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  1. #1
    Join Date
    Sep 2010
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    miami,fl.
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    624

    evap. temp drop vs humidity

    after doing some research for refrigeration equipement the higher the evap temp drop is the more humidity its capable of removing! not sure if that's true. I think of it this way. the higher the temp drop the faster it will cool in turn running less and removing less humidity from the air. if I picture a basic ac system that's running with a 15 degree split will need to run longer than a unit operating with a 20 degree split to satisfy the thermostat setpoint, in turn removing more humidity. maybe for ac its backwards one guy told me I cant compare it to an ac system. anyways im trying to grasp it better on the refrigeration side of things if it makes a difference.

  2. #2
    Join Date
    Dec 2012
    Location
    columbus, OH
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    2,039
    You need less td across evaporator and larger coil for more humidity.

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,446
    Quote Originally Posted by anthonyac1 View Post
    after doing some research for refrigeration equipement the higher the evap temp drop is the more humidity its capable of removing! not sure if that's true. I think of it this way. the higher the temp drop the faster it will cool in turn running less and removing less humidity from the air. if I picture a basic ac system that's running with a 15 degree split will need to run longer than a unit operating with a 20 degree split to satisfy the thermostat setpoint, in turn removing more humidity. maybe for ac its backwards one guy told me I cant compare it to an ac system. anyways im trying to grasp it better on the refrigeration side of things if it makes a difference.
    Your are missing a couple facts. Yes, colder coils remove more moisture, but cool slower. Therefore run longer. Compressors have lower capacities at lower coil temperatures.
    It takes 20-30 mins. to load a coil/pan with moisture. The moisture left on the coil/pan at the end of the cooling cycle will evaporate back into the home. The key is have long cooing cycles with 30^F split to provide <50%RH in the space. During low/no cooling loads, a dehumidifier is a more practical of providing <50%RH in the space.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
    Join Date
    Dec 2012
    Location
    columbus, OH
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    I believe anthonyac1 is talking about refrigerators.

  5. #5
    Join Date
    Dec 2012
    Location
    columbus, OH
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    A walk in cooler will have a large coil and high velocity air flow to maintain 10* TD and 85% humidity. A reach in cooler with a relatively smaller coil and less airflow will have a 20* TD and 65% humidity.

  6. #6
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,370
    I believe Core_d has the correct answer to your question.

    For refrigeration evaporators, the amount of moisture removal hinges on the TD, or the box temp minus the evaporator temperature. A greater TD will result in a lower RH in the box and vice versa.

    Remember, the box RH is done at the equipment selection stage. Once you match up a given evap with a given condensing unit for a given box temperature, the RH is basically fixed, and cannot be changed without rematching equipment.

    A typical walk-in evaporator is rated for a specific capacity at a nominal 10F TD. Let's say a coil is rated at 10,000 Btuh at a 10F TD. At an 8F TD it would have a capacity of 8000 Btuh. At a 15F TD it would do 15,000 Btuh. Each one, if selected for a corresponding load would result in a different final box RH.

    We're more concerned with the total Btuh load and the final RH than we are how much the latent load is or how much moisture is actually removed. So once you select for the total Btuh with the proper coil TD, you're essentially done.

    Typical air conditioning coils aren't rated the same as refrigeration coils. They are rated based on AHRI standard conditions when match with specific condensing units and give you the resulting sensible and latent capacities so you can then properly match them up with your design load. To satisfy both the sensible and latent loads you may have to change the equipment matchups, vary airflows, etc to accomplish that.

  7. #7
    Join Date
    Sep 2010
    Location
    miami,fl.
    Posts
    624
    Quote Originally Posted by icemeister View Post
    I believe Core_d has the correct answer to your question.

    For refrigeration evaporators, the amount of moisture removal hinges on the TD, or the box temp minus the evaporator temperature. A greater TD will result in a lower RH in the box and vice versa.

    Remember, the box RH is done at the equipment selection stage. Once you match up a given evap with a given condensing unit for a given box temperature, the RH is basically fixed, and cannot be changed without rematching equipment.

    A typical walk-in evaporator is rated for a specific capacity at a nominal 10F TD. Let's say a coil is rated at 10,000 Btuh at a 10F TD. At an 8F TD it would have a capacity of 8000 Btuh. At a 15F TD it would do 15,000 Btuh. Each one, if selected for a corresponding load would result in a different final box RH.

    We're more concerned with the total Btuh load and the final RH than we are how much the latent load is or how much moisture is actually removed. So once you select for the total Btuh with the proper coil TD, you're essentially done.

    Typical air conditioning coils aren't rated the same as refrigeration coils. They are rated based on AHRI standard conditions when match with specific condensing units and give you the resulting sensible and latent capacities so you can then properly match them up with your design load. To satisfy both the sensible and latent loads you may have to change the equipment matchups, vary airflows, etc to accomplish that.
    A typical walk-in evaporator is rated for a specific capacity at a nominal 10F TD. Let's say a coil is rated at 10,000 Btuh at a 10F TD. At an 8F TD it would have a capacity of 8000 Btuh. At a 15F TD it would do 15,000 Btuh. Each one, if selected for a corresponding load would result in a different final box RH.
    so with a higher td the unit is capable of removing more humidity, Is it because with a higher temp difference the coil is a lot colder than the dew point (not sure if that's the right term) causing more moisture to condense on the evap coil in turn lowering the humidity in the box. sorry to go back and forth with this post but im trying to understand the theory behind it so I can really see why.

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