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How to Measure Sensible vs. Latent Capacity for AC Equipment

lance_p | Posted in Energy Efficiency and Durability on

Looking at the latent vs. sensible capacity for air conditioning equipment, the basics are well covered but the specifics are a little confusing:

If an AC unit is rated 80% sensible and 20% latent, what happens when it’s operating in a dry environment?  Does the sensible load increase beyond 80%?  If so, does the efficiency (SEER rating) also increase?

I’m trying to understand how this works.  I know units with higher SEER ratings generally operate with higher coil temperatures and lower latent capacity to increase efficiency.  There must be a specific set of operating conditions this is tested in, but I cannot find this information.

Dehumidifiers are rated using 80F and 60% RH (portable units at 65F and 60% RH) test conditions, for example.

Thanks!

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    I guess if you want the exact details, you can purchase the standard:

    https://www.csagroup.org/store/product/C656-14/

    From what I gather, the SEER rating is tested with an 80F 50%RH room.

    In terms of your question, if you are not removing moisture, the effective SEER rating will be higher. This point might be moot as most high SEER units get their super rating by cranking up the airflow to the point that there is almost no latent removal.

  2. walta100 | | #2

    The numbers you read in the spec sheets are calculated from test units in a laboratory under very controlled conditions measured by a third party. They are only useful for comparing the perfect example of brand X to the perfect example of brand Y and the numbers are not reputable in the field.

    You can measure the wet and dry bulb temperature of the air entering and exiting the unit and with some math arrive at a number. Since the air temp and humidity both indoors and outdoors is very unlikely to match the requirements for the standard measurement your numbers can’t be compared to the spec sheets numbers.

    https://www.contractingbusiness.com/archive/article/20865137/practical-standards-to-measure-hvac-system-performance

    Walta

  3. charlie_sullivan | | #3

    Some equipment will have extended data tables in the back that show the SHR for different operating conditions, including temperature, humidity, airflow, and compressor power if it's a variable or multi-stage unit.

    Yes, operating in a dry enough environment, any unit will go to 100% sensible. However, that will not make the COP go up. The "sensible COP" (sensible cooling per unit power input) will go up, but the quoted COP is total cooling per unit power input. That could actually get worse in a dry environment, because for the same room temperature and air flow rate, the air will be cooled to a lower temperature. That means the coil will run at a lower temperature, increasing the "lift" needed to pump the heat from that low temperature to the high outdoor temperature.

    Summary of COP effects: If the unit causes a high SHR by running the coils at a high temperature, the result is high COP because of the high coil temperature. If the environment causes a high SHR, that causes the coils to run at a lower temperature, making COP worse.

  4. DC_Contrarian | | #4

    The unit doesn't do sensible and latent cooling separately. They both happen when air contacts a cool surface, in this case an evaporator coil. The amount of each that happens depends upon the temperature and humidity of the incoming air, and the temperature of the coil. The temperature of the surface will vary, it settles where the amount of heat being removed from the air balances the amount of heat being removed from the coil, which is determined by the amount of air flow and the amount of refrigerant flowing in the coil.

    All other things being equal, the colder the coil the lower the SHR. A low SHR is therefore a proxy for a low coil temperature.

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