Someone explain the answer to this please.

[moondawn]

Question. If the condensor fins are fouled or blocked, the HP increase but the LP also increases.
In this case why does the LP increase.

Answers 1. The Compressor draws in less vapour
2. More liquid passes through the capillary
3.Both suggestions are true
4. both suggestions are false.

WHICH IS CORRECT ANSWER
thanks in advance

[sweimaker]

l.p will be high as temperature hasnt been taken out of the hot gas in the condensor coil..

expansion valve reduces valve pressure/temperature, but hot/gas liquid is at a high temperature/pressure than it should

[moondawn]

hi sweimaker cheers for reply which answer would you go with then?

[simon@parker]

also you have to remeber this is a chemical reaction/process taking place the suction and discharge pressures of any gas can only be so far apart so as the discharge rises so does the suction
oh and to answer yr question both are wrong lol

[chillerman2006]

Hi Moondawn

a system has to balance and you have to match the heat absorbed in the evaporator to the heat rejected by the condensor

with a bocked/dirty condensor the head pressure will rise due to increased suction pressure and the heat rejected is not enough to reduce the refrigerant to equal or below its liquid phase temperature for that pressure

you then have a saturated vapour/liquid mix entering the evaporator instead of liquid

a liquid can absorb more heat per area of evap due to its increased mass and any available subcooling

lets just make up some figures for example

as a liquid the evap holds a kg of refrigerant and x amount of heat is transfered into the refrigerant by evaporator

as a saturated vapour/liquid mix you have less mass for instance 3/4 of a kg

but you are still transfering x amount of heat into the refrigerant by the evaporator

x amount of heat added to 1 kg of refrigerant = 4bar
x amount of heat added to 3/4 kg of refrigerant = 6bar

R's chillerman

[chilliwilly]

The answer is 4.

Compressor can't pull less vapour as there is only vapour being drawn in the form of superheated and not sub cooled refrigerant.

There won't be a full column of liquid in the liquid line to be passed through the cap or XV to be flashed into a gas.

[chillerman2006]

The answer is 4.

.

Argh chillywilly

thats cheating mate, i just give him all the facts so as 'Mad' does - gives me enough info for me to learn

rather than the answer

R's chillerman

[chilliwilly]

I'm sorry beegli beegli I'm sorry.

I didn't want to give a too longer explanaition as there was already enough to chew on

[chillerman2006]

I'm sorry beegli beegli I'm sorry.

I didn't want to give a too longer explanaition as there was already enough to chew on

Haha ok mate

think there is plenty to chew on now you mention it

R's chillerman

[RANGER1]

Right or wrong I think its 1.
The compressor will become less efficient as the pressure differential across it increases.

The piston has to go down further before gas enters cylinder, due to top dead centre clearance volume.

As far as more or less refrigerant passes through capilliary, I'm thinking hard to say as probably inefficient
poor quality liquid/gas.

[Tayters]

Hi Lee,

I like these questions. Most of us know the symptons but getting inside the pipework and imagining what is happening and why sorts the men from the boys. That said, I'm still definatly a boy in that respect!

I reckon as HP increses by a larger proportion than LP then more vapour is drawn in. Also the vapour is more superheated.
There would be less liquid in the cap tube as less liquid available from the condensor. Also it is at a higher saturated temp so as it cools along the tube it flashes off earlier

Therefore answer 4 correct.

Not quite sure of my theory about the vapour but seems right through my 'boys' eyes. Needs a 'man' to confirm!

Cheers,
Andy,

[chillerman2006]

Hi Tayters

I am still only one of the boys but all correct to me

the system is a constant wheel that has to balance

suction pressure/temp increase, then increases discharge pressure/temp

higher pressure/temp at a blocked coil can not reject enough heat to reduce liquid phase

mixed vapour in evap has increased pressure/temp per kg of refrigerant

the compressor now shifts a lower mass due to increased superheat and superheats each kg of refrigerant more

and the cycle continues until the point of equilibrium is reached

eg: R407c abient 25*c & 12*c evap on temp = 19bar.g/4.4bar.g

now becomes with blocked coil 23bar.g/6bar.g

and thats without considering, fractionating with R407c before one of my peers start

saying I have missed a bit

R's chillerman

[moondawn]

hi all, thanks for your replies the question is off a program called kotza and i was going through the cycle as best as i could (limited knowledge although) and could not understand why kotza was giving the answer as C.
It tells me the anser is both suggestions are true.

