Condenser Fan Cycle Control vs Refrigerant Charge

[dogma]

Hey Guys,

Quick question. With the Dramatic Increases to the price of refrigerants here in Australia post June 30 2012, Would it be advantageousness to minimise the amount of refrigerant stored in receiver vessels by adding Condenser fan cycle controls to keep a constant pressure on the receiver vessel where condenser fan cycle controls are not already incorporated??? I tested my theory recently by disconnecting 1 third of condenser fans on a -30deg. C. freezer that I didn't have enough refrigerant for on the day. This was a vertical receiver, with approx 2 thirds of the required charge my sight glass was full of flash gas and struggling to pull down, once I disconnected 1 condenser fan the sight glass cleared fully and ran down to temp and cycled until I returned the following day to add more refrigerant. The mid sight glass was sitting just under on the receiver.

Comments and Arguments appreciated.


Regards


Dogma

[dogma]

Anyone?? :(

[Magoo]

Hello Dogma.
my pennies worth, I have found that 404a systems work brilliantly operating at +40 > +43 'C SDT, anything under all turns pear shaped, flashing liquid sight glasses etc., add more refrigerant probably for no reason, then system plays up with super high ambients later in year, Over charged. Staging condenser fans with switches is tediuous, but well worth the time and effort.

[Tesla]

Hi dogma & Magoo
I think the maximum charge should be the minimum to maintain a liquid seal at the metering device under rated conditions.
The max charge is there to ensure sufficient cooling to the windings whilst not slugging the compressor with liquid. By adding condensing temp control we only reduce running costs in lower load conditions. So we can't reduce the amount of refrigerant in the system for full load conditions. I agree there is a little extra in all systems but I'm sure the manufacturer has tested each system in the rated conditions.
For bigger systems like large chiller systems it is viable to remove some of the charge for one of the two seasons heat/cool, This saves energy and reduces the total refrigerant charge on average.
Then in the back of my mind I had thought of a separate vessel that would hold liquid excess refrigerant during lower loads with the fan control but this would only save on refrigerant and power during lower loads.

[ALWAYSTHINKING]

I would guess the system you are working on has a flooded condensor system for cold weather opertion. This is controlled by a head master control valve that floods the condensor with liquid refrigerant. By flooding the condensor, the condensor surface area is reducing the amount of heat it can reject. Thus the head master valve controls the head pressure.

If you are think of doing this, fan cycling controls are not the way to go. They work, they are cheep easy to install, but they have a fault with them. Hear is the problem with them. Let say we set the cut in at 250psi using R-22 the condensing temp is 117 degrees F and 10 degrees F of subcooling that would make the liquid line temp of 107 degrees F. Then set the cut out at 175 Psi using R-22 the condensing temp is 92 degrees F.

Ok now we have the pressures set. Here is the problem when the fan starts the pressure drops very fast as soon as the condensing pressure temp get below the liquid temp, you will see bubbles in the sight glass. What is happening, the pressure change in the system is thurout the high side. So the liguid line temp stays the same but the pressure does not. Do the math condensing temp 92 degree - liquid line temp 107= -15 subcooling = NO SUBCOOLING. This happen every time the fan cycles over and over. How is the TXV every going to control super heat. You will all so see your suction pressure swing.

This a extreme case but I have see it happen many times. Fan cycling works most of the time. But other times it can end in Flood back, frooze coils, hunting txv, leaking condensor coils and compressor failers.

I am done using fan cyclers I will not put them in. I only use motor speed controlers (JCI P-66)and VFD's. Using motor controls the head pressure stays constant. Alwalys liquid in the liquid line, more efficient, easy on condensor fan motor and blades. Alway control them off of pressure not temperature. I don't know who thought you can control head pressure off of liquid line temp.
Yeh in theory it should work. But does not work well in cold weather, well then why but it on.

To answer your question. Yes install motor speed controls if single phaze or VFD if 3 phase and control them off head pressure.