Enthalpy

[anisaamm]

working in HVAC feild more than 14 years as technician, never had to deal with Psychrometric Chart, ( well thats how we worked in Middle east) recently moved to UK, plus developed interest in theory of HVAC, start reading related books. I do feel lost some time when i try to learn on my own from books, like now, i have under stood say about 70% or more of Psychrometric chart and can use it too, what confusing me is The word ENTHALPY, dictionary tells me its meaning as THERMODYNAMIC QUANTITY EQUIVALENT TO THE TOTAL HEAT CONTENT OF A SYSTEM. some one please explain what that means, if you can give couple of examples that will greatly help. thanks

[Edited by anisaamm on 12-15-2005 at 07:26 AM]

[coolwhip]

Enthalpy is a measure of total heat content of outdoor air, and considers humidity as well as temperature. Temperature is measured by an adjustable tstat called an outdoor air thermostat or OAT. Enthalpy is measured by an adjustable device call the Enthalpy Control or EC.

More effective use of outdoor air can be made if we consider a total heat content instead of just its drybulb temperature. This can be done by using an Enthalpy Control instead of an Outdoor Air Thermostat.

[anisaamm]

Thanks for your help, would u please give a couple of example linking enthalpy with rest of phsychrometric chart, or its applications

[anisaamm]

for instant rnthalpy of saturation, BTU per pounf of air, kindly explain

[coolwhip]

Originally posted by anisaamm
Thanks for your help, would u please give a couple of example linking enthalpy with rest of phsychrometric chart, or its applications

The EC senses both temp. and humidity. The EC has four fixed positions A B C D. Each setting defines a range of conditions at which the control will operate. The operating ranges can be better visualized if you look at a psychrometric chart. you can look at the shape of the bands and tell that as humidity decreases the allowable temperature for free cooling increases, which is why Enthalpy is a better measure of suitability than temperature alone.

My email is in my profile. I can send you a jpeg with the info.

[Carnak]

As coolwhip has been explaining it is the total heat content of the air. When you sensibly/cool heat moist air or change the amount of moisture in the air you are changing the enthalpy

It compares the total heat to the air to a reference point. For easy math they sometimes base it on 0 degrees F.

On a 'per pound of dry air basis', it is the sum of the specific heat of air times the temperature increase above 0F, the latent heat to evaporate the mositure content( moisture per pound of dry air), and the specific heat of the water vapour(moisture per pound of dry air) times the temperature increase above 0F.

[coolwhip]

Crap! Hope those pegs were ok. My scanner is on the fritz again.:mad:

[Carnak]

In SI they will sometimes base it on 0 degrees C, on a 'per kg of dry air' basis. The specific heat increases above 0C.

[cvw]

Great to see a person that is interested in the science of the trade. Don’t for get the word Latent. Aren’t we basically talking about state change, like when the body creates water on our skin for a state change to take place (saturated liquid put in to an evaporator to boil) to lower our body temp. Where is John D. when you need him? This thread could go longer then the Serverroomcooler theard.
Got to run.

Cvw

I know enough to be dangerous


[Edited by cvw on 12-15-2005 at 01:22 PM]

[oogene]

Norm Chris could really enlighten you, ooohhh Norm ????

[hvacpope]

http://www.handsdownsoftware.com/Overview_of_Psychrometrics.pdf

[coolwhip]

Originally posted by hvacpope
http://www.handsdownsoftware.com/Overview_of_Psychrometrics.pdf


Very nice!!!

[jacob perkins]

yes thats a good link.

another is the "gray manual" from the honeywell site
I dont have the link,but I will try to help find it if need be.

[cvw]

hvacpope

I’ve got a scarymetric chart, actually a old Carrier Psychrometric chart that I’ve put under my pillow hoping to totally understand the thermodynamics of this chart with no luck. I’m going to print out a copy of this ASHRAE Journal article and give it a go again.

Thanks
cvw

I know just enough to be dangerous

[cvw]

Well that don’t work very well. After 6 hours of sleep with that journal artical under my pillow I still don’t know much. I guess my pillow must be too thick – or my skull. This may be off point but what’s the difference between 1lb of 60 degree water in a pan that evaporates over 10 hours and 1lb of 212 degree water that has 900 + BTUs added to it in 1 hour to get 212 degree steam?

cvw

I think I know just enough to be dangerous

[hvacpope]

[QUOTE]Originally posted by cvw
[B]Well that don’t work very well. After 6 hours of sleep with that journal artical under my pillow I still don’t know much. I guess my pillow must be too thick – or my skull. This may be off point but what’s the difference between 1lb of 60 degree water in a pan that evaporates over 10 hours and 1lb of 212 degree water that has 900 + BTUs added to it in 1 hour to get 212 degree


If you were a tree, what kind of tree would you be?

