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  1. #1
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    Measurement of Refrigeration Capacity


    Dear all,

    I've been tasked to evaluate the COP of an existing ammonia refrigeration plant and as a newcomer, I would like to seek advice on how I could actually proceed.

    Parameters which I would be able to obtain are the suction and discharge pressures of the compressor and the compressor motor power. I understand that I should measure the mass flow as well but unfortunately I do not have the tools to measure the mass flow of ammonia.

    I would probably be able to estimate the mass flow at full load capacity from the specifications based on the condensing and evaporating temperatures, but I'm not sure on how the mass flow would vary with capacity in a reciprocating compressor. would it be reasonable to assume the power is related to the capacity in a linear relationship?

    I understand my questions are amatuerish and I realize I have much to learn on refrigeration. Thus I would deeply appreciate if anyone could point me in the direction in which I could measure the refrigeration capacity / COP of ammonia compressor.


    Thanks.
    CK



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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by ck10 View Post
    Dear all,

    I've been tasked to evaluate the COP of an existing ammonia refrigeration plant and as a newcomer, I would like to seek advice on how I could actually proceed.

    Parameters which I would be able to obtain are the suction and discharge pressures of the compressor and the compressor motor power. I understand that I should measure the mass flow as well but unfortunately I do not have the tools to measure the mass flow of ammonia.

    I would probably be able to estimate the mass flow at full load capacity from the specifications based on the condensing and evaporating temperatures, but I'm not sure on how the mass flow would vary with capacity in a reciprocating compressor. would it be reasonable to assume the power is related to the capacity in a linear relationship?

    I understand my questions are amatuerish and I realize I have much to learn on refrigeration. Thus I would deeply appreciate if anyone could point me in the direction in which I could measure the refrigeration capacity / COP of ammonia compressor.


    Thanks.
    CK
    Hi This stuff is freely available in GOOGLE
    http://www.nt.ntnu.no/users/skoge/bo...oniaReport.pdf

    use it in future

    Norm
    My wife used to say you never listen to a word I say at least I think that what she said

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    Re: Measurement of Refrigeration Capacity

    Hi Norm,

    Thanks for the link. Digesting the information now.


    Best regards,
    CK

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by ck10 View Post
    Dear all,

    I've been tasked to evaluate the COP of an existing ammonia refrigeration plant and as a newcomer, I would like to seek advice on how I could actually proceed.

    Parameters which I would be able to obtain are the suction and discharge pressures of the compressor and the compressor motor power. I understand that I should measure the mass flow as well but unfortunately I do not have the tools to measure the mass flow of ammonia.

    I would probably be able to estimate the mass flow at full load capacity from the specifications based on the condensing and evaporating temperatures, but I'm not sure on how the mass flow would vary with capacity in a reciprocating compressor. would it be reasonable to assume the power is related to the capacity in a linear relationship?

    I understand my questions are amatuerish and I realize I have much to learn on refrigeration. Thus I would deeply appreciate if anyone could point me in the direction in which I could measure the refrigeration capacity / COP of ammonia compressor.


    Thanks.
    CK
    You should go to compressor manufacturer and ask for compressor engineering data.This is information about power use and capacities at different condensing and suction pressures.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by Toosh View Post
    Hi This stuff is freely available in GOOGLE
    http://www.nt.ntnu.no/users/skoge/bo...oniaReport.pdf

    use it in future

    Norm
    This is theoretical research. I don't think that it can be useful for practical application.

