Thread: Aircon woes

Pat I recon thats the way to go, similar heat and time of day to replicate conditions. This will if need be put me in a better position when talking to the stealer. However I don"t think my bar, lights front end set up would actually be that unique and am surprised this has not been previously observed.
It will be few days before I can try this as I"m back at work.

The basic vapour/compression refrigeration cycle.

Compressor.

As it's name suggests, it takes a low pressure gas and compresses it to a higher pressure.
As a gas is compressed it also increases in heat.

This now high pressure and hot gas, known as a super heated vapour is leaves the compressor, travels along the discharge line and into the condenser.
The discharge gas inside the condenser tubes cools as ambient air flows over the fins and coils and ultimately, cools it enough so that it condenses into its liquid state by the last pass of the coil, although its pressure is still (almost) equal to the pressure it left the compressor.

This now saturated liquid is pushed through the liquid line to the filter/dryer/receiver.
In a car AC system this item performs three functions (in house fridges or commercial refrig systems it doesn't act as a receiver)

It is designed to handle excess liquid capacity (necessary if using a TX valve type expansion device) as well as filtering and drying the refrigerant.
Usually the desiccant performs both jobs and is little balls of activated alumina, trapped in the dryer by screens and often glass fibre mats.

It's very easy to check for a partially blocked dryer, there will be a temperature difference between the inlet and outlet liquid temps.

Your touch is all it takes to sense this

The still high pressure saturated liquid continues through the liquid line to the expansion device, in this case it sounds like a Thermostatic Expansion valve, but may be a fixed orifice, capillary tube, electronic expansion valve, etc.
The expansion device can be anything that throttles the flow of liquid with a tiny orifice.

What happens as the high pressure liquid is forced through the orifice is that it will start to expand as it enters the larger volume of the evaporator tube/s and as it does so it will absorb heat from its surroundings.

As the now expanding liquid travels through the evaporator it continues to boil and absorb heat.
Air flow across the evap coil is critical here, any blockage or restriction of air flow across the evaporator reduces the ability of the expanding liquid refrigerant to absorb more heat.

The TX valve regulates the flow of refrigerant entering the coil to changing evaporator loads by means of a feeler bulb/sensor at the evaporator outlet that is filled and sealed with refrigerant.
A small capillary tube goes back to the TX valve where it opens into a large diaphragm atop the valve.
This diaphragm is connected by a pushrod and spring arrangement to the inlet needle of the valve.

This mixture of gas and liquid in the sensor (known as a power element) responds in a known way to changes in temperature and so exerts pressure on the diaphragm in the valve, modulating a tapered needle in the orifice to changes in evaporator outlet temperature.

Hopefully, if everything is setup, sized and adjsuted correctly all liquid refrigerant has boiled off by the time it gets to the end of the evaporator.

As an aside, systems that use a fixed length capillary tube instead of a TX valve for expansion duties (eg. House fridge) do not use a receiver in their system, they have a fixed amount or refrigerant in the system known as a critical charge and the dryer cannot be mounted horizontally but vertically, otherwise it acts as a receiver, which really upsets the systems balance, but I digress......

Anyway, the the now saturated vapour travels along the suction line from the evap to the compressor, enters the compressor and starts its journey again.

The things that can jigger a TX valve are
1. crap in the inlet blocking the inlet screen.
2. moisture (if there's excess moisture in the system that the dryer can't handle, it will instantly freeze in the TX orifice) This is the same as turning the valve 'off'. It won't restart until the compressor is turned off and the temp of the valve rises above 0*C
3. broken or perforated capillary or power element. This is the same as turning a valve off. The inlet needle shuts under spring pressure and no refrigerant can flow.
4. wear (many, many years of use)

Under extremes of temperature I've seen dryers on commercial refrig systems 'release' moisture into the system, instantly freezing the TX valve internally.

I'd reckon it must happen more frequently on cars as
a) the underbonnet temps can be insane
b) all due respect but from my observations most car mechanic's that work on car ac systems don't have the best of work practices.
AC/refrig systems are extremely sensitive to moisture and as soon as the system is opened it is absorbing moisture from the surrounding air.
This includes the gas and more particularly the oils. A lot of car mechanics IME don't appear to appreciate this.

I've deliberately neglected any pressure switches and temp sensors that are mounted along the way as they can vary according to application and can usually be checked pretty quickly with a multimeter if opened/closed.