[air1]

One thing that has always struck my curiosity, is if water boils and changes state at 212 F at atmospheric pressure, how is it that water can evaporate at a lower temperature.
The way I understand it is that the molecules can become excited and escape the surface tension of the water. My understanding is that this is the principle of the UV humidifier.

I’ve also heard that the ultrasonic humidifiers don’t produce a true water vapor. Rather, I’ve been told that the ultrasonic humidifiers produce an atomized water fog.

Can anyone provide some clarification to these curious phenomena?

[hvacpope]

[QUOTE]Originally posted by air1
[B]One thing that has always struck my curiosity, is if water boils and changes state at 212 F at atmospheric pressure, how is it that water can evaporate at a lower temperature.
The way I understand it is that the molecules can become excited and escape the surface tension of the water. My understanding is that this is the principle of the UV humidifier.

I’ve also heard that the ultrasonic humidifiers don’t produce a true water vapor. Rather, I’ve been told that the ultrasonic humidifiers produce an atomized water fog.

Can anyone provide some clarification to these curious phenomena?

Air1, read this, http://en.wikipedia.org/wiki/Evaporation

[jogas]

Thermostats sense dry bulb temperature.
Enthalpy Controllers sense both Dry bulb + Wet bulb temperature.
It's important to realize that dry air takes less BTU's to heat or cool than "wet" air". Therefore, enthalpy takes into account the total btu's held in air. This allows us to anticipate the amount of btu's required to change that air to a wanted condition.
Anybody who works with refrigerant already knows why water "boils" at a lower temperature when the pressure is lowered. YES, there is a Pressure-Temperature chart for H2O!<LOL>
Seriously, we're used to water boiling at atmospheric pressure. Lowering the pressure also lowers the corresponding boiling point of the water(refrigerant).
My 2 cents,
Jogas

[air1]

Thanks for the link hvacpope. Wikapedia is good resource, if we can only be assures of its accuracy.

[air1]

Originally posted by jogas
Anybody who works with refrigerant already knows why water "boils" at a lower temperature when the pressure is lowered. YES, there is a Pressure-Temperature chart for H2O!<LOL>
Seriously, we're used to water boiling at atmospheric pressure. Lowering the pressure also lowers the corresponding boiling point of the water(refrigerant).
My 2 cents,
Jogas


What I meant to say is, "how does water evaporate at low temps at atmospheric pressure".

[jacob perkins]

Originally posted by jacob perkins
yes thats a good link.

another is the "gray manual" from the honeywell site
I dont have the link,but I will try to help find it if need be.

Okay I found it...
it was posted by "hanknerd" and can be found in a thread called "Need psychmetric math help" in the Pro's section.

[kwillmech]

back when in a/c school a classmate of mine told me of his cousin (girl) named Enthalpy. his uncle was an old school a/c guy that was obsessed/crazy.

true story. manny knight where are you???

[hvacpope]

