor didnt taught us how
    to do this.
    Uhm.. if you have some free time, can you pls tell us how liquid overfeed works?

    In getting the mass flowrate in the system

    mass flowrate of refrigerant (latent heat of vaporization ammonia) = Heat added by the
    evaporator

    Sizing of compressor
    Suction pressure = Psat at -10C, ammonia
    Discharge pressure = Psat at 34C(ambient temp), ammonia

    work of compressor = m(delta h), mass flowrate (enthalpy at 34C - enthalpy at -10C)

    Sizing of condenser
    Condensing temp = 45C
    Heat rejected = work of compressor + heat added by the evaporator

    Cooling tower
    Air temp in = 32 C
    temp of water in reservoir = 34C
    temp of water entering condenser = 34C
    temp of water leaving condenser = 50C

    water flowrate
    water flowrate (sp. heat water) (50C - 34C) = Heat rejected by condenser

    Sorry if we ask so many, thanks for all your help and thanks for spending time with us. thanks masters.

  15. #115
    Join Date
    Oct 2001
    Location
    Nottingham UK
    Posts
    5,733
    Rep Power
    51

    Re: Liquid Overfeed Systems

    It's not one of my strong points (ammonia) but some of this needs explaining to be able to understand.

    The discharge pressure of the ammonia compressor will be the pressure of ammonia at the ambient temperature in the condenser.
    This equates to 34C?

    The refrigerant will be condensed at 45C by a water-cooled condenser.
    The liquid comes from the reciever at 45C, goes through the expansion valve, into the separator.
    How does the ammonia vapour condensing at 34C get up to 45C?

  16. #116
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by ipelatak
    New example:
    Ambient temp = 34C
    Condensing temp = 45C
    Evaporating temp = -10C

    The liquid refrigerant in the separator is at -10C. The refrigerant will be pumped by a ammonia centrifugal pump. The pump will supply more liquid refrigerant into the evaporator than the amount that boils off.

    The pump provides the pressure to transport the liquid refrigerant. The pump will increase the pressure of the refrigerant thus increasing the boiling point so
    we need a hand expansion valve to bring back the saturation pressure of the refrigerant.

    The liquid refrigerant will pass through the evaporator and leaves at the same temperature of -10C but a mixture of liquid and vapor. The mixture of the refrigerant will go to the Separator to separate the liquid refrigerant and the vapor refrigerant.
    OK, so far... Except, the ambient temperature does not matter if you are using water-cooled condensers. I want you to clarify the condenser type. Is it a shell & tube condenser, or a plate heat exchanger?

    The discharge pressure of the ammonia compressor will be the pressure of ammonia at the ambient temperature in the condenser.
    No. The discharge pressure of the compressor is the condensing pressure (pressure at 45C) plus the discharge line pressure loss. Ambient dry bulb temperature only matters if the condenser is air-cooled.

    The refrigerant will be condensed at 45C by a water-cooled condenser. The refrigerant still under high pressure leaves the condenser and passes to a high-pressure receiver where it is stored for later use for supply in the separator.

    We expansion device after the high-pressure receiver to feed the liquid into the separator to maintain the liquid level. The liquid comes from the reciever at 45C, goes through the expansion valve, into the separator.
    That part is OK.

    sorry our professor didnt taught us how to do this.
    He should have. If he did not teach this, then what is he doing?

    The mass flow calculations are a little complicated for an overfeed system. It is different in various parts of the system.

    In general terms, the liquid supply to the separator provides sufficient liquid to maintain the level. The refrigerant that boils off in the evaporator is this amount.

    The refrigerant pump circulates about 300-400% this amount to the evaporator. The 300-400% mass flow is flowing through the liquid line from the pumps, to the evaporator, and back to the separator.

    I think you are getting the idea.

  17. #117
    Join Date
    Jan 2006
    Location
    Croatia
    Age
    68
    Posts
    2,299
    Rep Power
    31

    Re: Liquid Overfeed Systems

    Hi,
    Quote Originally Posted by ipelatak
    what are the valves we need and where to place them? sorry out professor didnt taught us how
    to do this.
    Uhm.. if you have some free time, can you pls tell us how liquid overfeed works?
    I think the best for you is to visit those links:

    http://www.danfoss.com/Asean/Products/Categories/
    http://www.danfoss.com/Pacific/Busin...+Conditioning/

    here you can find all products you need for refrigeration or AC plant. There you can find also some brochures and so on...

