Psychometric exercise

[Peter_1]

Does anyone has a copy of a complete psychometric theoretical exercise (Carrier chart) all plotted on the chart with a cooling, heating, humidifying and dehumidifying process?
I need it for classes and don't have much time to make one myself and.. it's a long time I made those calculations manually.

[US Iceman]

Peter,

I may have some old material (about 1960-70), but as you can probably guess it is in Imperial units.:o

[Peter_1]

That's no problem USIceman. It will be a good 'conversion excercise' for me
It's just that I need to be sure of the results.

The purpose of the exercise should be that we have an internal load with a given sensible and latent heat.
We need a constant fresh air intake of +/- 30%, in winter cold and dry (also need an electrcial heater to prevent cold battery from freezing) and in summer hot and humid, preheater, then a cooler, after-heater, filter

[Brian_UK]

Peter, this may save you some drawing time,try this 30day free trail software which draws up a chart quite easily.
http://www.linric.com/psycpro.htm

The file size is 19.3MB.

[Abby Normal]

trane has a nice one, laminated, it explains all the basic processes on the back with examples

[Abby Normal]

state all the parameters, I will give you a screen shot

[Lc_shi]

Hi peter,
Pls have a look if it can help you.
http://www.trane.com/commercial/software/index2.asp

regards
LC

[Peter_1]

I know how to use the chart, that's not the problem, I just need a real life exercise with numbers so that I'm sure the numbers I give are right.
In the Trane AC manual I have here in front of me are also some of these exercises.

[Abby Normal]

Some metric default examples from an old program I have

heating/humidification, note it is not quite at sea level

http://i32.photobucket.com/albums/d41/a_bee_normal/heating.jpg

A cooling dehumidification example, at sea level

http://i32.photobucket.com/albums/d41/a_bee_normal/Cooling.jpg

[chillerman2006]

Hi Peter
i have a psychometric programme from daikin - its 1.13mb. Is there a way to upload to the site for you or would you like me to email it to you
chiller2006

[Peter_1]

Thanks already for the input Abby normal, see you tookefforts to scan the pages for me.

Chillerman2006,can you try to upload it via www.megaupload.com or via my regular emailaddress [email]tld-nv@skynet.be[/email
Thanks .

[chillerman2006]

Hi Peter

i have just e-mailed it to you

chillerman2006

[Pooh]

Peter
I will have a look on monday what we have in the college for you and send it on to you.

Ian

[Josip]

Hi, Peter_1 :)

Is this of some help? Some charts for download;)

http://www.alder.co.za/down.htm

Best regards, Josip :)

[Dan]

Abby, in the distant past I had a grasp of what your charts show. I am interested in an explanation of how you plot the process. I recall the mixed air process being proportional to the overall cfm, but I am more than rusty on seeing the coil process and the room process completing the loop. Would it be possible for you to put into words what the cooling jpg is showing?

[Abby Normal]

Dan

The room point is the condition you want to maintain in the space.

The supply point is how cool and dry the air must be to compensate for the heat and humidity gains that are just created in the space. (or come in through the ceiling, exterior walls, the windows)

The return point shows that the room air is slightly warmed. Maybe it uses a plenum ceiling and the lights are heating up the air a bit.

The 'return' air and the outside air are mixed, the typical system is providing some fresh air. The lengthof the line segment between "return" and "Mix" is proportional to the outside air percentage. The length of line segment between "mixed" and "outdoor" is proportional to hown much return air is mixed with the outside air.

This process most likely is a draw through fan, and the fan and motor will heat the air. There could also be supply ducts in unconditioned space, so even if the duct work is insulated it will still pick up heat. Therefore the coil must over cool the mixed air down to a bit cooler than the required supply condition to compensate for the reheat.

The line segment between "leaving coil" and "supply" is reheat after the air is cooled. Could be a deliberate reheat or something you have limited controil over such as fan heat or ducts in unconditioned space.

