Energy Cost and Faster Cooling

[roelescence]

I always argue with the manager to start the chiller and let it cool down near set point and below before starting the AHUs . I wanted to explain it to him why I always insist with my procedure to minimize energy cost and get faster cooling. If I am right, please help me explain. If I am wrong, please tell me why.

[dkemper]

I'd suggest running a few "time to temp" tests monitoring amp draw. Track power usage and how long it takes to pull temps down to set point.

Your technique should result in a longer pull down time, but with a lower instantaneous heat load so should use less peak power, but over a longer period of time.

Your manager's technique should result in a shorter pull down time, but the compressors will work harder to get there using higher peak power, but for less time.

If your condensers have no trouble rejecting the heat from the faster pull down, you may find that the power usage is only slightly higher and with a shorter time period, may be more efficient.

Or, if your condensers do have issues rejecting the heat and your head pressures climb fairly high during the pull down, then gradually stepping up the heat load may well be the more efficient path.

Either way, it's best to prove it with data.

[chemi-cool]

you are wasting electricity if you just let the chiller run without heat load.

You don't gain anything. The unit must be designed for the heat load so its better if the chiller is running, use it to cool the air and not the AHU.

[roelescence]

I'd suggest running a few "time to temp" tests monitoring amp draw. Track power usage and how long it takes to pull temps down to set point.

Your technique should result in a longer pull down time, but with a lower instantaneous heat load so should use less peak power, but over a longer period of time.

Your manager's technique should result in a shorter pull down time, but the compressors will work harder to get there using higher peak power, but for less time.

If your condensers have no trouble rejecting the heat from the faster pull down, you may find that the power usage is only slightly higher and with a shorter time period, may be more efficient.

Or, if your condensers do have issues rejecting the heat and your head pressures climb fairly high during the pull down, then gradually stepping up the heat load may well be the more efficient path.

Either way, it's best to prove it with data.

The pull down takes 30 mins to 1.5 hrs if there are only two AHUs running and it takes 4.5 hours to 5 hours with 6 AHUs running. Average RLA is 95% both