    Here you can find almost all you need valves, systems&vessels, accessories

    http://haphillips.com/
    http://haphillips.com/products.html?pc=67#77


    Pls, don't show that to your professor

    Hope this will help to finalize your project on time

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

  18. #118
    Join Date
    Aug 2006
    Location
    manila
    Age
    39
    Posts
    6
    Rep Power
    0

    Re: Liquid Overfeed Systems

    Quote Originally Posted by us iceman
    OK, so far... Except, the ambient temperature does not matter if you are using water-cooled condensers. I want you to clarify the condenser type. Is it a shell & tube condenser, or a plate heat exchanger?
    shell and tube condenser.

    Quote Originally Posted by us iceman
    No. The discharge pressure of the compressor is the condensing pressure (pressure at 45C) plus the discharge line pressure loss. Ambient dry bulb temperature only matters if the condenser is air-cooled.
    thanks

    Quote Originally Posted by us iceman
    The mass flow calculations are a little complicated for an overfeed system. It is different in various parts of the system.
    for mass flowrate, pls correct us if we're wrong
    the flow will be the same at the separator to pump, pump to hand ex to evaporator to separator.

    In separator (vapor) to compressor to condenser to high-pressure liquid reciever.

    High-pressure receiver to ex valve to separator.

    Quote Originally Posted by us iceman
    In general terms, the liquid supply to the separator provides sufficient liquid to maintain the level. The refrigerant that boils off in the evaporator is this amount.
    aahh so this is the flowrate in the high-pres receiver to separator? thanks

    Quote Originally Posted by josip
    Pls, don't show that to your professor

    Hope this will help to finalize your project on time
    thanks for the link
    haha dont worry, we wont
    Thanks alot masters, ur our savior. thanks for spending time with us and for helping us. can we ask again if we have a problem somewhere again in our project? thanks master us iceman, master josip and to all
    Last edited by ipelatak; 25-08-2006 at 03:16 AM.

  19. #119
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by ipelatak
    for mass flowrate, pls correct us if we're wrong
    the flow will be the same at the separator to pump, pump to hand ex to evaporator to separator.
    Here is how the mass flow rate works.

    From the refrigerant pump to the hand expansion valve, to the evaporator, and back to the separator the mass flow is higher. This is where the mass flow will be about 3 or 4 times higher than the other part of the system.

    The mass flow from the separator (gas) to the compressor, to the condenser, to the receiver and out to the hand expansion valve on the separator (maintaining the liquid level) is based on the mass flow that boils off in the evaporator, PLUS...

    The flash gas that is generated when the 45C liquid flashes off down to 10C, which is being fed into the separator to maintain the liquid level.

    So, what you have coming out of the separator as gas (back to the compressor) is the flash gas + the gas boiled off in the evaporator coils.

    The excess liquid that is not boiled off in the evaporator, just simply recirculates. Someone may say you need to recirculate more liquid than 3-4 times what boils off.

    I say you can design the system for less, so you do not have to pump as much liquid. We will save that discussion for another time.

    It sounds like you guys are getting close to being finished. I hope you get a big A++ on your project.

    Best Regards,
    US Iceman

  20. #120
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    The flash gas that is generated when the 45C liquid flashes off down to 10C, which is being fed into the separator to maintain the liquid level.

    So, what you have coming out of the separator as gas (back to the compressor) is the flash gas + the gas boiled off in the evaporator coils.
    how can we calculate the gas that flashed in the expansion valve??? we're computing it for our compressor capacity...

    we're still confused in getting the compressor capacity because we have computed it using the enthalpy differencence between the discharge and suction times the mass flowrate(we assumed that the refrigerant that will vaporize in the evaporator is 25% of the overfeed massflowrate of the refrigerant that enters the evap. we havnt included the mass of the flash gas) is this correct??