To determine mixed air properties, plot the condition of the two air streams on a chart and connect with a straight line. The mixture will be on that line.

A quick way of determining the mixture, with reasonable accuracy is to do a weighted average of the dry bulb temperatures and the volumetric flows. Puts you in the ballpark.

Tmix= (T1 x Airflow1 + T2 x Airflow2)/(Airflow1 + AIrflow2)

The error for doing this based on volume and not mass is minimal usually. Once you calculated Tmix, locate where that dry bulb temperature cuts the mixing line you drew, and read off the wet bulb, the RH, the enthalpy etc.

[Abby Normal]

Thanks already for the input Abby normal, see you tookefforts to scan the pages for me.

Chillerman2006,can you try to upload it via www.megaupload.com (http://www.megaupload.com) or via my regular emailaddress [email]tld-nv@skynet.be[/email
Thanks .

You're welcome Peter, I do not work in metric much these days, so I just printed and scanned a couple default examples from an old windows 3.1 program I have.

The chart is easy a simple screen shot. To get all the state points and the process values I have to print and it usually takes a couple pages.

I live and breathe pyschrometrics, focussed mainly on the cooling end now, since I have been in the tropics for 8 years. Have not had to worry about humidifying for a long time.

Hardest one to teach is the humidifying, the ASHRAE chart and the 'energy protractor'

Trane sells an laminated one like I mentioned before, some one gave you a link to the electronic one in this thread. The laminated one has nice instructions and examples on the back. It is easy to read the dewpoints off of the trane chart too.

[Abby Normal]

Air conditioning, meaning heating, cooling, humidifying and dehumidifying all revolves around pyschrometrics.

It is the most important and the most ignored aspect of the HVAC industry.

[Dan]

Thank you Abby. That is exactly what I needed. I will be sharing this with a customer who asked me to show him the other day the effects of 100% outdoor air in a supermarket and I balked. Now I can do it.:)

[Abby Normal]

You're welcome Dan

[Peter_1]

Hi Peter

i have just e-mailed it to you

chillerman2006

Chillerman2006, got the file, thank you.
See most of the world is working with the Carrier chart. We in Belgium use mainly the Mollier chart. This is the same chart but the mirror view of the Carrier chart.http://www.ib-rauch.de/Beratung/mollier.gif

[Abby Normal]

The metric chart bases the enthalpy as compared to O degrees celsius, the IPS chart compares it to O degrees F.

Just makes for some simpler math, you start counting the specific heat of the air/water vapour from when the temperature and the specific heat mutliply out to zero.

[Peter_1]

The purpose of the exercise should be that we have an internal load with a given sensible and latent heat.
We need a constant fresh air intake of +/- 30%, in winter cold and dry (also need an electrical heater to prevent cold battery from freezing) and in summer hot and humid, preheater, then a cooler, after-heater, filter
To quote my own post: let's say that we have an internal load of 50 kW (170607 BTU/h or 14.21 TR) from which is 20% latent heat.
In the room are 30 peoples and local regulations states that there has to be a fresh air intake of 15m³/h/person (8.8ft³/min/person)
The RH inside the room must be kept at 65% and temperature 22°C (71.6°F)
Outside conditions can go in winter from -10°C/50% (14°F) and in summer to 35°C/80% (95°F)
Bypass factor of coils are all 0.2

Calculate:
needed cooling battery,
heating battery,
is a supplementary preheating or after-heating necessary,
how much moisture will be removed in the worst condition,
the humidification must be done with water kept at 10°C, as second case with water which takes the same wet bulb temperature as the air passing by, with water kept at 50°C, with steam at 110°C (230°F)
how much supplementary energy will be needed in the most humid situation to reheat the air to the desired room condition,

I'm sure that this is a difficult exercise for many amongst us, especially because this isn't real refrigeration but belongs anyhow to the basics.

[Abby Normal]

Pick a realistic summer outdoor design and I will give you a screen shot.

