# Psychrometric Chart problem

QUESTION
In navigating the AHU cycle ( summer) through the psychrometric chart I was always using the method whereby the Room Ratio Line ( RRL) was plotted on the chart to pass through the ‘Room’ point (state) and then the actual ‘Supply Air’ point would essentially be where the RRL crosses the line drawn from the ‘off cooling coil’ air state.
Simply put, on a hot and humid day the AHU is likely to go through sensible cooling, dehumidification (cooling below DP) and some sensible reheat to arrive at the required ‘Supply Air’ state.
My problem is the situation where the latent loads are zero or very small.
The ratio between the room sensible and total load is in that case =1 so the RRL is practically horizontal. The RRL will never cross the sensible heating line drawn from ‘off cooling coil’ state therefore the ‘Supply Air’ state cannot be found.
Where am I going wrong?
The above goes for the winter cycle also.

I’ve drawn the process on a psych chart:

When the air is cooled down to about the coil temperature (7 °) it ‘drops’ below the RRL. Unless I just stop dropping the air temperature at the point where RRL crosses the saturation line ( around 10° C on the chart). The 10° C supply temperature appears low and outside of recommended band compared to room temperature.
Any latent lads would increase the required coil capacity but would also angle the RRL a bit and let me pick the supply temperature 2-3 deg hire (…with some reheat)

REPLIES

GT-EGR
You are looking at the problem as if it is just a recirculating unit. You have to take your room air conditions and mix it with the quantity of outside air that you are saying is hot and humid. Then you’ll see that the coil has to dehumidify to reach that supply air temperature you need.

Looking at your chart, for some reason you are over cooling your cooling coil. The leaving air temp should be where your dashed line RRL intersects.

With the way you have it drawn, since you over cooled from your coil (went further from your RRL intersectional), you over dehumidified, and your space will exist at a lower RH, and will need some reheat to offset the extra cooling you get from having your cooling coil discharge a lower temperature.

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=466618
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