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the temperature difference of an evaporator coil is supposed to be 15 degrees F. you measure only 5 degrees F. all operating pressures check out normally and so does the superheat and subcooling. what is wrong?
a. the superheat measurement you took must be off
b. the ambient temperture is excessive
c. the humidit level is low
d. the humidity level is high
(please select best answer and comment if possible -- thanks!)
Do you mean Delta T? (change in temperature - of the air)? Dt is not TD (temperature difference - of coil and air).
If the DT is supposed to be 15 degrees, it would be assumed that the enthalpy of the air would be inclusive in order to specify what it should be.
If the TD is to be 15 degrees, it would not be affected by humidity.
A high ambient temperature would suggest a decrease in condenser or compressor capacity. Such a scenario would likely be determined by merely hooking up a gauge set prior to having to do any other extraneous troubleshooting, I would guess (considering the discrepancy that you pose).
:)
Sounds like you are making up a quiz.Quote:
he temperature difference of an evaporator coil is supposed to be 15 degrees F. you measure only 5 degrees F. all operating pressures check out normally and so does the superheat and subcooling. what is wrong?
But I would dismiss this question from a quiz because it is ill thought out. It has a "borrowed" smell to it.
An evaporator coil is not "supposed" to do anything. It is a tangle of copper and aluminum. Who supposes?
What the hell does "check out" mean?
What the hell does "what is wrong mean?"
Regardless, here is my answer:
Nothing is wrong, of course, unless there is some food spoiling, or people sweating.
How did I do?:)
Dan, I laughed for two minutes straight after reading your post. I thought the same thing after reading the quiz; E nothing is wrong.
- John K.
Confusing dT with TD is common.
Delta-T (dt) refers to a change in temperature of a single medium. In this case, a drop in air temperature as the air flows through the coil.
RA - SA = dT
Temperature difference (TD) refers to the temperature difference between two mediums. In this case, the difference between the return air temperature and the low side refrigerant saturation temperature (saturated suction temperature aka SST).
RA - SST = TD
A coil does two things: It cools and it de-humidifies. The cooling shows up in the form of increased dT (lower SA temperature), but the de-humidification does not. When a higher proportion of the refrigerating effect is used to de-humidify, the dT is lower (more de-humidification means less cooling).
If the question applies to dT rather than TD, then the answer is D.
Darn, Gary. I missed that possibility. That must have been what the question was asking. Leave it to you to come up with a good answer to a bad question.:)
Quote:
the temperature difference of an evaporator coil is supposed to be 15 degrees F. you measure only 5 degrees F. all operating pressures check out normally and so does the superheat and subcooling. what is wrong?
a. the superheat measurement you took must be off
b. the ambient temperture is excessive
c. the humidit level is low
d. the humidity level is high
The question states that there is a problem, because the observed DT (or whatever it is, we don't know) is incorrect. Merely by the statement
one would have to assume that "what it is supposed to be" is determined in consideration of all the things that would affect it (the humidity thereby inclusive). IN THAT RESPECT, ANY SPECULATION REGARDING THE LOAD IS NULL!Quote:
"is supposed to be 15 degrees F. you measure only 5 degrees F"
Actually the question reads like this, "Nothing is Wrong. So what of the following is wrong?"
Answer "D", essentially negates the premise of the question (that the observed (?-difference) is wrong), because a higher humidity would "jusify" the observed (difference) as what it "is supposed to be". But again, you are asked to respond on the basis that the observed (difference) is supposed to be (15degF), not (5degF).
In the light of the above paragraph, answer "C" would be more fitting, "Low humidity level". Why? Because logically, if a higher humidity level would result in a lower DT when the system is operating properly, and nothing is wrong - alas, however something is wrong - it must be that the humidity level is low, because if the humidity level was high the only thing that would be wrong is that the difference is supposed to be 15 degrees!.
... What is the difference between a Duck?.....
answer: One foot is both the same!
:D
On the other hand, perhaps we should consider the use of the word "supposed". Upon further evaluation of the question, I pondered the use and definition of suppose:
1. Assume as true
2. expect to act as stated
The definition(s) would imply that one was conveyed the information that the DT is 15degrees, hence assumed it true. By definition, suppose is not synonomous with a "guess", but implies some resource as a reference for the supposition. There is no qualification of the relationship of the information to any supposition that may possibly be made. As such, would you "suppose" that since birds fly ........... Emus fly?
Well really, only birds that fly can fly.
:confused:
Which really, this is the basis of the difficulty with the question. In the case of the bird that does not fly example, most (varieties of) birds fly. With no familiarity, if one is informed of the fact that an emu is a bird, one could "suppose" that it flies, referencing the information provided. However the supposition would be incorrect because a lack of information pertaining to the physical characteristics of the bird, those of which enable flight (wing structure, body weight, etc).
The supposition that the difference is supposed to be 15 degrees may be a reference to the original design criteria for the application, in consideration of the fact that the machines are operating properly. However the supposition may be proven incorrect due to the ommission of information pertaining to the change in the other factors from which the difference is derived.