From the RSES Journal 1999

Mastering Psychrometrics



An understanding of psychrometrics and use of the psychrometric chart is essential to the process of designing thermal systems and sizing the coils that are part of these systems. Whatever the type of coil, the air stream going through it can be plotted on the psychrometric chart, providing important information learned about the air stream.
With this knowledge, a designer can answer questions and make decisions during the coil selection process. Troubleshooting also can be facilitated by this knowledge. This article will cover some of the basic principles and concepts of using the psychrometric chart.
The psychrometric chart contains a great deal of information in a relatively compact format. Because of this, many are intimidated when first introduced to it. Once a few basic concepts are understood, the chart is really quite simple to use. Anyone who will keep an open mind and is willing to learn these basic concepts, can successfully use a psychrometric chart.
A psychrometric chart is an attempt to show relationships in the various properties of moist air. The chart includes all of the following properties for moist air: dry bulb temperature, wet bulb temperature, relative humidity, dew point temperature, humidity ratio, total heat (enthalpy) and specific volume. If any two of the seven properties are known, the remaining properties can be obtained.
Before one can understand the psychrometric chart, an understanding of each of the properties is required. The property definitions and how they are plotted on a psychrometric chart are listed below. Refer to the skeleton chart shown to clarify the descriptions
Dry bulb temperature (DB): The temperature of a substance as read by a common thermometer. The dry bulb temperature is an indication of the sensible heat content of a substance. On a psychrometric chart, dry bulb temperatures are shown as vertical lines originating from the horizontal axis on the bottom of the chart.
Wet bulb temperature (WB): The wet bulb temperature is used to measure the water content of moist air. It's obtained by passing air over a thermometer that has a wet wick over its sensing bulb. The drier the air, the more water will evaporate from the wick, which lowers the reading on the thermometer.
If the air is saturated (100 percent relative humidity), no water will evaporate from the wick and the wet bulb temperature will equal the dry bulb temperature. Wet bulb lines originate from where the dry bulb lines intersect the saturation line, and slope downward and to the right. Wet bulb lines are nearly but not exactly parallel to enthalpy lines.
Relative humidity (RH): This is the ratio of the amount of water vapor in a given sample of air to the maximum amount of water vapor the same air can hold. One hundred percent RH indicates saturated air (the air cannot hold any more water vapor), and 0 percent RH indicates perfectly dry air. (Note: The above definition is accurate for all practical purposes. The precise definition of relative humidity is the ratio of actual water vapor pressure in a sample of air to the water vapor pressure in saturated air at the same temperature.) The 100 percent RH line is the saturation line and lines of lesser RH fall below and to the right of this line.
Dew point temperature (DP): The temperature to which air must be cooled before condensation will begin. As a sample of air is cooled, its RH climbs until it reaches 100 percent RH (saturated air). This is the dew point temperature. At saturation, dew point temperature equals wet bulb temperature, which also equals dry bulb temperature, and the RH is 100 percent.
If air is passed over a surface that is below its dew point temperature, moisture from the air will condense on the surface. It's the dew point temperature of air going over a cooling coil's fins (and the surface temperature of the fins) that determines if the fins will be wet or dry. Dew point temperatures are shown on the saturation line.
Humidity ratio (W): Sometimes referred to as "specific humidity,² this is the actual weight of water vapor in a pound of dry air. Humidity ratio is expressed in pounds (or grains) of water vapor per pound of dry air. Humidity ratio lines are horizontal on the chart and originate from the vertical axis on the right hand side.
Enthalpy (H): This term is used to describe the total heat of a substance and is expressed in Btu per pound. For moist air, enthalpy indicates the total heat in the air and water vapor mixture and is shown as Btu per pound of dry air. Dry air at 0° F has been assigned an enthalpy of 0 Btu/lb. Enthalpy values are found on a scale above and to the left of the saturation line. Lines of constant enthalpy slope downward and to the right and nearly parallel the wet bulb lines.
Specific volume (SpV): The reciprocal of density, specific volume is expressed as cubic feet of air-water vapor mixture per pound of dry air. Lines of specific volume start on the horizontal axis and slope upwards and to the left.
As stated earlier, if any two of the above seven properties are known for a sample of moist air, the start point of the air can be plotted on a psychrometric chart and the remaining five properties can be graphically determined. (The two properties typically determined are the dry bulb temperature and the wet bulb temperature.)
Any process that involves heating, cooling, dehumidifying or humidifying air can be plotted on the psychrometric chart. The following statements apply:
Any sensible heating or cooling process is shown as a horizontal line on the chart. The humidity ratio and the dew point are constant in this process.
Any latent heating or cooling process is shown as a vertical line. The dry bulb temperature is constant in this process.
A typical cooling/dehumidifying process is represented as a line that goes down and to the left. This process would theoretically move horizontally to the left until the dew point is reached, and then follow the saturation line to the end point.
The actual process is more accurately portrayed by a curved line moving down and to the left. This is due to the mixing process of some parts of the air stream that have reached the dew point with other parts that are still being sensibly cooled.
A heating/humidifying process is represented by a line that rises and moves to the right. The actual process line depends on the type of humidification involved, but the end point will always be above and to the right of the start point.