You won't see 35C and 80% RH in the Beglium Congo

People always note the morning RH and think it stays that high in the heat of the afternoon.

[autt]

Answer the question:)
Hope this would be helpful.

[Peter_1]

It's just an exercise which must show what moisture can do associated with high temperatures.
Let's take then 60% RH.

Same for very low humidity associated with low water temperature to show that you can't just simple spray water in the air and thinks that the problem will be solved. You first have to preheat the air in some occasions.

You learn it the best to show extremes which don't fit right away in the chart.
To be honest, this is also difficult for me.

[Abby Normal]

Soon come Peter.

The cooling scenario is strange tho, fairly humid and cool indoor condition to be aiming for. As you cool this air with an SHR of 0.8, you come up to the saturation curve pretty quick, so you need high air flow. If you were maintaining lower RH,and a bit higher temperature like 24, you would have more room before you hit the curve, lower air flow.

The percentage of outside air compared to the total air flowing is going to be small, I will double it to 30 cubic meters per hour per person, as it will show a little better.

So here is a chart for your 50 kW room load about 80% sensible, and 900 cubic meters per hour fresh air.

http://i32.photobucket.com/albums/d41/a_bee_normal/Forums/RE/peter_chart.jpg

http://i32.photobucket.com/albums/d41/a_bee_normal/Forums/RE/Peter_Numbers.jpg

The computer program is a little off, but pretty, close. Now bear with me, my brain functions in inch pounds.

To do this manually, the first thing to do is to plot the room point of 22C & 65% and the outside air point 35C and 60% RH on a chart.

Now you said the room load was 50 kW and it was 20% latent and therefore 80% sensible. So you have a senisble heat ratio of 0.8.

When doing it by hand, you have to look at how the chart you use is set up to deal with the sensible heat ratio or SHF as Carrier calls it. The chart gives you a method of coming up with a sloping line. The slope of the line is set by the sensible heat ratio.

So your next manual step is to draw a line with the correct slope through the room condition that you want to maintain, and extend it until it hits the 100% RH curve. Where this line hits that curve is your apparatus dewpoint, the average coil surface temperature needed. In this example it would be somewhere between 13 and 14 C.

The next step is to determine the air flow rate and the supply air temperature.

The less air you move, the closer your supply temperature will be to the ADP, the more air you move, the farther you will be from the ADP.

I mentioned before that you would be hitting the curve fast because you are maintaining a high RH/cool temperature, so the air flow is high. I ended up telling the computer to supply air at a temperature 7.5 degrees C cooler than the room.

So the supply air temperature will be 22-7.5=14.5C DB, and where this intersects the process line, you read off all the other info like the wet bulb and the enthalpy.

So now, off the top of my head, I can't remember specific heat values of air in metric, but what you would do is take the sensible heat load and divide it by the specific heat of air, the density of air, and the temperature differential to get your air flow.

So its faster for me to convert to BTU/hr and CFM than to look up the SI specific heats and density.

program says 40,601W (i know its supposed to be 40 kW) sensible.

40,601 x 3.412= 138,531 Btu/hr

Use 1.08 for specific heat of air (.24) times the hours to minutes factor of 60 times density of 0.075, with the 7.5x1.8=13.5F differential,

Air flow is 138,531/(1.08 x 13.5)=9,501 CFM

9501/2.118=4,486 l/s x 60/1000=269 cubic meters per minute, and my program says 271. Close enough, I used volume some inexact conversions.

So you know the air flow and the supply air conditions.

Next step is to get the mixed air condition, so draw a line to connect the room point and the outside air point, the mixture will be on that line.

So I mentioned that I doubled the outside air requirement, as we had high airflow on this example.

So we have a total of 271 m^3/minute flowing and 15 m^3/min is outside air and 271-15=256 return air is being mixed.

So again using a quick volumetric average, the mixed air temperature can be estimated by a weighted average.

