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What "inlet air temperature" does the coil frost/freeze warning in OS/Eplus refer to?

asked 2018-10-23 13:27:20 -0500

jbatt gravatar image

updated 2018-12-27 10:14:21 -0500

The following 3 warnings seem to always come together. I've bolded the items that I am particularly confused about. My understanding of the warning is that there is a risk of frost on the condenser based on the temperature of the air coming off of the condenser. Scrolling down, you can find reference to an outdoor air temperature and an inlet air temperature (both bolded). I am confused as to why the inlet air to the condenser would not be roughly equal to the outdoor air temperature. Also, the inlet air temperature is significantly warmer than the outlet air temperature. This makes no sense to me - is the inlet air temperature here somehow referring to the evaporator coil (it is roughly what I would expect the return air temperature to be)? If so, what does this have to do with frost risk? Thanks.

* Warning * CalcDoe2DXCoil: Coil:Cooling:DX:SingleSpeed="1 SPD DX CLG COIL" - Air volume flow rate per watt of rated total cooling capacity is out of range at 2.479E-005 m3/s/W. * ~~~ * Environment=RUN PERIOD 1, at Simulation time=01/02 07:00 - 08:00 * ~~~ * Expected range for VolumeFlowPerRatedTotalCapacity=[2.684E-005--6.713E-005] * ~~~ * Possible causes include inconsistent air flow rates in system components, * ~~~ * or variable air volume [VAV] system using incorrect coil type.

* Warning * CalcDoe2DXCoil: Coil:Cooling:DX:SingleSpeed "1 SPD DX CLG COIL" - Air-cooled condenser inlet dry-bulb temperature below 0 C. Outdoor dry-bulb temperature = -5.60 * ~~~ * ... Occurrence info = RUN PERIOD 1, 01/02 07:00 - 07:03 * ~~~ * ... Operation at low ambient temperatures may require special performance curves.

* Warning * CalcDoe2DXCoil: Coil:Cooling:DX:SingleSpeed "1 SPD DX CLG COIL" - Full load outlet air dry-bulb temperature < 2C. This indicates the possibility of coil frost/freeze. Outlet temperature = -1.24 C. * ~~~ * ...Occurrence info = RUN PERIOD 1, 01/02 07:09 - 07:12 * ~~~ * ... Possible reasons for low outlet air dry-bulb temperatures are: This DX coil * ~~~ * 1) may have a low inlet air dry-bulb temperature. Inlet air temperature = 21.582 C. * ~~~ * 2) may have a low air flow rate per watt of cooling capacity. Check inputs. * ~~~ * 3) is used as part of a HX assisted cooling coil which uses a high sensible effectiveness. Check inputs.

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answered 2018-10-23 13:35:29 -0500

Inlet air temperature is at the inlet node entering the coil. The last warning here says the inlet air temperature entering the coil is 21.6C. So for the coil outlet temperature to be lower than 2C, the air flow through the coil might be lower than expected. Normal DX coil delta T is about 25 F or 14 C. If air flow is lowered, the delta T will increase. This is just a warning telling you to investigate why the outlet temperature is so low. Given a fairly reasonable inlet air temperature of 21.6 C (70.8 F) I suspect air flow is low, but that is a guess at this point. It would have been nice for that warning to also tell you the air flow per ton for help diagnosing the warning.

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The nature of my confusion is that I am unsure as to whether the warning is referring to the condenser or evaporator. The initial warning suggests we are looking at the condenser (though intuitively, the 21C speaks to it being the evaporator coil).

If it were the evaporator coil though, I should see this very very outlet temperature in my supply air, right?

jbatt gravatar imagejbatt ( 2018-10-23 13:40:48 -0500 )edit

Each warning is independent of the other. First talks about evap air flow to evap capacity ratio, second talks about condenser entering air temp and third talks about evap entering and leaving temps (with mention of air flow and configuration with HXs).

rraustad gravatar imagerraustad ( 2018-10-24 08:20:09 -0500 )edit

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Asked: 2018-10-23 13:27:20 -0500

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Last updated: Oct 23 '18