Help! having trouble with college question

[MadApprentice]

Ok, i have read my text book, and have consulted with tradesmen and my boss. But the smart mouth college teacher still wasnt happy with my result

We had to explain the effects of 5 different states on a refrigeration system. I know i should have this covered but i work on A/C and this is just making me not want to do refrigeration anymore!

We have to comment on the effects of having a normal system compared to having a blocked condenser, a blocked evaporator

I nailed these two situations, but the next two just have me stumped. What effect does having a high superheat have on the system?

And what effect does having a low superheat have on the system (looking at the tx valve mainly but they want a system overview)

Could people out there please explain to me these last two question and this teacher keeps flapping on about mass flow rate

Help is appreciated

Mad Apprentice

[slingblade]

Most direct effects are as follows.
1 high superheat, loss of duty in evaporator as not fully utilised and poor compressor motor cooling.

2 low superheat, possible liquid return to compressor resulting in oil dilution or slugging and smashed valves etc.

Mass flow rate is exactly what it says. the rate at which refrigerant flows round a system to give best heat exchange at any given load. the txv will effect the flow rate as it opens and closes but will control the superheat and give the best duty for the conditions most of the time, other factors aside ( blocked evap. etc )

[MadApprentice]

these are the readings i got

Condition: Low tx superheat

Airon coil 2degrees C
vapour into comp 8.9Degrees C
Liquid line into TX: 7.4DegC
Tx Bulb Temp -5.5degC

Superheat 2.5k
Suction 280kpa
Hp 1600kpa
Pressure at evap outlet: 300kpa

Condition: high tx superheat

Air on Evap 2degC
Vapour into Comp: 12.3DegC
Liquid line into TX 3.1degC
tx bulb 3.1degC
Superheat 11.4k
suction 240kpa
Hp 1600kpa
Pressure at evap outlet 250kpa



The hp is constant because it has fan speed controller. But the annoying teacher wants me to explain why i get these figures in these conditions. Its giving me the s.hits

Ihave written down what you have said and any further help is gratly apreciated

[Ronny]

Ok, i have read my text book, and have consulted with tradesmen and my boss. But the smart mouth college teacher still wasnt happy with my result

We had to explain the effects of 5 different states on a refrigeration system. I know i should have this covered but i work on A/C and this is just making me not want to do refrigeration anymore!

We have to comment on the effects of having a normal system compared to having a blocked condenser, a blocked evaporator

I nailed these two situations, but the next two just have me stumped. What effect does having a high superheat have on the system?

And what effect does having a low superheat have on the system (looking at the tx valve mainly but they want a system overview)

Could people out there please explain to me these last two question and this teacher keeps flapping on about mass flow rate

Help is appreciated

Mad Apprentice

Hello,

Superheat to high::eek:

compressor inlet temperature will rise and at the same pressure level the ref will have a lower density. This will lead to less mass-flow and in addition less cooling capacity:mad:
The further the hot gas temperature does increase according the isentropic behaviour.

Superheat too low::eek:

liquid ref can reach the compressor suction valve. Danger of liquid slugging:mad:
Liquid ref may change the oilbalance inside the compressor if there is no oilseperator inside.

[MadApprentice]

Hello,

Superheat to high::eek:

compressor inlet temperature will rise and at the same pressure level the ref will have a lower density. This will lead to less mass-flow and in addition less cooling capacity:mad:
The further the hot gas temperature does increase according the isentropic behaviour.

Superheat too low::eek:

liquid ref can reach the compressor suction valve. Danger of liquid slugging:mad:
Liquid ref may change the oilbalance inside the compressor if there is no oilseperator inside.



thank you for the help, but could you elaborate further? I am still not quite there(Iam only a 2nd year apprentice), with my understanding but what you have just said makes for more sense than wat the teacher was craping on about


Thank you again for the help it is much appreciated

[frank]

Hi MadApprentice.

