Thirty years ago, when I actually worked on a few cars, the only thing I knew about was the receiver / drier and the little sight glass, but now I learn I've got an "accumulator" on my 96 F150, and I'm a bit confused as to the theory of operation between the two.

Both serve as a mechanism to absorb moisture from the system, but they're located in different places and so I'm stumped about an aspect of how they do their jobs.

The receiver / driers is typically located so that it receives high pressure cooled and condensed refrigerant from the condenser. It can "store up" a certain amount of this and it's passed on to the evaporator via an orifice tube or other metering device.

But once this fluid hits the low pressure of the evaporator it boils away, turning into a gas and carrying away passenger compartment heat.

This is great and all, but what I don't understand is why you'd want an accumulator / drier instead. It contains gas and not liquid, right?

I can imagine there would be times when a reservoir of condensed refrigerant would be handy, like when you idling in traffic on a miserable hot and humid day with a car full of passengers. There are times when the compressor just can't keep up, and I would think a bit of reserve would be very welcome. It would also eliminate the need I feel sometimes to shift into neutral and rev the engine for more fan and compressor speed in hopes of gaining efficiency.

Last question: Other than avoiding fluid ingestion into the compressor, is there any reason you couldn't have BOTH a receiver / drier AND an accumulator? There's more stuff to break and service, of course, but other than that, I can't see a downside.

 

Words NOT from a pro.
The difference is in the orifice control. Accumulator systems have a fixed orifice somewhere between the condenser and the evaporator. Which happens to have the 'just so' diameter to match the compressor/condenser/evaporator needs & products. It will vary based on the size of each, and between R12 and R-134a.
The receiver/dryer systems have a variable orifice mounted on the evaporator. It generally has a little tube that connected to a probe mounted on the evaporator inside the case that uses capillary(?) action to vary the orifice based on evaporator temp. When the evap gets down to ~33, the sensor contracts enough to make the orifice close, and pinch off the refrigerant flow, and avoid frost/ice buildup. It is called an 'expansion valve system', IIRC.
The accumulator system uses a pressure switch mounted on the accumulator to make sure that the pressure does not go below 32f by turning off the compressor clutch power. It also has a 'vacuum break' built in so that to get liquid out and slug the compressor, the whole accumulator would have to be full almost to the top with liquid refrigerant.
In theory, only gas should escape either one. In theory only liquid should be 'presented' to the expansion valve or the orifice, directly from the condenser or the receiver/dryer.
I suppose you could add a receiver dryer to an accumulator system, but what's the point? There is a dessicant internal to the accumulator, just like the receiver/dryer has. They both perform the function of being a 'reservoir' of refrigerant, in one case, liquid, in the other, gas. You can adjust the function of an accumulator system, again, in theory, by adjusting the amount of refrigerant in the system. Starve it, and it will 'walk back' the flash point into the tube leading from the condenser. Stuff it, and it will not be able to get the pressure low enough to flash, and thus will not perform as it cannot transfer heat. Both are 'extremes' IMO.
tom

 

This is easy enough. I guess one of my questions is about why a receiver / drier could not have it's output metered by an orifice tube versus the variable orifice systems? As long as you had the original orifice tube working with the original evaporator, it seems to me that you'd never supply too much refrigerant to the evaporator, since the orifice tube would never let that much in.You mean the temperature going below 32F? Another member here on another forum said it had a window of operation between 24-45 PSI.

This is great, but I thought the accumulator was full of low pressure gas. If it was high pressure I guess the canister itself could dissipate enough heat to condense some liquid. How full do they typically get during operation? I guess that over 45 PSI condensing can occur. I'm still confused about this. When the compressor restarts and begins pumping in from the accumulator, that's got to lower the pressure and boil off any refrigerant, right? My not-so-hypothetical objective was identified at the start of the thread: generate a reservoir of condensed refrigerant ready hit the evaporator during critical periods. It occurs to me that the accumulator system may not be ideal for this purpose, if the accumulator only stores gas.Do you mean that the boiling point will occur before the orifice tube / expansion valve?Do you mean that your low side pressure will be too high, and the condensed refrigerant could not boil off?

It seems to me that adding reservoirs for refrigerant after the condenser would be fine so long as you got the right amount of refrigerant into the system. I'm sure it would be a bit tricky with plenty of trial and error, but it seems that it might work on the drawing board at least!

The other things that tells me there might be some wiggle room here is that an F150-350 to an E350 Minibus, all use the FS10 compressor. The F150-350 use the same accumulator (in my model year) but the passenger heavy Minibus uses a different part -- no doubt a different condenser as well.

 

Accumulators are used on Fixed Orifice Tube systems to prevent liquid refrigerant from being returned to the compressor. This could easily happen during times of light load, such as low ambient temp or low cabin temp on a mild day. The liquid refrigerant may not all flash off into a gas so it goes to the accumulator to finish boiling into a gas.
An overcharge condition can actually flood the evaporator and accumulator and cause the liquid-gas conversion to happen in the accumulator and suction hose.
Most FOT systems cycle the compressor to maintain the evaporator temperature just above freezing. The "off" pressure of 28psi correlates to 32.5 F evaporator temp. The 45psi "on" pressure allows the compressor head pressure to come down to a level that won't overload the compressor on startup. It's actually a very sloppy way of doing things, but it works and is cheap to implement.

Receiver/dryers are used in TXV to ensure a steady stream of liquid refrigerant to the expansion valve. It's possible for some refrigerant to leave the condenser as a gas, so it enters the R/D to finish condensing. The R/D also acts a bit like a shock absorber (or better, a capacitor) as changes are made to the TXV opening.

A receiver/dryer wouldn't serve any purpose on a cycling/fixed orifice system and could actually keep refrigarant flowing through the orifice tube when it is necessary to cut back the flow. FOT systems rely on the start/stop nature of the refrigerant flow to maintain the evaporator pressure/temp. (Like I said, sloppy but cheap).

A great place to pose your questions would be Automotive AC Information Forum - ACKITS.COM. There are a few folks there that could fill in all of the whys and wherefores in great detail. I "know" most of the theory behind it but explaining it intelligently is another story. TOMW did a pretty good job.

 

Many thanks for the link!

 

After visiting ACKITS.COM some related searches, I am becoming more enlightened, and now have another orifice tube question.<br /><br />Where is that critter?<br /><br />I had imagined that it was something occurring right at the evaporator, but read a post about it possibly being inside the accumulator??

 

The orifice must be very near the entry to the evaporator. The expansion device on any a/c system is where the action is. As the refrigerant passes through the expansion device and finds the low pressure after it, it evaporates. As we learned in junior high school science class evaporation is a cooling process. If the expansion device is not very close to the evaporator, it will not allow the heat to be removed from the evaporator coils.

In the cases where the expansion device is not right at the evaporator, find the point where the small diameter tube goes to a large diameter tube. That will be where the expansion device is found. If not right at the evap entrance, it will be very close by.

 

The orifice tube is most often located in the evaporator inlet tube, just inside the fitting for the liquid line.
Some Ford vehicles have the orifice as an integral part of the liquid line near the evaporator fitting.
Note: I'll muddy the waters just a bit. Some GM trucks, and I believe some Ford Trucks, have the orifice located at or near the condenser outlet fitting. On those the "liquid line" is actually not. The Cond-Evap line is on the low side of the system.

I'm glad you enjoyed the link. There's a lot of knowledge on that Forum. The forum sponsor is also one of my favorite suppliers.

 

I've emailed Arizona Mobile Air / ACKits.com for recommendations on my setup.

I'm sure they will set me strait on what can and cannot be achieved!