    BTW our overfeed is 400%

  21. #121
    Join Date
    Aug 2001
    Location
    NE Lincolnshire, UK
    Posts
    145
    Rep Power
    23

    Re: Liquid Overfeed Systems

    I'm stuck without my refrigerant tables but using steam as an example, I think this should help if I remember this right (sounds of scraping around in dusty corners of brain....)

    Flash gas is the excess heat energy caused by the reduction in pressure. So for steam condensate:

    7 bar = 721kj/kg
    0 bar = 419kj/kg
    Excess = 302kj

    So the percentage of flash gas is:

    (excess/enthalpy of evaporation kj @ 0 bar) x 100%

    (302kj/2258kj) x 100% = approx 13%, or 13kg of gas and 87kg of liquid.

    (Enthalpy of evaporation is the term that is now used for latent heat).

    So if you know your mass flow in kg through the regulator, you should be able to work out the amount of flash gas your compressor needs to deal with.

    Steve

  22. #122
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi to all,

    pls correct me if im wrong... what we did is, we calculate the percentage or quality by this equation: h= hf+xhfg
    this equation is from thermodynamics.. are we correct???

    also how can we measure the mass of the gas that leaves the evaporator?

    do the latent heat ammonia vary in different temp???

  23. #123
    Join Date
    Aug 2001
    Location
    NE Lincolnshire, UK
    Posts
    145
    Rep Power
    23

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer
    what we did is, we calculate the percentage or quality by this equation: h= hf+xhfg
    this equation is from thermodynamics.. are we correct???
    It is straight thermodynamics so the answer should come from the formula I posted up.

    Quote Originally Posted by corineramer
    also how can we measure the mass of the gas that leaves the evaporator?
    If you know the amount of liquid you are supplying to acheive a refrigerated affect, this volume of liquid will expand to vapour when it hits the evaporator.

    In simple terms, you supply say 0.05 kg/second of liquid to the evap and it will expand to produce say 0.05m3/hr of vapour. That vapour density at the evaporating pressure can be found in your refrigerant properties table.

    This means your compressor will need to deal with 0.05m3/hr from the evap plus flash gas generated at the hand regulator.

    Quote Originally Posted by corineramer
    do the latent heat ammonia vary in different temp???
    The answer to that should be in your refrigerant tables: http://www.engineeringtoolbox.com/ammonia-d_971.html

    Steve

  24. #124
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer
    also how can we measure the mass of the gas that leaves the evaporator?
    That's easy. The evaporator mass flow for the gas is based on the evaporator cooling capacity divided by, hg-hf (this is also listed as hfg in the refrigerant tables)

    Where,

    hf = liquid enthalpy at evaporating pressure/temperature

    hg = vapor enthalpy at evaporating pressure/temperature

    hfg = latent heat at evaporating pressure/temperature

    Quote Originally Posted by corineramer
    do the latent heat ammonia vary in different temp???
    If you look at the tables, you will find your own answer to this.

    Now, if you calculate the refrigerant mass flow into the separator, you need to use the conditions that exist for the compressor.

    hf = liquid enthalpy at condensing pressure/temperature

    hg = vapor enthalpy at evaporating pressure/temperature

    When you find the mass flow for one kW (or 1 Ton) using the above NRE (Net Refrigerating Effect), hg-hf, you will find this mass flow is higher than the mass flow for the evaporator (for a 1 kW or 1 Ton basis).

    The difference between the two mass flows (compressor mass flow minus evaporator mass flow - gas only) is the flash gas.

  25. #125
    Join Date
    Aug 2006
    Location
    manila
    Age
    39
    Posts
    6
    Rep Power
    0

    Re: Liquid Overfeed Systems

    Hi to all,

    Example

    evaporator cooling capacity = 178.2637 KW
    Temp of ammonia in separator = -10C
    Condensing temp of ammonia = 40C

    For evaporator mass flow for gas
    From ammonia table at -10C
    hf = 135.4 KJ/kg
    hg = 1433 KJ/kg
    hfg = hg - hf = 1433 - 135.4 = 1297.6 KJ/kg
    Mass evap gas = (178.2637 KJ/sec)/1297.6 KJ/kg
    = 0.13738 kg/sec

