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asmercer
05-20-2007, 01:23 PM
Hi I am a new tech still in HVAC School. I am having a hard time understanding the concept of the temperature difference of the airflow thru the evaporator and condenser when there are different symptoms.
An example, why does the temperature difference between air in and air out of the evaporator increase so much when you have a dirty evaporator, such as, air in 80 degrees, air out 52 degrees, delta t 28 degrees at the evaporator and only a small delta t at the condenser – air in 90degrees,air out 95 degrees, delta t 5 degrees?
Also does anybody know how to view the pro’s forum: training and education?

BigJon3475
05-20-2007, 01:31 PM
You have to post enough to become a pro and have some credentials I believe. It has everything to do with psychrometrics.

newbeetech
05-20-2007, 01:34 PM
that is a very good question about the T.D. ON THE EVEP COIL. what does that mean over 20 under 15, lets the icemiester,

BigJon3475
05-20-2007, 01:37 PM
It has to do with Btu's contained in the amount of air....if the air moves slower over the coil it gives it more time to remove heat......But it doesn't mean that your removing the most possible. You have to think in terms of volume. if you have lets just say 400 CFM.....Now you are moving 300 CFM due to a blocked coil or clogged fan....You have less heat to remove because of the Btu's per volume of air. Doesn't mean it's working as it should though because it's colder coming out of the vent.

asmercer
05-20-2007, 02:02 PM
what does that mean over 20 under 15, lets the icemiester. sorry newbeetech, i did not understand this question.

BigJon3475
05-20-2007, 02:10 PM
It means you need to have a full understand of psychrometrics. I'm a learner myself and the more I know the more it explains how and why to sooooooooo many questions.

vet
05-20-2007, 03:01 PM
That under 15 & over 20 degrees is just your delta-t across the evap coil and is only a guide line to look for a certain problem but not by its self, you need to check subcooling and or superheat along with wet bulb etc. which you will learn more about in school. Look it will take some years to be a good tech and you will make alot of mistakes along the way, but if you really want this then you will suceed with alot of training and you will never stop learning, luckly you have this site to talk over your problems. Most of us didnt so we researhed until we found the problem, lots of luck new guy.

newbeetech
05-20-2007, 03:55 PM
im sorry i should have read my typing , what i meant to say, was there is a tech name icemiester in this site that i know tends to be very knowledgable his help me out. when i really needed help

hvacbear
05-21-2007, 05:51 AM
Hi I am a new tech still in HVAC School. I am having a hard time understanding the concept of the temperature difference of the airflow thru the evaporator and condenser when there are different symptoms.
An example, why does the temperature difference between air in and air out of the evaporator increase so much when you have a dirty evaporator, such as, air in 80 degrees, air out 52 degrees, delta t 28 degrees at the evaporator and only a small delta t at the condenser – air in 90degrees,air out 95 degrees, delta t 5 degrees?
Also does anybody know how to view the pro’s forum: training and education?

In addition to what Big John and others have said:

Observe the difference in the fans, your evaporator fan is centrifugal (higher pressure) and your condenser fan is a propeller type fan (lower pressure).

The evaporator fan has to push air through or overcome resistance of a thicker coil and through duct work so higher pressure is needed. Less air volume (CFM) can be moved inside to satisfy setpoint because its delta T is higher.

The condenser fan moves more volume (CFM) at lower pressure so a lower delta T is fine due to the higher flow rate of air (CFM). There is also no regard for moisture removal (latent heat) in a condenser like there is in an evaporator (latent and sensible heat removal).

The bottom line is more flow (CFM) at a lower delta T can equal more heat exchange than a higher delta T at lower flow rate.

Psychrometrics and air flow are often not well explained. Laminate a psychrometric chart and plot several system points on it. This should be part of your course learn it even though it seems like you might not use it.

One last note: Your question shows you are really thinking keep it up and learn all you can.

If I confused you I will break this down some more just let me know, good luck.

Shophound
05-21-2007, 08:20 AM
Hi I am a new tech still in HVAC School. I am having a hard time understanding the concept of the temperature difference of the airflow thru the evaporator and condenser when there are different symptoms.
An example, why does the temperature difference between air in and air out of the evaporator increase so much when you have a dirty evaporator, such as, air in 80 degrees, air out 52 degrees, delta t 28 degrees at the evaporator and only a small delta t at the condenser – air in 90degrees,air out 95 degrees, delta t 5 degrees?
Also does anybody know how to view the pro’s forum: training and education?

If I'm reading your question correctly, you're wanting to know why if you're seeing a very large delta T over the evaporator due to a dirty evaporator (restricted air flow) while at the same time you're not seeing very much of a delta T over the condenser under the same operating conditions, correct?

Answer, in addition to what others have already posted, is that the total amount of heat being rejected by the condenser is less with the plugged evaporator or an evaporator suffering from poor airflow than under the same conditions with design airflow and a clean evaporator. The large difference in temperature you're seeing over the evaporator is due to longer contact time of whatever air does manage to flow over the coil. The amount of air passing through the coil that is unaffected by the cooling process is lower as well (known as coil bypass factor).

Although a good amount of heat is extracted from the air to cause such a large drop in temperature, there is overall less heat being applied to the coil due to reduced volume of air being moved over the coil. This translates to less heat for the condenser to reject, so you will see a lower delta T at the condenser. An evaporator coil with low airflow is an excellent latent heat remover, but can't handle the sensible heat gain being applied to the house or building. Result = poor cooling.

pstu
05-21-2007, 08:36 AM
I am neither a pro nor HVAC student formally, but I would invite you to take a look at a good psychrometric calculator such as this one:

http://www.envirochex.com/psychro.htm

Those paper charts are intimidating to me and I imagine to most people who see them at first. It is my hope that plugging in a few points with a calculator version of the same info, may make it easier to read the paper charts and master the material.

Hope this helps -- Pstu

the dangling wrangler
05-22-2007, 11:03 PM
My husband is jealous because this website wasn't around when he started out 35 years ago. He had to go to the school of hard knocks and lessons learned the same way. These young ones are very fortunate. May God be with them as they learn the trade and carry on in our footprints. Let your conscience be your guide, as my husband's mentor, Jack Sutphen taught him.