  6. #6
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    Re: Measurement of Refrigeration Capacity

    What are you cooling, is a liquid or are you chilling rooms.
    If liquid it is easy (flow*td*specific heat)/power in = COP, I would do this over a long period to give you an average.
    If load is not known, (rooms are variable) then install flow meter in liquid ammonia, measuring and recording system temps/pressures.
    Calculate nett refrigeration effect/power in. Again do over time to give averaged COP.
    Then base the actual verses therory.
    If large difference the possible comp problems (worn valves etc).
    Also are you including all other power users, water pumps, fans etc. This could give a wide swing in COPs especially in low load situations.
    It is all about time and money, the more you but in the better the result

  7. #7
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    Re: Measurement of Refrigeration Capacity

    I didn't see any plant with ammonia flow meters.
    Compressor problem. Discharge temperature will jump if discharge valves leak. Suction valves leak will reduce energy use. Compare current energy use at certain operating conditions with compressor engineering data from manufacturer.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by Segei View Post
    I didn't see any plant with ammonia flow meters.
    Compressor problem. Discharge temperature will jump if discharge valves leak. Suction valves leak will reduce energy use. Compare current energy use at certain operating conditions with compressor engineering data from manufacturer.
    You can get a flow meter for just about any material.
    To calculate COP of a system you need to know your load (the bit you are cooling) In rooms this is quite difficult (practically hugh variables) By knowing the actual mass flow of the refrigerant and the conditions it is at, you can calculate refrigeration effect, if you then remove system losses either by detail measurement or theoretical calcs you will end u with nett refrigeration effect. (how detailed to determins accuracy)
    Damage to compressor parts valves, rings etc, effect volumetric efficiency, which inturn effects actual mass flow, thus makes changes to power draw (COP)

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by mad fridgie View Post
    You can get a flow meter for just about any material.
    To calculate COP of a system you need to know your load (the bit you are cooling) In rooms this is quite difficult (practically hugh variables) By knowing the actual mass flow of the refrigerant and the conditions it is at, you can calculate refrigeration effect, if you then remove system losses either by detail measurement or theoretical calcs you will end u with nett refrigeration effect. (how detailed to determine accuracy)
    Damage to compressor parts valves, rings etc, effect volumetric efficiency, which inturn effects actual mass flow, thus makes changes to power draw (COP)
    Did you see any ammonia refrigeration plant with flow meters?
    Theoretically we can do everything but practically.... Typically ammonia refrigeration has a few evaporative temperatures. You need flow meter for every temperature. To compare apple and apple suction pressures and discharge pressure should be kept constant. Efficiency of the compressors should not be changed(no unloading for screw compressors). I didn't get about system losses. What do you mean?

