Psychrometric Chart help with Air conditioning. 3

[jasno999]

This may sound like a silly question but I have totally forgotten how to use the Psyh- chart to follow a air conditioning system problem.

For example if I start with an outdoor condition of 95 degrees F and 80% Relative Humidity and I mix that air with return air that is at 80 degrees F and 50% relative humidity. The mix is 80% Return air and 20% supply air.

I then take that mixed air and run it thru a evaporator to cool it down to a disscharge temperature of 36 degrees F.

My question is how to properly do this but more importantly how to graph this on a Psychrometric Chart. I am confused as to how the lines would appear. I thought that in cooling you went horizontally from right to left on the chart and then followed the due pointline down to your disscharge temperature. As you did that you would lose water at the evaporator. Then when you got to the line associated with 36 degrees F at 100% realtive humidity you stoped going down and then moved horizontally to the right on the chart until you readed your room tempererature which in my case is 80 degrees F. But I don't knwo if that is the right way to do it or not.

I am confused right now and need soem explanation.

 

[jasno999]

What it comes down to is the fact that I do nto see anywhere that my latent load for the room is used in any of my equations???

I mean I use equations to find my latent load associated with infiltration air and I use equations for fidn the latent load due to human sweat.

However I never use the latent load anywhere in any of the future equations. Somehting is wrong with that cause that has to be factored itno the evaparator load but it is not.

In the end the only things that affect evaporator loading in my worksheet are the mixed air and supply air conditions. Mixed air in my situations is jsut a mix of outdoor air and return air. But return air is being defined by the suppliers as a temperature and humidity they are telling me they can maintain.

I need to factor my latent loads into this worksheet somebow and the calculations that oy uhave given me don't do that.

What do I do?

 

[Yorkman]

Some of these numbers are way off the map, 102 degrees, 100%Rh. And I'm having a hard time keeping up with your train of thought. Several statements caught my eye, for example;
#1. "I realize that the evaporator is supposed to be cooling the air and getting rid of that moisture but I am confused as to how I lose moisture as my supply air gets heated back up."
When you cool hot moist air you remove sensible and latent heat (de-humidify), when sensible heat is added to the supply air due to space loads it's temperature is raised this process line will be a straight horizontal line. The humidity ratio remains the same, and the %Rh will drop, under normal conditions.
#2. However I have that one condition that is a high temperature and a high humidity. Say 102F and 100%RH -

When I do the math I get a discharge temperature of around 64F.

However I was told the room conditions could be maintained at 80F and 50% RH. When you go and plot this on the psyh chart you get a line that slopes downward from left to right. Meaning somehow between the supply air and the room air we are losing humidity???? That is not possible cause we should be gaining humidity here.

You are correct, it is not possible. I think it’s possible that you have exceeded the design of the coil Or the high temperature and high humidity have made SHR to steep. To find out what will actually happen, plot your SHR line from the 64 degree 95 %Rh supply air condition up and to the right and where it intersects with the 80 degree DB temperature line. Ideally it should intersect at 80 degrees 50 %Rh. I don't see that happening with your given conditions. This will be the room condition based on the supply air and space loads.
The Trane Air Conditioning manual has some examples on this type of problem and how to solve them starting in Chapter 5 page 106 through111. Take some time to read that and get familiar with what their saying. Examples 5-31 and 5-32 may answer some of your questions.


I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[jasno999]

I have been readign the book and tryign to figure this all out but I am just spinning my wheels at this point. I will look again tomorrow but I have been going nuts abotu this stuff.

I tried to break things down again. I was lookign at it from jsut a design standpoint. Jsut tking a CFM and using it to get all my other figures. And trying to set the equations so that everyhintg was affected by the cfm changes. I don't know. I am hoping by doign that I can look at my required evaporator Q for each temp/humidity range on outdoor air. Then I can see what kinds of Q i have at each condition.

I jsut don't know. I just can't tell is my sheet is workign right and if I have it configured correctly or not.

 

[quark]

Jasno,

I dare say you didn't try to understand the basics. Revisiting Yorkman's FAQ will help you once again.

With a discharge temperature of 64F (and nearly saturated), it is thermodynamically impossible, without including any other equipment in the system, to get to a control condition of 80F and 50%, even in the case of purely sensible room loads.

It is a superfluous design to consider the worst possible condition in selecting a refrigeration device. ASHRAE did these kind of studies with respect to the ambient conditions and you will get the details from weather data. If you are in a tropical region, selecting 1% conditions of WB and MCDB gives you a good design. If you are in a dry area, you can base your selection on either 0.4% of DB and MCWB (mean coincidental wet bulb temperature) or 1% of the values depending upon your willingness to spend.

I reiterate, again, that you can acheive the off coil temperature constructing your psychrometric process backwards. In simple terms, by drawing a line from room condition to the saturation curve with a slope of SHR.