Use of Dry/Wet bulb when design a ac system

[happychan314]

I need to design a AC system

i search many books and manual said i need wet bulb and dry bulb to design AC system

But actually what is the use of dry bulb and wet bulb?

i search data on my local weather department
but how to use it in calculation on loading?

[amitsaxena]

These DBT and WBT will decide the capacity of the unit as in extreme weather situation you will design. These air DBT/WBT are out door condition for your AC design and also if you dont have 100% recirculation then this will matter while calculating the load.
thanks
amit

[nike123]

I need to design a AC system



Do you designing it for your personal use, or you designing new product for massive production?

Don't you guys, there ,in China, have mentors or senior designers? Or you every time inventing things that are invented 100 years ago.
There are lot of design questions of basic type coming from there here on this forum.
Sometimes reinventing "hot water" could be beneficially, but in most cases it is waste of time and money.
Copying existing solutions, analyzing and improving them, is better way, as we know from many Japanese products which are made after 2nd WW.

P.S. Sorry if I sound rude. This is not intended to be criticism of you. It is only observation after some number of posts of same type.

[technomad]

Water vapour is a part of the air pressure and has its own specific energy capacity. Atmospheric pressure is made up of two parts one is the water vapour pressure and the other is the air pressure. The highest possible amount of water vapour is the saturation pressure when this is reached you start to get condensation this is 100% relative humidity. As you pass the wet bulb temperature 100% rh you will condensate water out of the air and lose energy capacity. For example if you have a server room that starts out with 60% at 25°C humidity and you do not open the door for any set time the relative humidity will drop to 30% 20°C this means a lose of 26 KJ/Kg dry air. Reducing the effective cooling capacity of your evaporator. To determine this you must also have a look at your min temperature in the evaporator to determine the amount of water you are losing per hour and may need to replace as the processors also lose cooling capacity because of a lack of water. You will also get the same problem trying to cool down a room in the desert with a air condition unit that worked perfectly in a rain forest. Use a h,x diagram also known as a mollier diagram to determine your specific energy capacity of the “wet” air and your “dry” air. Maby try this link for more understanding of this problem en.wikipedia.org/wiki/Relative_humidity

[happychan314]

i just want to know more about use of dry bulb and wet bulb temperature only, and because i am a institutional ME student, i hope i can get some information in this forum, so that i can learn more. I get few advise from my tutor, i try to rescue myself



i read AC design manual from ashrae, but not understand

sorry
:(

[nike123]

i just want to know more about use of dry bulb and wet bulb temperature only, and because i am a institutional ME student, i hope i can get some information in this forum, so that i can learn more. I get few advise from my tutor, i try to rescue myself



i read AC design manual from ashrae, but not understand

sorry
:(

Are you familiar with sensible and latent heat terms?

[technomad]

Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking action. Such an instrument is called a wet-bulb thermometer. The wet-bulb thermometer reports the thermodynamic wet-bulb temperature if the sock is shielded from radiant heat exchange with its surroundings. Air flows past the sock quickly enough to prevent evaporated moisture from affecting evaporation from the sock. The water supplied to the sock is at the same temperature as the thermodynamic wet-bulb temperature of the air. In usual practice, the value reported by a wet-bulb thermometer differs slightly from the thermodynamic wet-bulb temperature because the sock is not shielded so well from radiant heat exchange. Air flow rate past the sock may be less than optimum. The temperature of the water supplied to the sock is not controlled. At relative humiditys below 100 percent, water evaporators from the bulb which cools the bulb below ambient temperature. To determine relative humidity, ambient temperature is measured using an ordinary thermometer, better known in this context as a dry bulb thermometer. At any given ambient temperature, less relative humidity results in a greater difference between the dry-bulb and wet-bulb temperatures; the wet bulb is colder. The precise relative humidity is determined by finding one's wet-bulb and dry-bulb temperatures on a psychrometric chart (or via complex calculation). Psychrometers are instruments which contain both wet-bulb and dry-bulb thermometers. A wet-bulb thermometer can also be used in combination with a globe thermometer (which is affected by the radiant temperature of the surroundings) in the calculation of the wet bulb temperature.

[technomad]

By the way that was qoute and the wet bulb temperature in the h,x diagram and the dry bulbe is on the left on the axis/X

[technomad]

dry bulb is on the ordinate marked y

[technomad]

sorry transelation mistake:-(

[nike123]

Here (http://www.mediafire.com/?jgzm0yzzwt3), I prepared one chapter from book which name I will not mention.
Pasword for download is sent to you by visitor message.

[Abby Normal]

A design dry bulb and coincidental wet bulb temperature are used to design the peak cooling load. Ventilation air and/or infiltration are a major portion of the cooling load.

The design dry bulb and wet bulb temperature will allow you to determine the sensible and latent cooling requirements of the outdoor air, when you are experiencing a "hot day"

The wet bulb temperature indicates the total heat content of the air.When used in conjunction with the dry bulb temperature you can determine how much of the total heat is sensible and how much is latent.

What usually tends to happen though is the amount of humidity in the outdoor air is never at its maximum when the outside air is at its hottest,

For this reason, since about 1993, ASHRAE has been publishing dehumidification design data also, because a system designed just to meet the design dry bulb and wet bulb, may not always be capable of controlling humidity.

The dehumidification data is given usually as in the amount of moisture per unit mass of dry air or as a dew point. You are also given a coincidental dry bulb to expect when the outdoor humidity levels are at a maximum.

It is usually significantly cooler outside when the ambient humidity is at a maximum. It can also be over cast.

So with lower outdoor air temperatures and reduced solar load, an air conditioning system that is basically controlled to respond to temperature, can get over whelmed with mositure.

So a designer who has done his job properly has a system that can keep a space cooled and dehumidified during the design dry bulb and wet bulb condition, and has also designed a system that can prevent the space from becoming too humid when a design dehumidification conditions exist oustide also.

[Abby Normal]

If there are no published wet bulb and dry bulb temperatures, then you could analyze local data collected at the weather station over a period of time, as in some years. Publish design conditions are probably based on 20 years of statisitics.

So you would determine what temperature is about the hottest it gets for 99% of the year so for all but 88 hours and you have the design dry bulb. I believe then you would have to work out the avergae wet bulb temperature that coincided with what the design dry bulb temperature is.You could use the relative humidity that coincided and convert it to a wet bulb

For humidity control you would see what mositure level in the air, perhaps measured as kg water per kg of dry air is higher than anything you would experience for all but 88 hours out of the year. They would call this the 1% design level.

You can also design to the 0.4% level which covers you for all but 35 hours of the average year.

[dougheret0]

The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) publishes design climate data for hundreds of China locations. The data for each site inclues dry bulb, mean coincident wet bulb for .4%, 1%, and 2% annual occurrence. Also included are design wet bulb with mean coincident dry bulb for the same annual occurrence, to be used for dehumidification design. Google ASHRAE and go to their site where you may be able to get just the data for your location, or where you can, for a small fee, purchase a CD with data for all of the 4500 sites they cover in the world.

If you plan a career as a contractor or designer of HVAC systems, I strongly recommend you join ASHRAE. One membership benefit is the ASHRAE handbook series, in your language and preferred units.

Finally, for a comprehensive discussion of controlling humidity in buildigns, I recomment Humidity Control Design Guide for Commercial and Institutional Buildings, published by ASHRAE and available at their site.