i could not understand how "more liquid passes through the capillary" as it would be flash gas more than liquid.

i to think the answer is D and i think that the answer on Kotza is wrong. its good to see how others have worked the answer out as that also teaches my way of thinking.
as tayters says its hard to see whats going on inside the pipes and these questions do challenge.

cheers lee

[chillerman2006]

Hi moondawn

I have 'kotza' mate and I agree with you

Kotza has some gremlins/false answers

less vapour at compressor inlet by higher superheat

mass flow through the cap is dependant on state of refrigerent

velocity will increase due to higher pressure

but liquid can only increase 'if' the sate of refrigerant is still liquid

and not a vapour/liquid mix

R's chillerman

[frank]

Draw the circuit on a PH chart and you will notice that the quality of the refrigerant entering the evaporator reduces as the discharge pressure goes up, (greater 2 phase) which results in the evaporator not being full of liquid, and this increases suction superheat.

With a high temperature suction gas entering the compressor, you then add the heat of compression to it, resulting in a higher than design discharge temperature.

Don't forget that the first 3K of superheat is useful but any additional superheat is un-useful (wasted energy)

[moondawn]

well thanks all will continue to use it and any more questions i aint sure abouts will post and discuss.

cheers

[Josip]

Hi, moondawn

Question. If the condensor fins are fouled or blocked, the HP increase but the LP also increases.
In this case why does the LP increase.

Answers 1. The Compressor draws in less vapour
2. More liquid passes through the capillary
3.Both suggestions are true
4. both suggestions are false.

WHICH IS CORRECT ANSWER
thanks in advance

3. Both suggestions are true ... why...

Drawing this cycle on the Mollier chart .... having condensing pressure above normal (both for water and air cooled condensers) due to...

a) air or noncondensable gases in system
b) partially blocked or dirty condenser
c) high ambient temperature
d) high temperature cooling water
e) insufficient cooling water
f) high ambient air temperature
g) insufficient air flow

result is ....

1) Condensing pressure - increase
2) Evaporating pressure - increase greatly in case of capillary tube, but superheat decrease, in case with expansion valve evaporating pressure increase a little, but SH remains constant

3) Compression ratio - increase
4) Specific volume of suction vapor little decrease
both are giving reduced weight of circulated refrigerant i.e. cooling capacity decrease

Refrigerating effect decrease....
C.O.P decrease and due to higher condensing pressure running current increase..



Best regards, Josip

[chillerman2006]

result is ....

1) Condensing pressure - increase
2) Evaporating pressure - in case with expansion valve evaporating pressure increase a little, but SH remains constant

Hi Josip

I cant understand how superheat can remain constant

If the mass in evap is decreased (from liquid to mixed vapour) and heat transfer remains constant

Then pressure and superheat have to go up surely ???

As was the case with a unit I worked on just the other day

6bar.g suction and 18*c superheat with blocked coil

Once coil cleaned 4.4bar.g suction and 11*c superheat

With the same water outlet temp

(have also edited post #14 to be more accurate)

Regards chillerman

[mad fridgie]

Well, this is a tricky one.
Ok if both 1 and 2 are right, then the program is wrong, if you increase the liquid mass flow through the expansion device, then you must increase the mass flow through the compressor (this not about net refrigeration effect)
How ever your liquid/vapor % leaving the expansion valve will increase (more vapour) this then leaves less liquid to absorb load. Your heat ex-changer will reach equilibrium, so if you can not absorb the design amount of energy, then temp difference across the heat exchanger will reduce lifting the suction pressure. Depending upon the suction superheat, your actual compressor inlet mass flow could increase.
Then we have to consider the volumentric efficiency of the compressor, if it is a recip, then as compression ratio increases, the volumetric efficiency decrease (less actual mass flow through the compressor)
So really no a straight forward question.

[moondawn]

well thats me baffled again :-)
mad fridgie which one do you think is the answer or you thinking the question is not clear enough?