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 I have is a vendor that has selected a system for me and given me a quote. He has speced out all of the thigns liek the flow rate, the system overal BTUH and he has given me the room temperature and humidities that he says are possiable given specific outdoor air conditions.

So what Iam tryign to do is figure out if what he gave me works. So I am using the formulas to basically find the supply air temperature and humidity.

However since I know that there is moisture being added to the air via the people in the room I need to factor that into everyhting.

So if I get a guy that tells me the best indoor air condition he can get me when it is 100F and 80%RH outside is an indoor temperature of 80F @ 90%RH then I need to figure out what he is saying. I need to try to figure out what the supply air temperature is.

See if moisture was not being added then you could eaisly find the disscharge air temperature cause it should basically be a horizontal like from the end point (or room condition) right??? But when moisture is added you don't jsut go horizontally on the chart but you also go up. So you end up with a diagnoal line on the psyh chart. What I need to do is determine the length of that line or better yet determine the amount of moisture added lbw/lba so that I can figure out what the supply temperature really is.

It soudns confusing but I don't know how better to explain it.

I know that my supply air will be at aroudn a 95% RH. But I don't knwo what it is cause I need to look at the moisture added to the room and then go diagnally from the end condition of 80F @ 90%RH to the supply temp @ 95%RH....
In order to do that I need to figure out the actual moisture added and subtract it out. I jsut don;tknow how to do that. I know how to find my moisture added in lb/min but I do nto know how to relate that to the chart to find my moisture addition.

 

[jasno999]

For example let me simplify some.

When lookign at the psyh chart if you want to cool and dehumidify the following should occur. Please let me know if I am incorect in my thought process.

YOu start at a temperature and humidity. As you cool you move horizontally to the left on the chart until you hit the dew point line. The cooling doen in this phase is sensiable and deals on with cooling of the air temperature.

Now you have hit the dew point line you will start to ride the dew point line down and to the left until you reash your disscharge temperature out of your evaporator.

At that point you will have a disscharge temperature at around 95%RH coming out of the evaporator. This air is then heated back up as it enters the room and mixes with the room air. However since you are not addin any moisture in this part of the process the movment shoulw be on the horizontal line from left to righ across the chart, starting at your disscharge air temperature and humidity. You then go right until you hit the room temperature that is desired (Vertical line) and that is point at wich you can then see what your room humidity is.

Therfore if a supplier tells me that they can maintan the room temperature and humidity at 76F and 80%RH then that means to me that their supply air temperature must be around 69F.

But I think that I am missign somethign here. It confuses me - because if that is what I am told the room air conditions will be then I have to make those assumptions. But that makes little to no sense to me cause it means the disscharge air temperature is extreamly warm...

 

[jasno999]

I think I came to a conclusion and that is I am viewing the problem wrong. In reality when supply air coming from an evaporator enters a room you actually have a mixing process going on right?

Again I am not sure how to figure out the supply air temperature and humidity content.

If I know the following:
- Overall Room Temperature
- Overall Room Humidity
- Outdoor air temperature
- Outdoor air humidity
- Evaporator fan flow rate (lb./hr)
- Room Structural Load
- Room Human Load
- Room Solar Load
- Room electrical Load
- Room Infiltration Load

Then how do I figure out what the required supply air temperature and humidity are out of the evaporator?

I sort of understand the SHR. However you need to know your total heat load along with how much of it is related to latent load and sensiable load.

Now I am guessign that the total heat load would be the addition of the structural, electric, solar, human and infiltration loads. IS THAT CORRECT?

Now how would I be able to figure out what portion of that load is sensiable and what portion is latent?

If you can't do that without knowedage of the supply air temperature to begin with then is there another method to solve for the supply air temperature and humidity.

I actualyl own the Trane Air Conditioning Manual. I have been readign it but I am still confused. I have been lookign at your explanations and tryign to make them work as well. But the problem is no matter what I do it is liek I am missing a variable or the equations I have don't work or have a factor missign cause I don't know the supply air temperature and humidity.

I understand that the supply air to room air is a mixing process really but I just can't figure out how to work it all out. I have seriously been rackign my brain for the past week and a half tryign to figure this out. I have reached a lot of dead ends. I need soem more help before my brain goes wacky on me from frustration.