Tmix = ( 256 x 22 + 15 x 35)/(256 +15)
Tmix= (5632+525)/271
Tmix=22.72C same as program :)

So on the mixing line look where 22.72C dry bulb intersects and note all the mixed air properties especially the enthalpy value.

The mixed air enthalpy is shown as 51.63 kJ/kG, and the coil you use must cool it down to the supply air enthalpy of 39.83 kJ/kG

So take your air flow (may have to convert to a flow per second) multiply by the density to get a mass flow rate and then multiply by the change in enthalpy and hopefully come up with around 63.4 kW of total cooling needed.

[Abby Normal]

Looks like 19.83 is a conversion factor for a flow in m^3 per minute into kG per second and a factor of 1000 to convert KJ to J.

271x 19.83 x (51.63-39.83)=63,412W

[Abby Normal]

You would have to find a coil that could take the mixed air down to the required supply conditions using 10C water.

Bypass factor is pure Willis Carrier.

If you look at the mixed air scenario, the same thing happens when moist air is cooled. Some air contacts the coil and leaves saturated at the ADP. Other air passes (bypasses) through without contacting a fin or a tube. The supply air is a mixture of the air that makes contact with the air that bypasses.

[Abby Normal]

Instead of 'multiplying by the density' you could divide by the specifc volume you get from the chart

[Peter_1]

Abby Normal, many thanks for your big efforts.
I see that you fully understand this.

Perhaps I can help you to say that I use Uconeer from Karmasoft (it's free) for conversions.

Autt, I gave the students in school your website. I thank you also and I expressed already in the past my admiration/respect for you programming work.

[autt]

Hi Peter,

It's some hard for me to read all the posts, and people can not visit internet in our office, I envy people here have so much time talking to and helping each other, many intelligent people here and it is a good forum can learn more.
I also have many questions, I will join and ask for help if have time.

Regards,

[US Iceman]

Peter,

Here is a link to an article that is very good.
http://www.handsdownsoftware.com/Overview_of_Psychrometrics.pdf

I have not forgotten about finding those examples. I have been very busy trying to finish up projects (and also trying to watch over RE stuff;) too).

I will try to see if I can find those in the next day or so, before I leave for vacation.

[Dan]

Abby and Peter et al, this thread is a must view for those of us wish to utilize Psychrometric charts. Peter, you put forth a good challenge, and Abby, you did a fine job of explaining where assumptions versus givens come into play... and how to play with the variables. Terrific thread for both the newbies and the experienced. I will be revisiting this thread as I re-excercise manual calculations and regrasp the basics.

[Abby Normal]

You're welcome Peter and thank you Dan.

Thanks for the conversion tip Peter. I know both systems of measurements but have not been forced to work in SI for over 8 years now. I can't pull out "standard densities and specific heats" from the top of my head anymore.

SI is much simpler for the calculations though, it is just that at this point, it is easier for me to function in Btu's, cubic feet and "Grains of moisture".

Iceman had a nice link to the ASHRAE chart, about the only time I would use that chart would be for humidification using that 'protractor'.

Trane and carrier have a simple reference point and a sensible heat ratio/factor scale on their charts. I prefer the Trane chart myself, and it is easier to read the dewpoint from, especially for students.

Extending a cooling line to find the apparatus dewpoint is more important than one would think, especially with a large internal latent load. If the process line does not cut the 100% RH line, you have what is called an impossible process meaning, you cannot properly cool and dehumidify with a cooling coil alone.

You can only over cool the air to get the moisture out then reheat, or you can just cool the air to the desired temperature and use a dessicant.

Your stand alone mechanical dehumidifier over cools and reheats.

It is good to know how to manually do something rather than rely on a program. Programs even with a lot of 'error traps' are still prone to Garbage In = Garbage Out.

You put in the wrong information, you get an answer and you think that it has to be right because that is what the computer says.

[johnyfreon]

a little late but here a link to an exercise http://www.ageng.ndsu.nodak.edu/envr/asm368/Pschart.pdf