basics i know... but what is the best temp fot air con!

[totalacedude]

hi all,
im not an engineer, just some 1 learning and interested in refrigeration etc,

i have a cheap import heat pump that needs a regas,
i am wondering that as i have charged it to correct weight as per the label it seems to be way over charged!

what is the ideal temp on the line to the evaperator (from outside unit, ie capil tube is in there)
obviousally i dont want it below 0c or it will freeze, but it is a capaliry type and i know the pressures will change as per load etc etc so what should i go for as my mean value temp wise? (i have those guages with temps on!)

is 5-6c to high?

or could i drop it a little?

it is r407c btw....

many thanks


ian

[totalacedude]

forgot to say, low side is running at 4.5 bar!

ian

[Gary]

It's not that simple:

Since you have charged it to the correct weight, it has the correct charge. If the suction line is too cold, you don't have enough indoor airflow. Change the filter, clean the fan/blower blades, clean the coil, make sure the air is not obstructed.

The system needs the right charge AND the right airflow.

[totalacedude]

opposite is happening in fact.... way to high!

found the scale to be pants (borrowed from work) not refrigeration scales, but simular!!! or so i thought!

everything is nice and clean!!!

it is working, just as im new i was wondering what pressures / temp it should be nominal,

cheers for the speedy reply ian : o )

[Gary]

opposite is happening in fact.... way to high!

found the scale to be pants (borrowed from work) not refrigeration scales, but simular!!! or so i thought!

everything is nice and clean!!!

it is working, just as im new i was wondering what pressures / temp it should be nominal,

cheers for the speedy reply ian : o )

What is way too high?

[Gary]

found the scale to be pants...

What does that mean?

[The Viking]

First of all, don't blame the scales.
Even a set of bathroom scales @ £5 from Tesco will get you close enough for the unit to run.

Before you charged it did you get it to a deep vacuum?
When you charged it, did you pull the refrigerant from the cylinder as a liquid or as a gas?

[The Viking]

What does that mean?

:D:D:D
Aha, a language barrier!

On this side of the pond "Pants" in this context would translate to "not very good".
:cool:

[totalacedude]

lol pants indeed!!! : o D

yes i did vac it and pulled the refrigerant out in liquid form!
once it settled i measured the low side to be 5.5 - 5.75 bar equ to 8-12c ish i think...

i used an old fridge compressor to vac it out! wil this pulled enough of a vac? (just under .7 bar)

cheers all :)

[The Viking]

i used an old fridge compressor to vac it out! wil this pulled enough of a vac? (just under .7 bar)


Aha,
Now we have found the problem!


As the system operates on R407c, it will also have synthetic oil.

This oil is extremely hygroscopic (=it absorbs a lot of moisture) and any moisture in it will turn it in to acid.

The only way to get rid of the moisture is to get it down to a deep (and standing) vacuum, measured in microns not bar. Unfortunately, no compressor will come close to this level, you will need a proper vacuum pump.

(The micron gauge will start moving well after your suction gauge has reach it's minimum).


Sorry.

[totalacedude]

ahhhhh now i get it!!!!
i thought that would have pulled anough of a vacuum, but obv not lol...
thanks for the reply.... will keep taking in the info...

cheers ian

[Gary]

There is a catch-22 here: If you are doing this to save money, you will find that doing it on the cheap will make the system use more energy and will shorten it's life.

On the other hand, if you are serious about learning refrigeration, you will want to learn it right and you will need the proper tools. They are not cheap.

[totalacedude]

i am doing it as im interested in this!, even tho it can be very confusing :s that is in part what attracts me (yea yea im a glutton for punishment).

i think the only tool i now need is a vac pump! (will have to wait until next month to afford that im afraid)

allready have
guages
refrigerent
scales (faulty, but now fixed :o)
ir temp probe
recovery cylinder ( old propane tank)
flaring tools
differnet oils

just need pump....

and the knowlage how to use it all lolol...

i am learning every second atm, but have still yet to find any info that starts from the beggining, or at least explain things as they go not just expecting you to know allready!!! lol

any pointers? cheers ian

[Gary]

i am learning every second atm, but have still yet to find any info that starts from the beggining, or at least explain things as they go not just expecting you to know allready!!! lol

any pointers? cheers ian

Fair enough. Let's start from the beginning:

Wet your finger and wave it in the air. What you are feeling is a refrigeration effect. When a liquid turns to a vapor it absorbs heat. In this case it is sucking the heat out of your finger.

The opposite is also true. If that vapor then loses that heat, it will turn back into a liquid.

In a refrigeration system, we force a liquid to become a vapor in the evaporator, thus absorbing heat from the refrigerated space.

We then use a compressor to pump that vapor to the condenser.

In the condenser we force that vapor to reject the heat and thus turn back into a liquid so that we can re-use it.

We then meter the liquid back into the evaporator to complete the loop and do it all over again and again and again.

How do we force a liquid to become a vapor?... or a vapor to become a liquid? By manipulating its boiling point.