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by Segei View Post
    Did you see any ammonia refrigeration plant with flow meters?
    Theoretically we can do everything but practically.... Typically ammonia refrigeration has a few evaporative temperatures. You need flow meter for every temperature. To compare apple and apple suction pressures and discharge pressure should be kept constant. Efficiency of the compressors should not be changed(no unloading for screw compressors). I didn't get about system losses. What do you mean?
    Yes seen mass flow devices on ammonia!
    These flow meters are normally coupled to computers and give real time readings, they also totalise (add up the total flow, like a water meter)
    If a single plant (common refrigeration system)
    then liquid flow is common, compressor suction pressure and temperature are common. You can calculate your nett refrigeration effect.
    How you record the data can determine if instantaneous performance is calculated or long term averages.
    If you have a LT pot and a HT pot, then you need 2 flow meters and 2 sets of temp/pressure equipment.
    Let the compressor unload. we are measuring all, not a problem.
    System losses, total change in enthalpy gives refrigeration effect, but not all the change is related to chiling the product/area.
    Such as pressure drop down suction line and heat ingress to the suction line, to what level you go to is determided by the mighty Dollar, Euro, Pound, Yen etc. ( I sorted stated this already)
    On a practical note the bigger the plant, the more worth while.
    The original writer did not give size or application, only wanted to know how to do it.
    He state that the comps were recips, so will have some for of wear and thus reduced performance.
    The more actual correct info you have the more accurate your result.
    I am certainaly not detracting the skill of a good engineer, I would rather have one of these than a computer. The combination complements each other.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by mad fridgie View Post
    Yes seen mass flow devices on ammonia!
    These flow meters are normally coupled to computers and give real time readings, they also totalise (add up the total flow, like a water meter)
    If a single plant (common refrigeration system)
    then liquid flow is common, compressor suction pressure and temperature are common. You can calculate your nett refrigeration effect.
    How you record the data can determine if instantaneous performance is calculated or long term averages.
    If you have a LT pot and a HT pot, then you need 2 flow meters and 2 sets of temp/pressure equipment.
    Let the compressor unload. we are measuring all, not a problem.
    System losses, total change in enthalpy gives refrigeration effect, but not all the change is related to chiling the product/area.
    Such as pressure drop down suction line and heat ingress to the suction line, to what level you go to is determined by the mighty Dollar, Euro, Pound, Yen etc. ( I sorted stated this already)
    On a practical note the bigger the plant, the more worth while.
    The original writer did not give size or application, only wanted to know how to do it.
    He state that the comps were recips, so will have some for of wear and thus reduced performance.
    The more actual correct info you have the more accurate your result.
    I am certainly not detracting the skill of a good engineer, I would rather have one of these than a computer. The combination complements each other.
    It looks like laboratory but not real life plant. Probably, this is one or two plants in the world that have flow meters. I are lucky that you saw one. It is costs money. Assume that we install flow meters and measured the power use. What is the next? To determine COP. OK we have it. What can we do about this COP? That is the goal of all this investments and measurements. To determine these compressors are good or bad. I'm sorry. I didn't get about the money.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by Segei View Post
    It looks like laboratory but not real life plant. Probably, this is one or two plants in the world that have flow meters. I are lucky that you saw one. It is costs money. Assume that we install flow meters and measured the power use. What is the next? To determine COP. OK we have it. What can we do about this COP? That is the goal of all this investments and measurements. To determine these compressors are good or bad. I'm sorry. I didn't get about the money.
    Not that expensive, but yes your are right it is a measuring device. This have generally been used to prove a systems performance, (penalty clauses on new installations, where the contractor designs a system and guarantess efficiency).
    Once in along with all other devices that measure you can see other failings such insulation, compressor, fouling. It is just a tool.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by mad fridgie View Post
    Not that expensive, but yes your are right it is a measuring device. This have generally been used to prove a systems performance, (penalty clauses on new installations, where the contractor designs a system and guarantess efficiency).
    Once in along with all other devices that measure you can see other failings such insulation, compressor, fouling. It is just a tool.
    I think that it is expensive toy(meters, installation, computers software and etc). How you can guaranty efficiency? Is this efficiency at design conditions? What about efficiency at operating conditions? There are cost effective ways to check insulation, compressors, fouling and etc.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by Segei View Post
    I think that it is expensive toy(meters, installation, computers software and etc). How you can guaranty efficiency? Is this efficiency at design conditions? What about efficiency at operating conditions? There are cost effective ways to check insulation, compressors, fouling and etc.
    When you quote a 15Mw job, clients want to know the running costs, you have design figures and you have installed figures. Of course it is near on impossible to run plant under steady state at design conditions. I think all understand that, so methods have to be undertaken to prove actual performance, other wise you state what ever you wanted in your tender, and when it came to performance testing you could say not perfect design condtions, so we will not guarantee our figures.
    Lets look at this job 15Mw for ease say stated COP 3
    this make 5000Kw power draw, lets say calcls are out by 1% and you use 5050Kw or 50Kw more than stated. over 20 years full time running that would be.
    9,125,000Kwhrs at $0.15 Kwhr comes to $1,368,750
    Is it now an expensive toy?
    Does it fix any thing "NO"
    You need to put the thread in context!
    How would you measure COP on older equipment?
    As far as your other comments there are lots of ways to check.
    Knowledge is gained by gathering information.

  15. #15
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    Re: Measurement of Refrigeration Capacity

    Dear all,

    Much appreciation for the responses and advice and please accept my apologies if I've caused any confusion with my lack of information.