 

[jasno999]

Note my humidities listed above that are known are measured in %RH

 

[Yorkman]

Jasno999,
Sorry that I haven't been responding been out of town. Go to Trane Air conditioning Manual: Chapter V
Example 5-26; Pg 99 that is a pretty close representation of your problem if I understand it. The sensible heat loads are structural loads, equipment loads, some %infiltration load, and some %occupant load. The latent loads are moisture generated by cooking, washing, coffee pots, a % of the infiltration load and a % of the occupant load. (An ASHRAE Fundamentals manual can be helpful in determining these values). These two loads combined is your total heat load in BTU/Hr for the space. Take the sensible heat load in BTU/Hr and divide it by the total heat load to get your SHR. Use the sensible heat protractor and generate the SHR slope for the space load. Plot the desired Db temperature and % Rh for the space. Transfer the SHR line from the desired space temperature point to the 95% Rh curve on the chart. The intersection point of the 95% Rh and the SHR line will give you the required supply air Db temperature @ 95% Rh. Use the formula
BTU/Hr = 1.08 x CFM x (T1-T2) with the known sensible heat load for BTU/HR, T1 as the space temperture and T2 as supply temperature, you can now solve for the CFM.

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

 

[Yorkman]

I've posted this as a FAQ in the HVAC section I don't guarrentee it will make any more sense over there, but it may help.%-)

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

 

[jasno999]

No I really appreciate the help Yorkman. I have only been out of college for 6 years not but when you never use something for a period of time that long you foget stuff. And truthfully I am not sure how many realy world problems we did in college.

I think I understand what you are saying so today I wil lcrack the book back open and try to resolve this problem and see what I come up with. Again I am sort of doign this backwards because I am not really tryign to design the system I am just tryign to make sure that the design given to me will work. But I guess the best hting I could do is pick some points that I woudl liek to get to and then solve the equations and find the CFM required and compare it against what I am receiveing from my vendors.


I think it was a simple point that I was missing for the SHR. So now that you explained that to me I think I can use it. I still do nto fully understand it however. I knwo that as you add cold wet air to a room a few thigns happen. As that air heats up it becomes less humid. However at the same time you are mixing with room air that has a given temperature and humidity. YOu are also adding moisture via occupant sweat and infiltration. But my latent loading from the infiltration and occupants is only goign to be a small portion of the total heat load.

Let me work the problem and see what I come up with.

I may have some follow up questions about the evaporator loading after I get this portion figured out. You had helped me soem with that part already so I will go back to your previous posts but if I get stuck I will post some more questions.

Again I really appreciate it. YOur help is really fantastic. Wish there were more people like yourself out there. (This whole thing has got me thinking maybe I should take a class at one of the local colleges to refresh and learn more about the equations behind HVAC)

 

[Yorkman]

Jasno99,
I think you got it, the SHR line in reality shows the mixing process of the supply air with the rooms latent and sensible heat loads to arrive at the desired room condition. Because you used the rooms sensible and latent heat loads to find the SHR line, when you select your desired room condition and use the SHR line to find the required supply air condition, it should be a perfect match. The problem can get a bit more involved when the SHR line is steep (reflecting a high latent load in the room) because it does'nt intersect the 90-95%Rh line making the supply air unreasonably cold and usually requires some form of reheat. I believe Quark mentioned that point earlier in the thread take a look. Also check out the FAQ in the HVAC forum I posted a small point on finding the load on the coil due to mixed air condition. Goodluck

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

 

[jasno999]

Ok here is a good question.

If you are solving for your CFM supply rate how do you find items such as your infiltration latent and sensiable loading and how do you get your latent human loading.

see those equations can only be used if you know the following:

- % of recirculation air
- Infiltration of outdoor air flow rate (CFM)
- Humidity Ratio (grains/lb) of indoor air

Are you supposed to make assumptions for thee items? You need to know them so that you can get to the answers but it seems like each equation is based on or depends on the answer to other equations. So how do you figure it all out if all you know is your structural loads, electrical loads, solar loads but not your flow rates or internal conditions?

 

[jasno999]

Ok I used the formulas and tried to figure the whole thing out.

What I did was take several outdoor air conditions and I made some assumptions. For example I assumed a % for return air. I assumed a relative humidity of 60% for the room and know I needed to maintain that with an 80F temperature. I aso made an assumption for leakage.

What I found was that different conditions provided different loading requirments. Makes sense. The other thing that I found was that as ,y temperature increased so did the flow requirment that I was calculating.