The boiling point is the temperature at which the liquid turns to vapor when heat is added. It is also the temperature at which a vapor turns to liquid when heat is removed.

Boiling point = saturation temp = evaporating temp = condensing temp

When we think of the boiling point of a liquid it is the boiling point at zero psi pressure. If we increase its pressure we raise its boiling point. If we decrease its pressure we lower its boiling point.

In the evaporator we force liquid to become a vapor by lowering its pressure until its boiling point/evaporating temperature is lower than the air it is trying to cool.

In the condenser we force the vapor to become a liquid by raising its pressure until its boiling point/condensing temperature is higher than the air it is trying to heat.

Different substances have different boiling points at different pressures.

We can tell what the boiling point/saturation temp/evaporating temp/condensing temp is at various pressures for common refrigerants by checking a pressure/temperature chart.

Is this making sense so far?

[totalacedude]

i understand the basics....

its things like this that i dont get.

when guages connected to system lp and using chart change the pressure read into temp, i cant ever seem to find anywhere in my system that has a close temp to the corresponding value! lol

other things like in flooded systems i understand we want the temps above 0c or we get ice, but what is the best temp (if there is any) to keep the evaporator at.

superheat!!!

things like tev's, do they keep to a set temp, or is it a temperature differential from where it evaporates to where it measures

i have hundrends of questions, but am sure its better keeping shut atm till i learn more, and not look like an idiot lolol

cheers ian

[totalacedude]

ps forgot to say i understand that in flooded systems everything changes due to load etc, but just as a mean value....
cheers ian

[Gary]

i have hundrends of questions, but am sure its better keeping shut atm till i learn more, and not look like an idiot lolol

cheers ian

Tastes like chicken doesn't mean a thing unless you first know what chicken tastes like. All new knowledge must be linked to previously existing knowledge.

It is not a matter of looking foolish, it is a matter of learning in the proper sequence so that each new bit of information is linked to something you already know.

You are not ready for flooded systems.

[Gary]

Okay... let's go a step further: Superheat and subcooling.

If we boil off a liquid into vapor and then add heat to that vapor its temperature will rise above the saturation temperature. This is called superheating the vapor. When its temp is 10 degrees above the saturation temperature it is superheated 10 degrees. When its temp is 20 above saturation it has 20 degrees of superheat, etc, etc.

Similarly if we condense a vapor into liquid and then further cool the liquid this is called subcooling. When the temp gets 10 degrees below saturation it has 10 degrees of subcooling. When its temp is 20 degrees below saturation it has 20 degrees of subcooling.

Refrigerant flows very rapidly through the evaporator coil into the suction line. Many people believe that you can't have superheat until the liquid has all turned to vapor, but this is not true. Because of the velocity of the refrigerant flow it is possible to have liquid droplets surrounded by superheated vapor at the outlet of the evaporator... and in fact this is what happens. All of the liquid droplets are gone by the time there is 5-10F/3-5.5K superheat.

We want the superheat at the evaporator outlet to be low enough to ensure that we are fully utilizing the coil, thus maximizing its ability to absorb heat, but we do not want liquid droplets to be sent back to the compressor.

Similarly, it is possible to have vapor bubbles surrounded by subcooled liquid at the outlet of the condenser. All of the vapor bubbles disappear at about 10-15F/5.5-8.5K subcooling.

We want the subcooling to be high enough to ensure that we are sending sufficient liquid to the metering device, but not so high that we are backing up liquid into the condenser, thus reducing its ability to reject heat.

[totalacedude]

ok so why do so many ppl allow differning values of superheat to different refigerents, by what your saying we could all use the evap point of refigerant plus x degrees in every case!!!

i totally understand that we need to a. make sure we are not sending liquid back, and b. utilising metering device.

again am i being misslead by my friend

if this is the case id rather forget everything and learn it correctly :)
thanks ian

[Gary]

But wait, there's more...

[totalacedude]

lol..... go on...

by the way thanks :)

[Gary]

On a cap tube system there is a fixed amount of liquid flowing into the evaporator. When the load is heavy there is warmer air flowing through the coil and thus the liquid is all boiled off long before it reaches the outlet of the coil, thus the superheat is high when the load is heavy. If properly designed and charged, the superheat will be just right when the design temperature (design load) is reached.

Many people believe that a TXV will maintain a fixed superheat, regardless of load. This is just simply not true. When the load is heavy the superheat rises and more liquid is fed to the evaporator. The superheat remains high as long as the load remains high. And again, the superheat is just right when the design temperature (design load) is reached. But the design temp will be reached sooner because of the extra refrigerant feed.

[Gary]

As we see, when the load decreases the superheat decreases... so what happens when the filter gets dirty, or the evap coil... or the blower wheel? Less airflow means less load therefore the superheat drops, even though the refrigerated space may be at design temp.

When the load is high the superheat is high, and when the load is low the superheat is low... even with a TXV.

Everywhere, throughout the system, there are such opposing forces balancing against each other, and it can be very difficult to tell which of these forces is out of balance.

[Gary]

And yes, there is more... much much more... but that's enough for now.