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  1. #1
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    Condenser sizing question

    Hi,

    I have a question about sizing a condenser.

    When sizing a condenser, should we consider only an operating point at which compressor and evaporator match (working point), or should we also take into account that the suction pressure might be bigger after defrost cycle and therefore the condenser should also have a greater capacity?
    Thank you.

    Best regards,
    Bob

  2. #2
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    Hi,

    I am not an expert, actually I am new in this forum but from what I know the condenser must be able to handle the required value of heat rejection. The heat rejection is the sum of evaporator capacity and the compressor work.

    And I think it should be sized at the highest possible suction pressure because in this area is when the capacity is the highest possible thereby resulting a higher need for heat of rejection.

    All of that is theory, but I think it is the basis of selection.

  3. #3
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    A simple "load Calc" will tell how many BTU's are needed. A proper "load calc" will also take into consideration the required box RH% due to the product that is in the box. A AC/R system does not get sized by the "hot pulldown/start up"conditions because the comp/cond unit will then be oversized when the box/house is at its normal run conditions,which it cannot get to due to the oversized components.

  4. #4
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    Quote Originally Posted by BobFrost View Post
    Hi,

    I have a question about sizing a condenser.

    When sizing a condenser, should we consider only an operating point at which compressor and evaporator match (working point), or should we also take into account that the suction pressure might be bigger after defrost cycle and therefore the condenser should also have a greater capacity?
    Thank you.

    Best regards,
    Bob
    The short answer is that the condenser is typically selected at the normal design conditions for saturated suction and condensing temperatures (SST/SCT). The post-defrost pulldown is accounted for by decreasing the condenser TD (SCT - Ambient Temp), typically for low temperature systems where the post defrost pulldown load is most significant.

    In supermarket refrigeration for example, medium temperature system are selected at a 15F TD whereas low temperature are selected at a 10F TD, which effectively increase the condenser capacity by 50%.

    A side-benefit of the lower TD and lower condensing temperature is higher overall system efficiency for the low temperature system.

  5. #5
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    Hi, thanks everyone for the answers.

    Quote Originally Posted by icemeister View Post
    The short answer is that the condenser is typically selected at the normal design conditions for saturated suction and condensing temperatures (SST/SCT). The post-defrost pulldown is accounted for by decreasing the condenser TD (SCT - Ambient Temp), typically for low temperature systems where the post defrost pulldown load is most significant.

    In supermarket refrigeration for example, medium temperature system are selected at a 15F TD whereas low temperature are selected at a 10F TD, which effectively increase the condenser capacity by 50%.
    Could we also say that we oversize it in this way?

    Also, why is the LT condenser sized for lower TD?

    A side-benefit of the lower TD and lower condensing temperature is higher overall system efficiency for the low temperature system.
    Are you referring to the COP?

  6. #6
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    Quote Originally Posted by BobFrost View Post

    Could we also say that we oversize it in this way?
    Yes, decreasing the condenser TD is the same thing as oversizing the condenser.

    Quote Originally Posted by BobFrost View Post
    Also, why is the LT condenser sized for lower TD?
    The TD is deceased to increase the condenser capacity.

    Quote Originally Posted by BobFrost View Post
    Are you referring to the COP?
    COP (Coefficient of Performance) is a way of expressing efficiency, so yes.

  7. #7
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    Quote Originally Posted by icemeister View Post
    The TD is deceased to increase the condenser capacity.
    Sorry icemeister, I didn't ask my question well. English is not my native language, but I guess you noticed that


    You wrote:

    medium temperature system are selected at a 15F TD whereas low temperature are selected at a 10F TD
    so I am wondering why is the LT condenser sized at 10F TD and MT at 15F TD?

  8. #8
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    Quote Originally Posted by BobFrost View Post
    Sorry icemeister, I didn't ask my question well. English is not my native language, but I guess you noticed that

    so I am wondering why is the LT condenser sized at 10F TD and MT at 15F TD?
    Traditionally, low temperature condensers are sized at a 10F TD to oversize them for the higher post-defrost load and for the higher efficiency form the lower condensing temperature.

    Your English is excellent. Much better than many for whom it's their mother tongue.

    Where are you from? I'm a US native in Ecuador, living here for 6 years trying to learn Spanish. It's much more difficult than I expected.

  9. #9
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    Quote Originally Posted by icemeister View Post
    Traditionally, low temperature condensers are sized at a 10F TD to oversize them for the higher post-defrost load and for the higher efficiency form the lower condensing temperature.
    Thanks. Makes sense.

    I am from Europe and I was looking for a literature and couldn't find any definitive guidelines. Usually from what I see, condensers are sized for TD = 10 K (TD = 18 F) and even up to TD = 15 K (27 F)!. Hence the confusion when I read that some of you folks mention 10 F TD (5.5 C) or 15 F TD (8.3 C).
    Some condensing unit manufacturers undersize condensers on their units so much that their compressors regularly burn out during summer periods.

    Your English is excellent. Much better than many for whom it's their mother tongue.
    Thanks.

    Where are you from? I'm a US native in Ecuador, living here for 6 years trying to learn Spanish. It's much more difficult than I expected.
    I am from a small European country. I'll PM you where I am from.

  10. #10
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    OT: Is there a chance to send a personal message on this board?

  11. #11
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  12. #12
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    Not quite

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