Now here is the question. You said you need to solve for CFM but my CFM is different for each different condition. So in the real world I would think that you would select one CFM number (probabbly a lower CFM if possiable) and then use that for all the conditions.

What that meas is you set your CFM for all conditions and instead of solving for it you then need to use it to solve for your supply air disscharge temperature. Is that correct???

I see no other way of doing it but if you do it this way then you are throwing out the whole idea of the SHR and how that works on the psyh chart...

I have a little confusing in my mind about this.

 

[Yorkman]

Yes that is correct. The SHR is calculated at design load. Using this to find the design CFM, that means that it will meet the max. "reasonable" load your going to see. Some days you will be at design load, some days you will be less than design load and every once in a while your load will be over design. That's why G-D made thermostats! Most of the time you won't be running at design load in a comfort cooling application. Designing the system to meet a 100% design criteria means your paying for equipment that won't be run at that level, what maybe 70% of the time? The trick is finding a balance that wiil meet the load 97-99% of the time and still being able to justify the cost. That's why you guys get paid the big bucks!
By the way where are you located? A room design temperature of 80 degrees and 60 %Rh seems a bit high. Most customers I know start screaming when the space temp hits 76-77 degrees 45-50% Rh!

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

 

[jasno999]

Ha ha. I used to work for an HVAC company but never did loading calculation. I was a controls person. So I know that side of the buisness but never got involved with the actual engineerign side.

The project I am talking about is unique and it is not a comfort cooling application. I do realize that 80F would be hot for people in an office building - trust me when you are in controls the majority of your complaints are for people being too ho or too cold. But I am sure you know that.

Again I appreciate the help. I think I am on the right track now. I at least feel a lot more confident about my calculations and the numbers I am coming up with. I am sure a few more questions might pop up and when they do I will post them - but again your help has been tremendous.

Thanks

 

[jasno999]

HEre is another interesting question for you.

One thing I noticed is that manufactures will give you two different values for heat transfer.

They give you the condenser heat rejection value (BTU/hour) and they also give you the system cooling value in (BTU/hour).

The numbers that I am calculating for the evaporator Q - are they supposed to correspond with the overall system cooling load or do they correspond with the load of heat rejection at the condenser?

The heat transfer rates at the condenser are larger than those given for the overall system cooling in my documents. I need to know what to compare the number I am calculating against.

 

[Yorkman]

Glad I could help,
If you post these type of questions (HVAC) in the HVAC section you may get better attention, and a faster response.

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

 

[Yorkman]

The condensers' rate of heat rejection has to be higher than the system cooling load. Remember that for every 12,000 BTU/Hr picked up in the evaporator the condenser must reject about 15,000 BTU/Hr (or more on hermetic compressors) due to the heat of compression. This is the price of doing work on the gas. When figuring system load I would go with the evaporator Q values, with a package unit the condenser is sized to reject the additional heat of compression. On a built up system were you are selecting the condenser a quick discussion with the supplier should lead you to the correct condenser size, it will probably reject 15,000-17,000 BTU/Hr for every 12,000BTU/Hr picked up in the evap.

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

 

[quark]

Jasno999,

I hope, by this time, you got answers to all your question by well articulated suggestions from Yorkman. Regarding cfm, you have to choose the maximum cfm you have got out of various requirements.

We in Pharma, know the cfm well in advance before starting any psychrometric calculations as the no. of air changes per hour is critical.

Yorkman,

Excellent work. The ADP is apparatus dew point (generally referred to as dew point).

To all,

I sincerely feel that the efforts and knowledge of yorkman should not go unappreciated. We have stars handy with us, why wait?

 

[Yorkman]

Quark, Thanks.

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

 

[Welchita]

By the way, does anybody knows about available dynamic Psychrometric Chart program?

Thanks in advance.
welch

 

[Yorkman]

Is dynamic part of the title? Or is that a type of feature your looking for?

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

 

[jasno999]

How do I give somebody a star???

Also are there charts out there that represent the Psychrometric chart that I could use. My one fear is that if I read the enthalpy numbers incorrectly (or any number on the Psychrometric Chart)it can greatly affect my calculations. I noticed that even small error can greatly affect the final number. So I would rather do some interperlation on a chart and get my numbers closer to exact than just use my eye and a ruler on the Psychrometric chart.