Test online psychrometric calculator

[penderway]

New here and this is my first post. I've developed an online psychrometric calculator and I believe it is now working perfectly.

I would like someone or anyone who has expertise in this area to check my work.

The link is http://efficientcomfort.net/asp/PsychroCoilCalc_Web/PsychroCoilCalc_Web.asp

The calculator is geared towards evaluating coil performance rather than designing a coil. In particular I'm interested in knowing if I'm converting the input of standard or actual air CFM correctly to dry and moist air specific values (depending on selected option) for the calculations.

Any help would be greatly appreciated.

Wayne Pendergast

 

[Greg Bernhardt]

I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?

 

[penderway]

Thanks for the response.

I believe that the calculator is working correctly. It's a bit different than standard psychrometric calculators in that it can calculate with moist air specific units, instead of just the common dry air specific and standard air units. I had some doubt that I was correctly converting between these different units for calculating actual airflow to use in subsequent calculations.

Dry air specific units are used all the time for HVAC coil design, and standard air is used all the time to rate blowers and fans. Since my goal is not to build another design tool, as there are already many, I am incorporating units of real moist air in order to measure real accurate actual performance. There aren't many resources that I can find that do this.

Since I'm not a formally trained engineer, I thought this would be a good place to get some verification that what I'm doing is correct.

But after going over it again and again for the past couple days, I'm more confident in it.

 

[Mech_Engineer]

Hi penderway,

Have you considered testing it by inputting values from an available psychrometric chart? One could give you many values you could use for testing input/output combinations.

Good luck.

 

[penderway]

Hi penderway,

Have you considered testing it by inputting values from an available psychrometric chart? One could give you many values you could use for testing input/output combinations.

Good luck.

Thanks, I have, It's right on target with the many typical psychrometric charts available (depending if they incorporate enhancement and compressibility factor. Trouble is they are all "Dry Air Specific".

Now if I could find a moist air specific chart, that would be nice. :)

 

[Mech_Engineer]

What do you mean by "moist air specific"? Maybe I'm rusty on the subject, but my understanding is a psychrometric chart should be valid for all relative humidities, e.g. 0-100% RH.

 

[penderway]

Here's my understanding...and I welcome any corrections.

Dry air specific is the most commonly used unit of measure for HVAC design, It is the mass of dry air plus (and separate from) the mass of moisture it carries. Much easier to use in calculations and is accurate enough in the temperature and humidity ranges used in HVAC design. It's expressed, for example in density, as lb_da/ft³. When used in HVAC design it errors on the over-sizing side so it is very acceptable.

Moist air specific AFAIK is used in industrial drying processes and is practically never used for HVAC design. It is more accurate and is best for measuring actual performance. It is a measurement of the total moist air mass. Expressed as lb_ma/ft³ or Humid density. Also Humid volume and Humid enthalpy.

Then there is also Standard Air, which is always 0.075 lb_da/ft³. This is used for rating fans and blowers.

I've built my calculator to accommodate each of these units of measure as airflow inputs, and I convert them to either dry or moist air specific outputs. I haven't been able to find any references on this (except for standard air), but I believe I've figured it out correctly.

 

[Mech_Engineer]

When you say "more accurate," what is the typical percent error in an HVAC problem your method corrects for?

 

[penderway]

I've not quantified it. I believe it's insignificant in the common ranges of temperature, humidity and altitude. I am in no way disputing the practice of using dry air specific units for design purposes. I only advocate the highest accuracy possible when measuring and calculating the actual performance.

I have another calculator that shows the difference of using 1.08, 0.68 and 4.5 factors in the common formulas.
http://efficientcomfort.net/asp/PerformCalc_Web/PerformCalc_Web.asp
Accuracy depends on how close to standard air the actual air your measuring is. If the airflow you measure is 0.075lb_da/ft³ density and 0.24 Btu/ft³·°F specific heat, then the formulas will likely be exact.

 

[penderway]

I thought it was common knowledge that the cooling and dehumidifying process had an ADP (apparatus dew point) that was a straight line thru the state points to the saturation curve.

I've read in system design literature that if it did not, then it was an invalid cooling and dehumidifying process because the coil was dry.

But in the real world (not at design conditions) the coil can be wet even if the straight process line doesn't intersect the saturation line.

I found the following quote from “Handbook Of Air Conditionig & Refrigeration - Basics & Constant Volume System Design” In section 20.49 it states:

A straight line has been used to represent a cooling and dehumidifying process, and the intersection of this straight line and the saturation curve is considered the effective surface temperature of
the coil and is called the apparatus dew point. This is a misconception. First, the cooling and dehumidifying process is actually a curve instead of a straight line. Second, the effective surface temperature as a fictitious reference point is affected by many factors and is difficult to determine in heatand mass-transfer calculations.
It is more accurate to define the apparatus dew point as the dew point of the moist air leaving the
conditioning apparatus, the coil, the air washer, or other heat exchangers.

This further demonstrates the flaws of applying design concepts (such as density factors, 1.08, etc.) to performance measurement procedures.

I've updated my psychrometric and HVAC performance calculators to incorporate leaving air dew point as the ADP when the process line doesn't intersect the saturation curve. I've made the ADP invalid only if the bypass factor exceeds .5

Just when I think I really understand psychrometrics... it turns out I don't!

 

[rahul dixit]

hey! can i know why your are using specific enthalpy in calculation of cooling load. Can't we directly use enthalpy available fromthe psychrometric table corresponding to the given Dbt and RH.The difference in answers is huge.

 

[penderway]

An accurate value of Humid Air Enthalpy (whether moist air specific or dry air specific) is critical for calculating air conditioning load and/or equipment performance, especially when the highest accuracy is desired.

The formulae used in my calculator... (see page 6, 10 & 11 of this pdf download for examples)…are adopted, by the American Society of Heating Refrigeration and Air conditioning Engineers, as The Standard.

You could Google "Thermodynamic Properties for Real Moist Air, Dry Air, Steam, Water, and Ice (ASHRAE RP-1485)" for more details.

What is the source of the table you are using? Have you a link to it? I'm curious to compare.
:)

 

[rahul dixit]

I am using SI Units. i used you calculator and just converted the output say btu/lb to kj/kg. I compared this with direct calculation in SI units...the results were different.
I am just a student and not understanding why such a change is coming. Shouldn't just converting the final load from english to SI be giving the same answer as calculating the whole process in SI units. Please guide. I use the following book https://books.google.co.in/books/about/Engineering_Thermodynamics.html?id=91MZ2ZOb3n8C. You can get a pdf copy of this book easily in direct download. I'll try to upload.

 

[rahul dixit]

https://drive.google.com/file/d/0B6hjEAwn8lB-cm1IQ3B6Sk1XQWc/edit here you go!