    For compressor mass flow for gas
    From ammonia table at 40C (condensing temp)
    hf = 371.9 KJ/kg
    From ammonia table at -10C (evaporating temp)
    hg = 1433 KJ/kg
    hfg = hg -hf = 1433 - 371.9 = 1061.1 KJ/kg
    Mass comp gas = (178.2637 KJ/sec)/1061.1 KJ/kg
    = 0.16800 kg/sec

    Mass comp gas = Mass evap gas + Mass flash gas
    Mass flash gas = Mass comp gas - Mass evap gas
    = 0.16800 - 0.13738
    = 0.03062 kg/sec

    Pls correct us if we're wrong
    In getting the mass flowrate with overfeed of 400%
    = mass evap gas x 4
    = 0.13738 kg/sec x 4 = 0.54952 kg/sec
    In getting the volume flowrate with overfeed
    from ammonia table at -10C
    vf = 1.5338 L/kg
    Vol. flowrate = 0.54952 kg/sec x 1.5338 L/kg
    = 0.84285 L/sec

    In getting the work of compressor, Wc
    Suction pressure, Ps = Psat at -10C
    From ammonia table at -10C
    Ps = 2.908 Bar or 290.8 KPa
    hg = 1433 KJ/kg
    Discharde pressure, Pd = Psat at 40C
    From ammonia table at 40C
    Pd = 15.54 Bar or 1554 KPa
    hg = 1473.3 KJ/kg
    Wc = Mass comp gas x (hg at 40C - hg at -10C)
    = 0.16800 kg/sec (1473.3 KJ/kg - 1433 KJ/kg)
    = 6.7704 KW
    pls correct us if we're wrong.. thanks masters

  26. #126
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi to all,

    i have a question in our ageing process... we have to maintain the temp of our product to 4C... we're confused in getting the load because there's no temp difference... but we're to maintain this temp... how can we get the load in order for us to select the equipment.. we will use a water chiller in maintaining this temp... pls help, tnx

  27. #127
    Join Date
    Aug 2001
    Location
    NE Lincolnshire, UK
    Posts
    145
    Rep Power
    23

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer View Post
    hi to all,

    i have a question in our ageing process... we have to maintain the temp of our product to 4C... we're confused in getting the load because there's no temp difference... but we're to maintain this temp... how can we get the load in order for us to select the equipment.. we will use a water chiller in maintaining this temp... pls help, tnx
    Load comes from heat. Your product will heat up by respiration (biological activity) and heat will leak into the cooled space (heat transmission). Your refrigeration should carry away this heat. Heat will also come from any lights, fans, or people in the cooled space. Even if you load your product in at 4c, it will warm up eventually.

    You have to work out this heat load and this information is readily available in text books and on the internet.

    In simple terms, you have to work out the equipment needed to carry away this heat and from this you get your equipment requirements. Look for water chiller suppliers and download their information. A water chiller will be rated in kW and you need to supply it with enough refrigerant and enough water to move the heat.


    Steve

  28. #128
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer
    we're confused in getting the load because there's no temp difference...
    Steve mentioned where the heat comes from for your load. The remaining issue to determine is the desired temperature range for the water chiller flow and the evaporating temperature of the chiller.

    If you use a high temperature range (larger TD for the water), the flow rate will be smaller (smaller water pump).

    A lower temperature range (smaller TD for the water), the flow rate will be much higher (larger water pump).

    Depending on what you select for the supply chilled water temperature to the coil(s) and the return water temperature (from the coils), you are finding the TD required. This is a balance between the requirements of the cooling process and the costs of the equipment (operating and initial equipment costs)

    When you have the temperature range and flow rate established, then you can select the evaporating temperature for the chiller.

    Once this is completed, you have the load and the evaporating temperature, so now you can add this to the refrigeration capacity requirements and select the chiller.
    Last edited by US Iceman; 29-08-2006 at 02:37 PM. Reason: added text

  29. #129
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    im selecting the the compressor using the grasso site that you gave... what does refrigerating capacity at 2940 min^-1 mean??? min raised to -1, what does it stand for?
    Last edited by corineramer; 30-08-2006 at 11:18 AM.

