Iced Evaporator

The equipment is trying to do too much work, Guv. Try asking less of it.

Sorry I don't understand what do you mean .can you pleased to give more details

Thank you tcmech and G.A from me .But is there any more?

What about less or excess refrigerant charge . is it taken into consideration or not

"Package AC units"; this covers lots of different types. Is this a window type AC, a split type. How many tons of refrigeration? What type of metering device is this system using e.g. capillary tube, thermostatic expansion valve?

What is the humidity conditions when this happens? Is the thermostat calling for cooling continuously? Are the evaporator coils dirty, the filter?

Once ice starts to form on an evaporator, it acts both to prevent airflow, and insulate the evaporator, so the ice will build up. But more information is needed as to system type. Capillary tube or some other restrictor type metering device, these systems are refrigerant charge critical systems. When the charge is slightly low, the condensing pressure, and evaporator pressure drops, causing the evaporator to drop below the freezing temperature of water.

When the charge is slightly low, the condensing pressure, and evaporator pressure drops, causing the evaporator to drop below the freezing temperature of water.

I understand that cooling rate depends upon the amount of pressure drop before and after expansion valve.

for low charge pressure , it leads to less pressure drop ,it means lower cooling rate.

So it will not be a cause for less evaporator temperature , I think it may be right for condenser ,but not for evaporator.

Am i right

The type of AC system is important here. You did not indicate what this is.

No, cooling rate (BTU/hr) is based on the size of the whole unit, compressor pumping rate, evaporator size, condenser size.

For fixed flow restriction type metering device, the cooling rate will change based on condensing temperature, evaporator temp. Since water vapor is condensing on evaporator (your complaint of frost), this and the air temperature through the evaporator are part of what the evaporator temperature becomes.

You need to describe your system.

Nine times out of ten, when I have encountered icing on the coils, it has been due to the thermostat set too low, as mike_k has suggested. Most people do not understand what a thermostat is- it is simply an on/off switch. The unit must cycle off occasionally to allow the coils to warm up. If the thermostat is set too low, the compressor never cycles off, and ice builds up. As someone else pointed out, lower thermostat settings do not change the temperature of the cold air being injected onto the room, nor does it change the rate at which the room is cooled down. Many hotels here in the tropics will actually disable access to the thermostat control by room residents for this very reason.

If your system is properly charged the evaporator temperature will be around 4C.

Ten times out of ten when I have encountered icing on the coils, it has been due to low refrigerant charge.

Low thermostat setting results in a cold room.

On restricted air flow causing icing...,,total bollocks. The room will warm up and if the airflow is really restricted the A/C unit will stop the compressor on a high pressure setting or the compressor will flog liquid, stall, hummmmm, and turn off on over temperature (eventually... permanently or until the motor cools)

I think that if the air flow is completely stopped, the Low pressure switch will trip. Since there will be no heat input, the evaporator will not be anymore an evaporator. Liquid gas will not evaporate any more and the suction pressure will drop drastically.

The condensor will not overpressure.

The danger will be that some liquid will reach the compressor and will damage it if the low pressure switch is absent or not functioning.

Ice formation will develop at first and as long as air is flowing, bringing humidity. If no air flows, the icing will be limited to whatever was there as humidity ...

Yes, everything tcmtech, Yahlasit, and of course the SolarEagle said. If it's the charge it will fail as more leaks out BUT I have seen these split units used in the ME etc. just ice some when the room temp is low and they are running flat out [most use 'little' inverters to power the compressors], there are settings to vary how the louvers so you won't see it and ice detectors on the evaporator coils to slow down the compressor when it ices.

Aish, do you have to pop the top cover off to see the ice?

It may be normal as I've seen these units working normal have some ice on the evaporator before the compressor slows down to defrost it.

The chips that control these new units are good. I only kept a list of what to jump {like the old PID loop books} to get these units working when things started going out like charge. Then I had the indoor and outdoor units changed out, where you live it's not that much $.

Short answer, assuming this an AC unit that was working ok and is now icing up: Low refrigerant level.

Tighten up the unions in the refrigerant circuit and bump up the charge.

with all respect for all contributors , this for yours valuable suggestions.

but I'm still not understand why less refrigerant charge leads to this phenomena.

Say for no air stream restriction , good blower condition , also all other parameters are in good condition.

why less refrigerant charge causing this phenomena.
Can some one gives more illustration

For your answer, look at #14.

Low refrigerant charge, as explained, will cause very low temperature at the evaporator fins (There is still some gas in the system!), which will start icing the condensed water from the air, strait on the surface of the fins --> Blocks the air flow --> increases the icing etc.

Apart from checking the High and low pressures on a system, you must make sure that you have charged the correct amount of refrigerant as per nameplate. If the ambiant conditions change, the amount of gas must allways be enough so as not to cause too low a suction pressure, which will cause a higher pressure drop at the expansion valve, hence a possible too low a temperature at the evaporator.

the amount of gas must allways be enough so as not to cause too low a suction pressure, which will cause a higher pressure drop at the expansion valve, hence a possible too low a temperature at the evaporator

very good explanation LAA and G.A from me.

thank you

With too little refrigerant in the system the pressure is lower and results in the refrigerant boiling (evaporating) at a lower temperature in the evaporator and sometimes it even starts boiling in the high pressure line incoming. If it is really low it boils at the HP connection at the condensor and ices that up as well.

First I would check the air flow in & out to make sure there is no obstruction to the vents, intakes & any duct/channels. Then make sure the filters & coil Fins are clean & get enough air flow, also make sure the fan is working correctly. Next I would check the temperature setting making sure it is not too low forcing it to run continuously, then make sure the temp controller is actually working. You can do this by increasing the temperature setting to see if it actually cycles at the higher setting. If that is all ok I agree with the others check for low gas.

i am agreed with tcmtech it can happen because of blockage in fins/filter so the air cant collect the full cooling or can be happen when the gas is less also.

Either low air flow or low refrigerant charge will cause the suction pressure, along with suction temperature, to fall into the icing range.

The low air flow is most often due to dirty filters which restricts air flow. You can easily install clean filters.

Try that and if it doesn't correct the icing problem nake certain nothing is blocking any of the supply or return air diffusers. As a longer shot the supply fan may be turning too slowly and moving insufficient air.

If low air is not your porblem you will need the services of a reliable air conditioning service firm to investigate and correct the problem. Choose this firm carefully. Correcting low suction is not rocket science but there are a lot of charlatans driving cleverly labeled service vans. Rather than soliciting by phone and going with the lowest price get recommendations from friends. Good luck.

Lou Bindner