2079 Question Reference 04.01

[sparrow]

Hi people I am currently being used to assess both the Practical and online parts of 2079, I have come across a question that although the correct answer has been found I do not understand why?

Which of the following is most likely to increase refrigerant leakage potential from an Air to water heat pump system? Excessive

a) Expansion valve superheat
b) liquid line subcooling
c) discharge vapour superheat
d) expansion line subcooling.

I really would apreciate an explanation on the problem if anyone is interested.


Sparrow.

[Silhouette]

I think the answer is Expansion valve superheat!
Am i correct?

[TRASH101]

answer c)

Why? - I think the keyword is excessive. A significant increase in discharge superheat would increase the surface temp of the discharge pipe to, possibley, beyond reasonable parameters. As the question refers to specifically air to water heat pump I can only conclude that
A) the water could overheat.
B) the thermal expansion may become a problem.
C) any steam would be considered aggressive to the heat exchanger.

That is all I can think of. I hope someone else has a more concise answer.

[sparrow]

I think the answer is Expansion valve superheat!
Am i correct?

No silhouette but it looks like Trash101 is onto something here with the Excessive part, it also makes sense what he is saying.

Sparrow

[taz24]

No silhouette but it looks like Trash101 is onto something here with the Excessive part, it also makes sense what he is saying.

Sparrow

I got that question on my test and I answered Discharge superheat.

My reasoning is that if the discharge superheat is excesive then the pressure will be and that could result in somthing failing somewhere.

taz.

.

[Yuri B.]

Sorry, can "superheat" - by definition - refer to discharge vapour leaaving the compressor? What may be hotter than the vapour itself so as to add additional heat to it?

[Yuri B.]

Of course, the gas inside the comp is hotter.

[Yuri B.]

I am an electrician - not a "ref" mechanic - working in a HVAC firm these last few years, and untill now thought "superheat" was a specific term reflecting, representing the qwantity of refrigerant in an evaporator, describing its state, etc., during a refrigeration cycle (for one of hundreds other possible definitions). From the silence which followed my last post I infer the term's application field is wider.

[nike123]

I am an electrician - not a "ref" mechanic - working in a HVAC firm these last few years, and untill now thought "superheat" was a specific term reflecting, representing the qwantity of refrigerant in an evaporator, describing its state, etc., during a refrigeration cycle (for one of hundreds other possible definitions). From the silence which followed my last post I infer the term's application field is wider.

Refrigerant gases has their saturation point where they change state from gases to liquids and vice-versa.
For every pressure there is corresponding saturation temperature.
Compressor and expansion device serves to raise and lower pressure, and in that way change saturation temperatures to level which is suitable for our usage in particular equipment.

Also, in system gases could be superheated when all available liquid refrigerant in that part of circuit has changed state from liquid to gas and further heated.
That is what we call superheat and amount of that superheat could tell us how many, or if any, refrigerant is evaporated in that part of circuit.

Subcooling is simmilar. When all gas refrigerant has changed state and started to cool below its saturation temperature for corresponding pressure we call it subcooled. Amount of that subcooling tell us how many of liquid refrigerant is in that part of circuit.

These two values tells us where in circuit is refrigerant and ind what state, since we don't have mean to see his state by our eyes.

Interpreting these two values (along with others) serves to evaluate system operation and therefore all the time their usage in different subjects and relations.

[nike123]

Sorry, can "superheat" - by definition - refer to discharge vapour leaaving the compressor? What may be hotter than the vapour itself so as to add additional heat to it?

Vapor without presence of its liquid, is always superheated. Any amount of heat which gas has after change of state is superheat.

If only gas leaves evaporator it is superheated, gas leaving compressor is superheated.

[Yuri B.]

Hello Nike 123! Your last but one post is redundant, sorry for your effort, the last one is key one, elucidating me. Thus, I knew, liquids are subcooled, so water at 20 C and atmospheric pressure, thermodinamically speaking, is subcooled 80 C. I just missed, somehow, the point that it also refers (now as 'superheat') to gases - in ALL their states. Thank you.

[Yuri B.]

Self-precision: water at 20 C is subcooled 80 K.

[nike123]

Hello Nike 123! Your last but one post is redundant, sorry for your effort,

That effort is not wasted since other members of RE could learn something from that.
I learned a lot here, and it is my duty to spread that "disease".