    In brief, scope of interest is to evaluate the operating COP of existing compressors using ammonia as refrigerant. Parameters measured were the suction and discharge pressures of each individual compressor. Power consumed by each compressor was also measured.

    Ideally, as suggested by mad fridgie, is to install a flowmeter to measure mass flow of the ammonia from the common line. However, the intention is to have measurements over a few weeks period and the flowmeter would not be a permanent installation. Of course benefits from having accurate measurements would outweigh the cost of the flowmeter itself, but i'm afraid currently there's no provison for a flowmeter.

    I've explored using the tech specifications from the manufacturer as well, which is basically a catalog showing condensing temp @35deg C and evap temp @ -5 , -10, -15 and -20. Current operating pressures are close to design, however I'm unsure if the compressor is operating at full load. If I could address this concern then I would probably be able to compare the actual power consumed vs the design power to evaluate the performance of the compressors.

    Thus, it boils down to how I could estimate the refrigeration capacity without the use of flowmeter (hope I'm not asking for the impossible ). Would I be able to make assumptions on the mass flow derived from full load conditions (i.e. how mass flow varies with power, etc). Understand there's a lot of factors involved such as pressure drop losses, heat ingress, superheat, subcool, motor efficiency, age, wear and tear, volumetric efficiency), but I'm wondering if there's general rule of thumb or assumptions which would allow for a broader picture at a macro level for preliminary evaluation?

    Once again much appreciation for the advice and for taking your precious time.


    Thanks and best regards,
    CK

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    Re: Measurement of Refrigeration Capacity

    Yes use suction and discharge pressure and suction temp at compressor inlet and liquid temperature at the exansion device
    You can input this into your manufactures comp details,
    make note of power draw (and amps)
    Client maybe paying for KVA not kW
    Loading and unloading (force to 100% on controller) how manage your load to keep at required pressure maybe some what more difficult.
    If a recip then you can leave unloaded and reduce capacity by % unload (power use would be by % slightly higher (rotating mass etc)

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by mad fridgie View Post
    When you quote a 15Mw job, clients want to know the running costs, you have design figures and you have installed figures. Of course it is near on impossible to run plant under steady state at design conditions. I think all understand that, so methods have to be undertaken to prove actual performance, other wise you state what ever you wanted in your tender, and when it came to performance testing you could say not perfect design conditions, so we will not guarantee our figures.
    Lets look at this job 15Mw for ease say stated COP 3
    this make 5000Kw power draw, lets say calcls are out by 1% and you use 5050Kw or 50Kw more than stated. over 20 years full time running that would be.
    9,125,000Kwhrs at $0.15 Kwhr comes to $1,368,750
    Is it now an expensive toy?
    Does it fix any thing "NO"
    You need to put the thread in context!
    How would you measure COP on older equipment?
    As far as your other comments there are lots of ways to check.
    Knowledge is gained by gathering information.
    Let's talk about energy savings. I think that major energy savings should come from efficient operation of the refrigeration plant. Maximum energy savings can be achieved by operating plant at optimum set points and using optimum operating strategies.
    One example. Refrigeration plant has compressor power 2000 HP. It has compressor(low and high stage) VFDs, condenser fan VFDs, evaporator fan VFDs. Put flow meters and they will show that this plant has good COP. But..... This plant is operated at 150 psig(10 bars) condensing pressure all year around. If we take this plant without VFDs($200,000 savings) and lower winter condensing pressure to 120psig(8 bars), this plant will have the same(or better) efficiency than plant with VFDs. However, minimum condensing pressure can be lowered to 100 psig or lower. This is energy savings. This will be 10-20% not 1%. Optimum set points and optimum operating strategies is the way to improve efficiency.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by ck10 View Post
    Dear all,

    Much appreciation for the responses and advice and please accept my apologies if I've caused any confusion with my lack of information.