  30. #130
    Join Date
    Aug 2001
    Location
    NE Lincolnshire, UK
    Posts
    145
    Rep Power
    23

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer View Post
    hi us iceman,

    im selecting the the compressor using the grasso site that you gave... what does refrigerating capacity at 2940 min^-1 mean??? min raised to -1, what does it stand for?
    Means revs per minute (rpm) as in say 160m3/hr of refrigerant @ 2940 rpm

    Steve

  31. #131
    Join Date
    Mar 2006
    Location
    Merate (LC) - Italy
    Age
    52
    Posts
    2,554
    Rep Power
    24

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer View Post
    hi us iceman,

    im selecting the the compressor using the grasso site that you gave... what does refrigerating capacity at 2940 min^-1 mean??? min raised to -1, what does it stand for?
    min^-1 means 1/min
    so 2940 min^-1 is 2940 / min or 2940rpm

  32. #132
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer
    what does refrigerating capacity at 2940 min^-1 mean???
    Others have addressed the engineering notation for the 2940 min^-1, so I believe we can move past this point.

    However, I did want to clarify the statement itself.

    When the Grasso literature says capacity at 2940 RPM, are they stating the kW (refrigeration) at 2940 RPM? Or, does the literature list cubic meters per hour?

    Capacity is kW (or Tons), not m^3/hr. You may have already seen this, but I wanted to point out the fact that you must read the small notes sometimes hidden in the technical information.

    The other issue that sometimes gets overlooked is the use of subcooling. Some manufacturers will use subcooling on ammonia compressors to inflate the performance values quoted.

    Very seldom will you find subcooling from the condenser in an ammonia system, IF THE CONDENSER IS PIPED CORRECTLY.

    Capacity is capacity, but you have to be careful on how the manufacturer states you will get that capacity at a specific operating condition.
    Last edited by US Iceman; 30-08-2006 at 02:46 PM. Reason: text editing

  33. #133
    Join Date
    Mar 2006
    Location
    Merate (LC) - Italy
    Age
    52
    Posts
    2,554
    Rep Power
    24

    Re: Liquid Overfeed Systems

    Quote Originally Posted by US Iceman View Post
    Capacity is kW (or Tons), not m^3/hr. You may have already seen this
    Generally, capacity is directly proportional to swept displacement and (that is the same) to rotational speed.
    2940rpm is the synchro speed for 50Hz asynchronous three-phase 2-poles motors (50/s x 60 s/min = 3000/min), but at 60Hz it is more likely to be 3520 rpm, with a net increase in capacity of 20%.
    Which is why capacity is specified together with the swept volume or rotational speed.
    Last edited by NoNickName; 30-08-2006 at 02:57 PM. Reason: Clarity of terms

  34. #134
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by NoNickName
    Generally, capacity is directly proportional to swept displacement and (that is the same) to rotational speed.
    Not always... If the screw compressor speed is slowed down below about 50% of the full speed, the capacity drops off very quickly.

    In general terms though, I agree with what you said. But in no way did I intend to make a point that the capacity does not change with speed.

    corineramer, here is a little equation to help you with this...

    (Hz X 120)/ number of motor poles = nominal RPM

    Here is an example:

    (50 Hz X 120)/4 pole motor = 1500 RPM

    (60 Hz X 120)/4 pole motor = 1800 RPM

    (50 Hz X 120)/2 pole motor = 3000 RPM

    (60 Hz X 120)/2 pole motor = 3600 RPM

    The actual full load RPM will be slightly less than this due to slippage of the rotor in the stator. I thought this might help you understand the relationship with motor speed, Hertz, and motor construciton.

  35. #135
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi,

    thanks for that info... us iceman where can i get a clearer drawing of the evapco horizontal recirculator system?? we need it for our lay out, we're gonna use cad in drawing it... right now im selecting the equipments and following the procedure in the brochures but i dont know if im doin it right.. hehehe.. tnx again for the big help

  36. #136
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Hi corineramer,

    Try this as an example to start with. This example is for an open drive refrigerant pump, not a hermetic pump.
    Attached Files Attached Files

  37. #137
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    thank you so much sir... u have contributed a lot to our work... just a little push and where nearly finished..

  38. #138
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    I'm glad to help you guys.


    Push, push, push.... Did that help you get the project done?

