c and g exam question

[hoorain]

hello gents im new to ACR and have been reading the book air conditionog and refgn. technology and im trying to prepare for a 5day intensive course to hopefully get a first job as an entrant while i was going through the exam questions on the C&G website for 2079 i came across a question that to me seems to be misleading and challengable though i havnt had my first step into the ACR apart from working as a helper in installing residential split units but i think C&G need to rephrase the question: its a two part quesrion No.02.03 unit 301 candidates the answer to the scond part of the quesrion is given as (A) in the answers list.My personal oppinion is that you can also decrease the diffrence between the condensing and evaporating temperatures by increasing the ihdoor temperature so if thats done then do you think the answer given in the list is correct unless they specify a constant indoor temp. of with in design operating temperature e.g.70 F.

[Brian_UK]

hello gents I'm new to ACR and have been reading the book air conditioning and refgn. technology and im trying to prepare for a 5day intensive course to hopefully get a first job as an entrant.
While i was going through the exam questions on the C&G website for 2079 i came across a question that to me seems to be misleading and challengable though i haven't had my first step into the ACR apart from working as a helper in installing residential split units.

I think C&G need to rephrase the question:
its a two part question No.02.03 unit 301
candidates the answer to the second part of the question is given as (A) in the answers list.

My personal opinion is that you can also decrease the difference between the condensing and evaporating temperatures by increasing the indoor temperature.

So if thats done then do you think the answer given in the list is correct unless they specify a constant indoor temp. of with in design operating temperature e.g.70 F.

Hi Hoorain, welcome to the forum.

I have split your question a bit because it is extremely hard to read when it is a solid block of text.

As to your question it is difficult to answer because we don't know what answer A is, we haven't got the paper to hand.

[hoorain]

Hi Hoorain, welcome to the forum.

I have split your question a bit because it is extremely hard to read when it is a solid block of text.

As to your question it is difficult to answer because we don't know what answer A is, we haven't got the paper to hand.

ok the question goes like this;
(Quest.) As the difference between the condensing temp. and the evaporating temp. decreases:
(A) work done decreases and discharge temp. decreases.
(B) work done decreases and discharge temp. Increases.
(C) work done increases discharge temp. decreases.
(D) work done increases discharge temp. increases.
Correct Answer (A) in the city and guilds sample questions answers list.

you can view the sample questions if you type city and guilds f gas 2079 sample question.(on google).
I would await for all replies so pls reply all thanks

[Bones]

answer A sounds good to me.

whats misleading about that question?

[kasunyash]

yep i think so

[marc5180]

I had that on a test a while back, the answer is A.

When the room gets down to temp the suction pressure will fall as well as the discharge pressure, meaning that the work done decreases because there is less heat to move.

[acnerd]

I think the question may be misleading. It specifically says the difference between condensing and evaporating temperatures. As the work load decreases, both the condenser and evaporator temperatures decrease together. Do we assume that one falls more than the other to create a smaller difference in temperature?

You design guys are more clued up than me, but I'm sure I'll learn something from the answer!

[carlfoster]

Don't forget that refrigerating effect is created by the difference between the 2 pressures. So if that difference is reduced you will get less refrigeration taking place.

eg, when the compressor switches off the pressures equalise, the evaporator warms up and the discharge line cools down.

[marc5180]

The refrigeration effect is created by a difference of two temperatures, the heat of compression is the difference between two pressures.

[carlfoster]

The refrigeration effect is created by a difference of two temperatures, the heat of compression is the difference between two pressures.

I hate to have to disagree but I do. Although it is true that when the vapour is compressed it heats up as the pressure increases, but it heats up into a highly superheated vapour, which is useless until is cools down through the condenser and eventually starts condensing into a liquid.

So the heat of compression can not be the thing that creates refrigerating effect. The high pressure liquid on the other hand has the capacity to absorb a lot of heat when it passes through the restrictor, has it's pressure drastically reduced, and therefore boils off. The higher the pressure difference between the liquid line and the evaporator, the more cooling is taking place.

If you see different, I'm quite happy to be corrected

[philfridge]

My personal oppinion is that you can also decrease the diffrence between the condensing and evaporating temperatures by increasing the ihdoor temperature so if thats done then do you think the answer given in the list is correct unless they specify a constant indoor temp. of with in design operating temperature e.g.70 F.

No you are incorrect if you increase the indoor temperature you will increase the load raising the operating pressures / temperatures.

[marc5180]

I didnt sat the heat of compression makes the refrigeration effect i said the heat of compression is the difference between two pressures.

Look at a p/h chart and you will know what i mean.

The 4 points around the system are refrigeration effect, heat of compression, condensing effect and expansion.

[carlfoster]

I didnt sat the heat of compression makes the refrigeration effect i said the heat of compression is the difference between two pressures.

Look at a p/h chart and you will know what i mean.

The 4 points around the system are refrigeration effect, heat of compression, condensing effect and expansion.

Sorry mate, I misread your post. My bad.

[paul thompson]

100% the answer is A. I.VE COMPLETED THE 2079-101 knowledge assessment today (after 1 and a half days in the classroom ).oh and the good news is ive passed the gola on line 40 question test ,

[frank]

I hate to have to disagree but I do. Although it is true that when the vapour is compressed it heats up as the pressure increases, but it heats up into a highly superheated vapour, which is useless until is cools down through the condenser and eventually starts condensing into a liquid.

Don't forget that this high pressure vapour is also needed to produce the momentum to circulate the refrigerant around the circuit.