This post just reinforces my previous reply to your post in the other thread.

If you studied hard and didn't consider that all theory was a waste of time, you would be like all the other engineers on here - able to visualise what would happen inside a system given a set of operating conditions.

Your comment about not wanting to do refrigeration but just to concentrate on air conditioning - air conditioning utilises the refrigeration circuit at the heart of it's operation. You must have a good understanding of the theory before you can do either.

[Brian_UK]

Mind you, I am interested in the five states of the system, I thought that there were only two changes of state so can't figure out where the other three come from.

[Josip]

Hi, MadApprentice :)

this is about blocked condenser thus having high condenser pressure and some other problems

http://www.refrigeration-engineer.com/forums/showpost.php?p=47154&postcount=4

Best regards, Josip :)

[MadApprentice]

Brian to carify, we had to run a refrig system in normal operation conditions and draw up a ph chart

then we had to block the condenser and take recordings and draw a phchart over the top of te original one

then the same with blocked evaporator

then low tx superheat

then high tx superheat

and you would eventually have 5 system states ( or 4 faults and 1 normal) system situations.


Then we had to explain how these changes made a differenceand i was having troulble with the superheat comments.

Make sense?

[Bones]

hey Mad,

what TAFE College do you go to?

With the 5 states thing...

Are you supposed to state down a ph chart on a working fridge system, then add faults to it...

ie. block condenser, block evap, open tx, close tx etc and compare readings to the correct working system and explain how the system differed from the correct operation with each of the faults?

[MadApprentice]

i go to qld yeah thats pretty much it states was probably the wrong word to use.
It would be better referd to compare for possible faults against a correctly operating system

[Ronny]

Hi Mad???,

it is not easy for me to explain it in an easy way, cause I am not a teacher and I do not know the education system there.

We start with superheat and a system equipped with a TXV (thermal expansion valve):

I expect from you for this that you are able to read the logp-h-diagramm and to have the basics with the thermodynamic properties.

We use for explanation a car system (I am working in this area)!

Beginning is the suction inlet of the compressor. Normal condition (ps = 3 bar, ts = 1o°C). This does mean you have approx 10 K Superheat. On this point you will have a density of the R134a phase.(go in logp_h) You will find rho ~ 14.3 kg/m³, now we increase the superheat extremly to 50 K!!! (low charge of the system, maybe). (go in logp_h) You will find rho ~ 12.5 kg/m³. The displacement of your compressor is still the same, but the density reduction is approx 13%. That means that the compressor will deliver less refrigerant and the massflow does decrease. In the first step you can say it is a linear function, which does mean your cooling capacity will drop at those 13% too. So far, so good?
Now we expect that the isentropic efficiency of the compressor will not change (in reality he does!!, to a worser value).

Compression line:
(go in logp_h) at an assumed hp (pd = 13.2 bar approx 50°C with 10K Superheat) NOW 50K Superheat with the same isentropic behavior (pd = 13.2 bar approx 100°C with 50K Superheat). Here you can see the influence on the hotgas. That describes high temperatures inside the compressor (higher than 100 °C!!) and more thermal stress for the other parts on the hp-side. Keep in mind that oil is sensitive to high temperature from 150°C you can crack it maybe.

So now I stop and wait of your reply if you get it before we discuss the other things.

Have a nice weekend,
Ronny

[MadApprentice]

Woah, man you were just speaking another language!!! We havent talked about isentropic behaviours yet. But i had another attempt at the task and with the help from guys here i nderstood it and nailed it! ( the teach even made a comment that it was one of the best he read so far!)

Thanksfor the help
Happyapprentice

PS had an airconditioning exam and got 92% i am fking STOKED

From the Happierapprentice

[MadApprentice]

Hey all i need help again, I am in desperate need of a blank commisioning form. I need to see what they contain, as i am supposed to make on for college! (i have never seen one before)

Please could someone direct where i could find one?

Thank you