[cvw]

The ASHRAE article is great but the math ====side bar. When a person off the street is shown a 6E pump he see a hunk of metal, when I see a 6E, I see a mechanical elegance, Armature, Crankshaft, Connecting Rods, Wrist Pins, Pistons, Valves, a compressor. When you show me math that is more complicated then (I=E/R,P=IsqR) I start to see the math as that hunk of metal, something my math teachers did or didn’t do to me. Anyway this is my problem.==== I still would like to understand what is happening while you’re adding 900+ BTUs to 1lb of 212 degree water to change it’s stat or when I put room temperature water in a beaker, place it under the bell jar of a vacuum pump that our high school physic lab owned and pump it down till flash over.
What is the Latent BTU release in this stat change. I understand the BTUs at this low pressure does not equal 800+BTUs. Does the temperature of the water in the beaker relate to Saturated liquid subcooling?
I think after looking up boiling point on Kikipedia, ( go source hvacpope, do you teach HVAC courses? ) the water that flashed over went into a stat of vaporization as I approached the Triple point. Part of my confusion was not realizing that evaporation is different than vaporization. Does R22 evaporate in the coil or does it go thru the stat of vaporization, boiling in the coil?

cvw

I’m a 63 year old tree still trying to grow.


[Edited by cvw on 12-17-2005 at 11:11 PM]

[shelby]

****en hot + ****en humid =lots of enthalpy (>25 btu/lbs)

[Andy Schoen]

This document has some thermodynamic terms defined in simple terms, though the thermodynamic equations may not make much sense unless you are familiar with calculus. http://www.sporlan.com/5-162.htm

[Carnak]

Here are the enthalpy formulae without the calculus.

Enthalpy of moist air with respect to air at 0F

enthalpy= 0.240 x T + W x( 1061.2 + 0.444 x T)

where enthalpy is in Btu/pound dry air, T is the temperature in degrees F and W is the humidity ratio in pounds moisture per pound of dry air. It is the specific heat of air plus the latent heat to evaporate water plus the specific heat of water vapour.

In metric the equation for enthalpy with respect to 0 degrees C is

enthalpy = 1.0 x T + W x (2501.3 + 1.86 x T)

where enthalpy is in kJ/kg dry air, W is the humidity ratio in kg moisture/kg dry air and T is the temperature in degrees C

[Carnak]

Originally posted by air1
One thing that has always struck my curiosity, is if water boils and changes state at 212 F at atmospheric pressure, how is it that water can evaporate at a lower temperature.
The way I understand it is that the molecules can become excited and escape the surface tension of the water. My understanding is that this is the principle of the UV humidifier.

I’ve also heard that the ultrasonic humidifiers don’t produce a true water vapor. Rather, I’ve been told that the ultrasonic humidifiers produce an atomized water fog.

Can anyone provide some clarification to these curious phenomena?


Water evaporation below its boiling point is explained by "Dalton's Law of Partial Pressure".

Water has a T & P relationship just like a man made refrigerant. At 212F, the vapour pressure of water equals atmospheric pressure, so as long as there is a heat source available water can rapidly change state.

Below 212F the vapour pressure of water is less than atmospheric pressure. Dalton's Law basically says that if you mix several gases into a container, each gas contributes a certain amount to the total pressure, however each gas behaves as though it is the only gas present at its partial pressure.

Water can evaporate into the air until there is 100% RH. At 100% RH the partial pressure of water vapour in the air is equal to the vapourpressure of water saturated at that temperature.

At 72F water has a vapour pressure of 0.3887 psia. Sea level pressure is 14.696 psia. Water can evaporate into 72 degree air until it's vapour pressure in the air is 0.3887 psia. The remaining pressure of 14.31 psia is from the oxygen, nitrogen, CO2, helium , argon, and all other gases even CO in the air.

[TitleLess]

So why doesn't a big red brick evaporate ?

If there is no big red brick pressure in the air then why doesn't the red brick evaporate?

[anisaamm]

I cant be more gratefull to all of you guys for the help in entholpy subject, especially coolwhip,carnak,hvacpope,air1, cbw, jogas and Andy Scheon, and thanks to cvw for encouragment. I am really getting much more than i asked for, its seems to me as if i found a gold mine of knowledge on Entholopy and related subject. What was confusing me about water and its evaporation below its boiling point, but wasnt sure how to put it to you guys, that was done by 'air1' nicely, and the explanations from carnak, also the link provided by hvacpope, explained the subject perfectly, thanks once more. I hope this discussion will carry on little longer.