The tragedy in this "test question", is that the relationship between the load and temperature difference is backward. Even worse, the condition is taught as a WRONG!!! Can you imagine the rookie (fresh out of tech school) spending all day on the job trying to "make it right"?
________________________________________________
Actually, the question points back to itself.
A discrepancy is observed:
No discrepancy is observed:Quote:
"is supposed to be 15 degrees F. you measure only 5 degrees F"
You may conclude that the question itself is wrong, but then you would be assuming the intent of the author. Because of familiarity with the subject we can make assumptions about the question based on the answers provided, and the common misuse of the terms.Quote:
all operating pressures check out normally and so does the superheat and subcooling.
__________________________________________________
What makes it impossible however, is that no appropriate answer is provided. Answers such as:
You were too lazy to measure the other 10 degrees and knocked off early
Nothing is wrong
You did not calculate the difference with wet bulb readings
You did not calculate the difference at all before taking readings
The 15 degree supposition is wrong
Of course the last of the potential correct answers is the Real Answer
:)
My answer would be A, (measurement off). Humidity shouldn't make a difference and if ambient temp is indeed excessive then pressures would be off. :)
Now Robearbam, If the ambient was high, the pressures would not be "OFF", the pressures would be "higher", however CORRECT for the operating conditions (in a properly operating system, which essentially is what is described).Quote:
Originally posted by Robearbam
My answer would be A, (measurement off). Humidity shouldn't make a difference and if ambient temp is indeed excessive then pressures would be off. :)
In regard to your observation of the effect of humidity, the Delta-T of the airflow across the EVAPORATOR coil decreases with higher humidity due to the increase of latent work of dehumidification.
:)
I guess "excessive ambient" could mean the condensing unit just sits in the sun on a hot day, "or", it could mean that it is somehow parked next to other equipment that attributes to the word "excessive". I've seen 10 - 3 ton condensing units in a basement with no ventilation. A rather small area to boot. You can imagine the outcome of that!:)
How about...the system isn't stabilized and readings shouldn't be taken yet? :confused:
How about...dantoubiana dropped out of tech school and never visited this sight again.:D
Quote:
Originally posted by herefishy
The question states that there is a problem, because the observed DT (or whatever it is, we don't know) is incorrect. Merely by the statement
one would have to assume that "what it is supposed to be" is determined in consideration of all the things that would affect it (the humidity thereby inclusive). IN THAT RESPECT, ANY SPECULATION REGARDING THE LOAD IS NULL!
Actually the question reads like this, "Nothing is Wrong. So what of the following is wrong?"
Answer "D", essentially negates the premise of the question (that the observed (?-difference) is wrong), because a higher humidity would "jusify" the observed (difference) as what it "is supposed to be". But again, you are asked to respond on the basis that the observed (difference) is supposed to be (15degF), not (5degF).
In the light of the above paragraph, answer "C" would be more fitting, "Low humidity level". Why? Because logically, if a higher humidity level would result in a lower DT when the system is operating properly, and nothing is wrong - alas, however something is wrong - it must be that the humidity level is low, because if the humidity level was high the only thing that would be wrong is that the difference is supposed to be 15 degrees!.
... What is the difference between a Duck?.....
answer: One foot is both the same!
:D
On the other hand, perhaps we should consider the use of the word "supposed". Upon further evaluation of the question, I pondered the use and definition of suppose:
1. Assume as true
2. expect to act as stated
The definition(s) would imply that one was conveyed the information that the DT is 15degrees, hence assumed it true. By definition, suppose is not synonomous with a "guess", but implies some resource as a reference for the supposition. There is no qualification of the relationship of the information to any supposition that may possibly be made. As such, would you "suppose" that since birds fly ........... Emus fly?
Well really, only birds that fly can fly.
:confused:
Which really, this is the basis of the difficulty with the question. In the case of the bird that does not fly example, most (varieties of) birds fly. With no familiarity, if one is informed of the fact that an emu is a bird, one could "suppose" that it flies, referencing the information provided. However the supposition would be incorrect because a lack of information pertaining to the physical characteristics of the bird, those of which enable flight (wing structure, body weight, etc).
The supposition that the difference is supposed to be 15 degrees may be a reference to the original design criteria for the application, in consideration of the fact that the machines are operating properly. However the supposition may be proven incorrect due to the ommission of information pertaining to the change in the other factors from which the difference is derived.
The tragedy in this "test question", is that the relationship between the load and temperature difference is backward. Even worse, the condition is taught as a WRONG!!! Can you imagine the rookie (fresh out of tech school) spending all day on the job trying to "make it right"?
________________________________________________
Actually, the question points back to itself.
A discrepancy is observed:
No discrepancy is observed:
You may conclude that the question itself is wrong, but then you would be assuming the intent of the author. Because of familiarity with the subject we can make assumptions about the question based on the answers provided, and the common misuse of the terms.
__________________________________________________
What makes it impossible however, is that no appropriate answer is provided. Answers such as:
You were too lazy to measure the other 10 degrees and knocked off early
Nothing is wrong
You did not calculate the difference with wet bulb readings
You did not calculate the difference at all before taking readings
The 15 degree supposition is wrong
Of course the last of the potential correct answers is the Real Answer
:)
:p