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Psychrometric Chart problem

Psychrometric Chart problem

Psychrometric Chart problem

Greetings All,
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? Can’t see what am I missing here!
The above goes for the winter cycle also.
Kind Regards

RE: Psychrometric Chart problem

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.

RE: Psychrometric Chart problem

Can you draw the process on a psych chart? If I am understanding, you are basically looking for sensible only cooling, which would be a horizontal line. As GT-EGR said, you probably have ventilation air adding a latent load, but let's ignore that for now. I am not understanding why your RRL line and "off coil" do not cross.

RE: Psychrometric Chart problem

Hi BronYrAur

I uploaded the chart with the process shown. Basically, 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)


RE: Psychrometric Chart problem

Thanks GT_EGR,

It's mostly re-circulation with some fresh air in - I uploaded the chart above.


RE: Psychrometric Chart problem

Your process line will not hug the saturation curve due to coil bypass. Supply air temp should be at the intersection of the RRL line and the line drawn from the Mixed Air Temperature to the Appratus Dew Point. Unless I'm missing something, it seems pretty straight forward.

RE: Psychrometric Chart problem

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 overcooled 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.

RE: Psychrometric Chart problem

That "over cooling" is the apparatus dew point. Not the actual leaving air temperature. The required leaving air temperature should be where are the rrl line intersects.

When drawing the modifications I made to the Chart, I left the apparatus dew point as had originally been shown. That may not necessarily be the correct point. The coil bypass is the difference between apparatus dew point and actual leaving air conditions from the coil.

RE: Psychrometric Chart problem

I read the original post as more of a theoretical psychrometric question, I think bringing coil bypass into the equation is going to complicate beyond what the original post was asking. I don’t disagree with you at all though.

RE: Psychrometric Chart problem

Many thanks - both of you.
BronYrAur - yes you saw correctly what I had in mind.The 'real' cooling process does intersect the RRL. I, sort of, overlooked that.
Will have to stop drawing it along the saturation line - only doing it to show the physics behind the drop in moisture content.


RE: Psychrometric Chart problem

I am with GT-EGR on this one. Cooling down to 7C to offset bypass indicates the bypass percentage as 15 which is usually high (mixed condition is 17C so, coil bypass air at 17C should bring the cooled air at 7C to 10C).

If the SHR line runs in a nonintersecting way to the saturation curve then cooling the air to required temperature, to reduce moisture and reheat, as suggested by GT-EGR. If it is purely sensible load now, cool air down to 10C.

RE: Psychrometric Chart problem

If you cool down the air to 7 C (which is very cold, you might want to check your air distribution) and throw the air into the space, assuming the space load analysis is correct, you will end up colder and dryer than your estimated return air temperature conditions, so this point is not correct. Since you are overcooling the space your options are to reheat or to reset the supply air temperature (assuming constant volume system).

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