Charge amount at altitude?

[awilkinsonuk]

I have a strange one here.....

If we charge a capillary tube system with 180g of R404a at sea level the charge amount is perfect. If we charge the exact same unit at altitude with 180g it appears to be under-gassed?

Help!

Andy

[Brian_UK]

Welcome to the forum.

There was another thread relating to charging at altitude which may be of interest....

http://www.refrigeration-engineer.com/forums/showthread.php?t=10765&highlight=altitude

By what critirea are you saying that it appears undercharged, can you give us some readings?

Did you actually re-evacuate and reweigh in the charge at altitude or are things being done in a test chamber?

[awilkinsonuk]

Basically we have a plant in Mexico (@6500ft ASL) and a plant in Dallas and the UK. We have a unit we have been producing in the UK for years that is a cap tube system. We weigh in 180g and the unit has frost on the suction return about 1" away from the compressor. This is what I would call a critical charge (I am no refrigeration expert by the way).

If we charge the unit in Mexico (on different scales) the unit looks undercharged and has no frost anywhere near the compressor. To gain the same result as a UK unit we need to use about 250g of refrigerant. If we then ship this unit to the UK it is overcharged and we have frost on the compressor.

Sorry if the explanation/terminology isn't perfect

Andy

[Brian_UK]

Explanation not bad, thanks :)

What are the ambient temperatures in the different locations ?

If the dew point temperature changes then the pipes may or may not start freezing/frosting.

[awilkinsonuk]

We have a test lab in the uk and the ambient temp is both 24 deg C in both locations, humidity is virtually identical too. In Dallas we shipped a unit and again similar conditions and the unit worked perfectly with 180g, The only obvious difference is altitude.

We shipped some units back to the uk from Mexico and these we reclaimed the gas and got 230g to 240g. The guys here in Mexico had been critically charging the units and this is the amount it took.

I am puzzled!!

Andy

[awilkinsonuk]

I should also say that this is a coiled well unit and 3/4 of the well has frost the last third does not unless I add more R404a.

[nike123]

Basically we have a plant in Mexico (@6500ft ASL) and a plant in Dallas and the UK. We have a unit we have been producing in the UK for years that is a cap tube system. We weigh in 180g and the unit has frost on the suction return about 1" away from the compressor. This is what I would call a critical charge (I am no refrigeration expert by the way).

If we charge the unit in Mexico (on different scales) the unit looks undercharged and has no frost anywhere near the compressor. To gain the same result as a UK unit we need to use about 250g of refrigerant. If we then ship this unit to the UK it is overcharged and we have frost on the compressor.

Sorry if the explanation/terminology isn't perfect

Andy

This is what I think about your problem.
You need to pressurize charging plant at sea level pressure of 1013 millibars to achieve simmilar density of air there and in, say, UK.
Because of thinner air in your altitude evaporator gets lower amount/density of air and therefore it have lower superheat (and heat absorption capacity) and subcooling (and heat rejection capacity) than at sea level. That is why you need more refrigerant at higher level to compensate for lower density of air.

If your products are for market at that high sea level then they should be calculated and charged for that air density.

[awilkinsonuk]

That does make sense but would you expect the 6500ft altitude to give as much as 30% differential in the charge amount? I would have thought 10% but we are charging a unit that normally has 180g with 230g or more!

Andy

Thanks for the answer by the way

[Gary]

I'm thinking a comparison of data might reveal the differences.

Low side:

Evap air in temp
Evap air out temp
Saturated suction temp (SST) converted from pressure
Suction line temp at evap outlet

High side:

Cond air in temp
Cond air out temp
Saturated condensing temp (SCT) converted from pressure
Liquid line temp at receiver outlet

[nike123]

That does make sense but would you expect the 6500ft altitude to give as much as 30% differential in the charge amount? I would have thought 10% but we are charging a unit that normally has 180g with 230g or more!



I don't know. I am not designer of refrigeration equipment.
That was only my logical thinking when I consider different air density on different altitudes.
Inside the circuit, different altitudes cannot have any influence, because system is sealed from environment pressures.

P.S.
Do you have any difference in frequency and voltage on that locations?

[awilkinsonuk]

There is one difference.....


The units are designed for 230VAC 50Hz and we run them here at 230VAC 60Hz.

This is a small 10cc single phase compressor.

Andy

[Gary]

There is one difference.....


The units are designed for 230VAC 50Hz and we run them here at 230VAC 60Hz.

This is a small 10cc single phase compressor.

Andy

That's a BIG difference. The compressor and fans have 20% more capacity on 60Hz.

[nike123]

There is one difference.....


The units are designed for 230VAC 50Hz and we run them here at 230VAC 60Hz.



Andy

OK, as Garry already said, that is what is your problem!

You cannot use same compressor, evaporator, condenser and same amount of refrigerant at different frequencies because mass flow of refrigerant is not same with different frequency. Imagine that as you have inverter compressor.

[Brian_UK]

There is one difference.....


The units are designed for 230VAC 50Hz and we run them here at 230VAC 60Hz.

This is a small 10cc single phase compressor.

AndyThere you go, typical designer.

Only give you half the facts and save the deciding information 'til last :D:D

At least we're all learning here Andy ;)

[Gary]

P.S.
Do you have any difference in frequency and voltage on that locations?

Good call, Nike.

[awilkinsonuk]

Yes, I thought we were running a genorator but I only found out today we run a transformer and don't alter the frequency! As we say in the UK - assumption is the mother of all f**k-ups!!

[nike123]

Good call, Nike.

Thanks Gary!
When it comes from authority like you, I am more than honored.

[awilkinsonuk]

One thing though....

We ran a unit in Dallas at 208Vac @ 60Hz and this ran ok and the charge amount was correct. If we run the same machine here in Mexico at 230Vac @ 60Hz it works totally different! I wouldn't have thought dropping 10% on the voltage would make up for the 20% increase in frequency would it?

Andy

[icecube51]

if a system is designed to run on certain specifics be aware if you change something on what makes it to go to work ,power or pressure,the whole outcome will change.remember we are working whit thermo dynamic fluid and gases.

Ice

[icecube51]

have seen it on two units,one next to the other,working on the same power line. stand alone they worked perfectly,together yore worst nightmare. giff them each a power line of their one and problem solved.

we learn surprisingly each day,if we let it happen.;)

Ice

[Gary]

One thing though....

We ran a unit in Dallas at 208Vac @ 60Hz and this ran ok and the charge amount was correct. If we run the same machine here in Mexico at 230Vac @ 60Hz it works totally different! I wouldn't have thought dropping 10% on the voltage would make up for the 20% increase in frequency would it?

Andy


On a motor under load, dropping the voltage increases the slippage and therefore decreases the speed.