    In brief, scope of interest is to evaluate the operating COP of existing compressors using ammonia as refrigerant. Parameters measured were the suction and discharge pressures of each individual compressor. Power consumed by each compressor was also measured.

    Ideally, as suggested by mad fridgie, is to install a flowmeter to measure mass flow of the ammonia from the common line. However, the intention is to have measurements over a few weeks period and the flowmeter would not be a permanent installation. Of course benefits from having accurate measurements would outweigh the cost of the flowmeter itself, but i'm afraid currently there's no provison for a flowmeter.

    I've explored using the tech specifications from the manufacturer as well, which is basically a catalog showing condensing temp @35deg C and evap temp @ -5 , -10, -15 and -20. Current operating pressures are close to design, however I'm unsure if the compressor is operating at full load. If I could address this concern then I would probably be able to compare the actual power consumed vs the design power to evaluate the performance of the compressors.

    Thus, it boils down to how I could estimate the refrigeration capacity without the use of flowmeter (hope I'm not asking for the impossible ). Would I be able to make assumptions on the mass flow derived from full load conditions (i.e. how mass flow varies with power, etc). Understand there's a lot of factors involved such as pressure drop losses, heat ingress, superheat, subcool, motor efficiency, age, wear and tear, volumetric efficiency), but I'm wondering if there's general rule of thumb or assumptions which would allow for a broader picture at a macro level for preliminary evaluation?

    Once again much appreciation for the advice and for taking your precious time.


    Thanks and best regards,
    CK
    Compressor capacity you can get only from flow meters or from compressor manufacturer. What is the goal of your evaluation? You've got COP 3. Is it good or bad? COP will be diffferent at different operating conditions.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by Segei View Post
    Let's talk about energy savings. I think that major energy savings should come from efficient operation of the refrigeration plant. Maximum energy savings can be achieved by operating plant at optimum set points and using optimum operating strategies.
    One example. Refrigeration plant has compressor power 2000 HP. It has compressor(low and high stage) VFDs, condenser fan VFDs, evaporator fan VFDs. Put flow meters and they will show that this plant has good COP. But..... This plant is operated at 150 psig(10 bars) condensing pressure all year around. If we take this plant without VFDs($200,000 savings) and lower winter condensing pressure to 120psig(8 bars), this plant will have the same(or better) efficiency than plant with VFDs. However, minimum condensing pressure can be lowered to 100 psig or lower. This is energy savings. This will be 10-20% not 1%. Optimum set points and optimum operating strategies is the way to improve efficiency.
    I could not agree with you more, if every piece of refrigeration had just a small increase in efficiency, think of how big the energy saving would be world wide.
    In these hard times many(most) do not want to spend the money on these type of projects.
    The 1% was only an example, you have to prove performance, unfortunatley this has to be done by wide of instruments. As I stated the main use of flow meters was for one party to prove to another party actual results, more than presumed calculated results.

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    Re: Measurement of Refrigeration Capacity

    Quote Originally Posted by mad fridgie View Post
    I could not agree with you more, if every piece of refrigeration had just a small increase in efficiency, think of how big the energy saving would be world wide.
    In these hard times many(most) do not want to spend the money on these type of projects.
    The 1% was only an example, you have to prove performance, unfortunately this has to be done by wide of instruments. As I stated the main use of flow meters was for one party to prove to another party actual results, more than presumed calculated results.
    Unfortunately, many companies don't realize that optimization of a refrigeration plant operation is the best way to improve efficiency of every refrigeration plant. Just smart, efficient operation.

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    Re: Measurement of Refrigeration Capacity

    COP (HP/TR) is difficult to quantify and using manufacturing data only gets you close. In general, you can assign efficiency to the original system design (single stage two stage sub cooling belts etc .

    When I've been called to look at COP I look at the overall system design and installation. Bottom line is the cost to operate and maintain the equipment. Each industry has bench mark operating costs and what makes the owner or CEO happy is when they are below industry bench marks. Cost per cube or per pound however its tabulated.

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