[TitleLess]

But what about the brick ?

[socal]

I had a bad case of enthalpy once. My doctor gave me a cream to apply three times a day. It went away in a week.

[Carnak]

Originally posted by titleless
But what about the brick ? Red bricks are ruled by the fat eddy law of stupid questions I am afraid. Does not apply in this case.

[TitleLess]

I thought it may be because they are at saturation.

[cvw]

OK solid, liquid, vapor. What is the triple point of red brick anyway?

cvw

[Andy Schoen]

Originally posted by titleless
So why doesn't a big red brick evaporate ?

If there is no big red brick pressure in the air then why doesn't the red brick evaporate?

Amusingly, the second law of thermodynamics would predict this event, at least within an isolated system or a static universe. It does take a rather long time to happen, though... :)


[Edited by Andy Schoen on 12-20-2005 at 08:22 PM]

[icemeister]

I don't the believe the brick is evaporating. It is sublimating............verrrrrrrry slowly. ;):D

[Andy Schoen]

Good point icemeister... like dry ice perhaps. The second law says entropy must increase for any real world process. Therefore, the red brick must decay if it is left alone. Even a diamond must decay into some graphite dust... and then ultimately into heat. Yes, we are all dust in the wind :) Of course, DeBeers isn't too concerned about its diamonds disintegrating. We're talking billions of years for this to happen, and at that point, who will care if a diamond doesn't last forever?

[TitleLess]

So we all agree that a brick can evaporate right ? or subliminate, whichever

OK if it evaporates can it also condense back to its previous form ? or something close to it ?

[Andy Schoen]

Yes, but it would require energy, and perhaps lots of time. Our solar system started off in a gaseous state. Now we have rocks, diamonds, and the components to make red bricks.

[TitleLess]

OK, now you see where I'm going with this.

[socal]

Under the correct conditions, Pressure,Temp., everyting can and will evaporate/condense/combust/change from its previous state,solid to liquid,liquid to gas,and back again etc. etc. Of course with this change' BTU's are moved from one place to another. Just a transfer of BTU's. Enthalpy is all about BTU's and their change of form. Enthalpy is just a mesurement of BTU's, I think.

[Andy Schoen]

Not a bad way to look at it. :)


Originally posted by socal
Enthalpy is just a mesurement of BTU's, I think.

It's better to look at enthalpy as the useable energy in a substance. Btu is simply a unit of measurement for energy, like kilowatt-hours and ft-lbs.

[icemeister]

Enthalpy or heat content by itself as denoted by the arbitrary value we call the BTU (or whatever) is not measureable or quantifiable. It is when we consider the First law of Thermodynamics and the change in enthalpy between two points of energy intensity or temperature (as defined by another arbitrary value like Degrees F) that we can get a measureable and therefore useful quantity of heat.

This discussion of the red brick takes us into the realm of the concept of entropy and the Second Law. I don't think we really want to go there. ;)

[TitleLess]

Please go on what is the second law ?

[Andy Schoen]

The implications of the second law are many, and it is considered the fundamental law of science. It basically states that entropy must increase for any naturally occurring event. Explaining entropy can then becomes a bit of a problem. The definition I like is entropy is a measure of energy which is no longer available to do useful work. Any conversion of energy from one form to another cannot be 100 percent efficient. There will always be energy used in some fashion that did not perform useful work.

Take for example an a/c system that uses electricity to produce cooling. A second law analysis on such a system will show that it is not 100 percent efficient. You will lose useful work due to compressor motor heat, internal friction within the compressor, pressure drop in the refrigerant lines, etc. Even the expansion valve is a source of energy loss... the pressure drop across the valve is a source of energy that isn't used.

The formulation of the second law ended mankind's efforts to produce a perpetual motion machine, even though even nowadays one occasionally hears about some clown thinking they have developed one.

[cvw]

Where is Steven Hawking?

In the beginning there was only energy and time. The time thingy is a stretch but don’t we measure energy with 1-degree increase of one pound of H2O over an amount of time, our BTU.
At a point in time, roughly 18 billion of our earth’s orbits ago, the energy reached the saturation point and matter condensed out to form our dimensional space. The laws of Thermodynamics came into existence at this point.
All the energy, as we know it, in the Universe came from this single state change.

I guess.

cvw