  39. #139
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    yup!! nearly, we dont know bout the piping because there are a lot of things that we dont know yet, like the inclination of hte pipes the oil traps that must be "u" shape.. i saw it in the brochure.. we're not famaliar with that but will solve it, hehehe also do u have the price of surge drum, cooling towet and evaporative condenser of the EVAPCO??
    Last edited by corineramer; 01-09-2006 at 05:47 PM.

  40. #140
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer
    we dont know bout the piping because there are a lot of things that we dont know yet, like the inclination of hte pipes the oil traps that must be "u" shape...
    What oil traps are you talking about?

    Quote Originally Posted by corineramer
    also do u have the price of surge drum, cooling tower and evaporative condenser of the EVAPCO??
    Sorry, that is one area I can't help you with.

    But, it is nice to hear you are making progress. Keep up the good work.

  41. #141
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    yup, we also managed to get a hold of an ASHRAE copy for the sizing of the pipes, hehehe... we'll have to read it...

    do u know where can we find a tunnel freezer for ice cream and filling machine?? i didnt like the one that we got because im not sure about the sanitary and its second hand.. its for the quality.. as for the filling machine we need to fill them in our container... our ice cream does not come on sticks and wrapper.. its in bulk cans..

    thank you so much! rock on!!!\m/

  42. #142
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Quote Originally Posted by corineramer
    do u know where can we find a tunnel freezer for ice cream and filling machine??
    Sorry, I won't be much help there either. Josip or Andy may have some ideas, but since I have a very limited exposure to ice cream making I just don't know who the manufacturers are.

  43. #143
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    ok tnx.. i'll just pm them...

  44. #144
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    do u know something that can help us in piping our ammonia system??

  45. #145
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    What is your question? I will try to answer them...

  46. #146
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    for the suction diameter of the compressor and outlet diameter of the low pressure receiver are different... we dont know how to size them? our equipments have different diameters.. how can we pipe them? by using the larger diameter or the smaller or you can calculate for it??

  47. #147
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Always calculate the pipe size you need. The connections put on the equipment is the manufacturers attempt at balancing cost versus requirements.

    When you are designing the system you have to know the operating conditions for each piece of equipment.

    Let's look at an evaporator...

    If the evaporator is selected for a specific evaporating temperature, then you will have some pressure loss (due to flow) to the next piece of equipment. The difference in pressure between these two components is determined by the mass flow and pipe size.

    If the pipe is too small, you will have a higher pressure loss.

    If the pipe is larger (for the same mass flow), the pressure loss will be reduced.

    The pressure at the end of this pipe will affect the performance of that device.

    I never recommend using the connection size as the required pipe size.

    You do have to connect the pipe to the two components, so what you want to find is the required pipe size based on allowable pressure loss (this is what you use for your system design and equipment selections).

    After you know the required pipe size, you will probably find that you will need some pipe reducer fittings to make the terminating connections.

  48. #148
    Join Date
    Aug 2006
    Location
    philippines
    Posts
    24
    Rep Power
    0

    Re: Liquid Overfeed Systems

    hi us iceman,

    how can we compute for the amount of ammonia in the system??? what im trying to do is get the volume of the low pressure receiver and then multiply it with the density of ammonia and try add them up.. is this right?

  49. #149
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    Volume of system X density = mass

    You have to be careful if the volume you use is either gas or liquid (or both in a two phase line).

  50. #150
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: Liquid Overfeed Systems

    corineramer,

    How is your project developing? Are you guys done yet?

Page 3 of 4 FirstFirst 1234 LastLast

Similar Threads

  1. Suction gas <-> liquid line heat exchange and performance gains
    By DaBit in forum Technical Speculations
    Replies: 30
    Last Post: 18-05-2010, 01:52 PM
  2. ammonia suction piping risers
    By Volnei in forum NH3
    Replies: 53
    Last Post: 20-04-2009, 11:03 PM
  3. Liquid line for multi evaporators
    By Kevin Yeo in forum Supermarket Refrigeration
    Replies: 1
    Last Post: 29-12-2005, 03:29 PM
  4. someone please help!
    By dormy in forum Fundamentals
    Replies: 14
    Last Post: 10-01-2005, 09:43 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •