Low Ambient Temp Control

Eugene Silberstein 3

Thanks Jack.

There is a post in which a gentleman is looking for a one-ton split... Anything in your line for him?

Frank_20

Low Ambient Temp Control

What exactly does a low ambient temp control(for condenser fan motor)protect against? Also was told this control should only be used with txv not a fixed bore meter device. With a fixed bore device it may flood back to compressor.Is this correct? Thank You

Paul Fredricks_3

The low ambient control cycles the fan when the outdoor temperature drops. The idea is to keep the head pressure up. You see them more on office and computer room units that operate in the cold weather. They are starting to show up on the new higher efficiency units.

A fixxed orifice only uses what it is given. If the headpressure is up it will feed more refrigerant through. This makes sense in the summer when the indoor loads are higher. I suppose in a situation with a high inside load year round it may be OK. Read that as I'm not sure if it matters if it's fixed or TXV. Anyone else?

Empire_2

Lo Ambiant control

Mainly used in hi load Server rooms and the like, lo ambient control is used to raise the condensing temps of the A/C system to (Fake) out the system into thinking it's a normal average day. If you were to run the unit at lo outdoor temps the result would be a very lo Evap temp and pressure which would eventually freeze the coil as a result. "Delta P" is a very necessary fact of refrigeration and the very lo temps outside will produce the lo pressures and temps that will cause the equipment to operate with problems. Introducing the head control or fan cycling control.... I am now able to run my A/C unit at very lo temps, by keeping my head pressure up as if it were a 60* or 70* day. Since the "say 15*air" is flowing across the condenser coil, It is going to Drop your head pressure to non operational levels. As a result you can bet the evap will FREEZE. By keeping the head pressure and temps at workable levels, this is how and why it works. There is much more to it, but I hope this makes sense to you.

Mike T.

JackFre

The new inverter based

mini-splits cannot accept a low ambient control.

Empire_2

Jack

The mitsubishi mini-split (P) series uses it all the time. Down to 0*f. I'm not sure what you mean by inverter base....

Mike T.

don_156

Great reply

Mike.

Frank_20

Low Ambient Temp Control

Guys this is why I ask. I service a convience store. Building is 12 years old. Original condenser lasted 7 years(compressor went bad). In the last 5 years I changed 3 more compressors,same unit. Other guys I talk to say I should install low ambient control on this unit. I know this unit comes on in 30* and 40* weather. Several coolers inside building running keep it warm. I am talking AC unit. I can't understand how it will directly help compressor. What am I missing? Thanks for the help and replys.

Techman

Lo Head

Hi Frank,Thats a lot of compressors to lose!On AC and refrigeration units that have "factory installed" head pressure controls, the control is typically set for 95* condensing.At this "low" pressure the refrigerant mass is moving thru the system fast enough to bring back the oil to the compressor and the metering device still has enough pressure drop across the metering device to make it meter properly. Any refrig unit that has the cond unit subjected to low ambients should have a temperature control that cycles a liquid line solenoid valve and the compressor is controlled by a low pressure switch. Also a crankcase heater is required.AC manufacturers install fan cycle controlls on "orifice " and cap tube systems and the control operates at 95*condensing. r-22=180psi r-12=108psi r-502=200 psi minimum high side pressure

Marty

how bad ?

Bad ? whats bad ? Something is killing them.

Brad White_83

Inverter based

I think Mike T. means with a variable speed control (VFD) and that conventional low-ambient control should not be applied as such. Mike- am I correct?

Brad White_83

Some thoughts in no particular order

The kind of low-ambient control I see is known commonly as a "head pressure control damper" on the discharge of the condensing fan. The purpose as stated elsewhere in this thread is to slow down the condensing rate. When the damper closes the airflow rate naturally drops, the Delta-T goes up so the temperature of the refrigerant does not go too low.

30 to 40 degrees is too low for conventional refrigerants to work effectively without low ambient control. Absent that, the refrigerant can slug (turn from vapor into liquid too quickly and in the wrong place). Should the compressor ever see liquid you could lose it in a jiffy. Compressors compress gas only. Liquids are essentially incompressible.

Empire_2

You got it Brad

JackFre

By inverter, I mean....

The units are inverter (variable voltage)controlled so you are getting true variable speed ac. Go to fujitsugeneral.com and brouse the web site. there is a good graphic there to better explain how it works. When you look at the outputs (min-max) it is really incredible. If you are a fan of the modulating boilers...this is the same, but in ac. For instance The Model 12RLQ will modulate from 3,800-14,500 in cooling mode and because it is a heat pump it will make 3,100-21,000 btu of heat (ARI rates them at 47F). 21 SEER and 12.5/8 EER

Eugene Silberstein 3

The Way I Like to Look at it....

When teaching air conditioning theory, I like to start out by describing the system as a pass-through device, of sorts. Heat enters and heat leaves. Heat enters the system through the evaporator and leaves the system via the condenser. If there is a problem with the evaporator blower, the amount of heat being added to the system is reduced but the condenser is still able to reject heat.

If there is less heat being added to the system, but the system is rejecting heat efficiently, the operating pressures and temperatures will be lower (of course I am ignoring the effects of head pressure controls, AXVs and other pressure-maintaining devices).

In a similar train of thought, if the condenser fan motor is not operating properly, the system is not able to reject heat efficiently but heat is being absorbed into the system. If the system can absorb heat but cannot reject heat, the amount of heat contained in the system will rise and the operating temperatures and pressures will rise as well (Once again I am ignoring the effects of head pressure controls, AXVs and other pressure-maintaining devices).

So, in order to maintain the desired head pressure during low ambinet conditions, the trick is to keep heat in the system. This is accomplished by reducing the rate of condensing medium flow. The condensing medium can be wikther air or water, or, in the case of an evaporative condenser, air and water.

On air cooled equipment we can maintain head pressure by fan cycling, condenser flooding or by adding shutters at the condenser coil. For water cooled applications, the amount of water flow is controlled by a water regulating valve, which acts against the head pressure to close in the event the head pressure drops too low.

Sorry for the late post on this one. Must have slipped past me.

Christoph

efficiency/capacity loss using fan cycling control

I'm working with a 3.5 tonne split unit needing to run year round with outdoor ambient temps going down to 32F or lower.  There is a fan cycling control installed to maintain head pressure as well as a crankcase heater.   This is for a computer server room and according to my calculations the heat load is around 50% of total capacity based on room size and power consumption.



When outdoor ambient temps are around 70F and above, all is well.  The room is able to reach and maintain 71F with apparent consistent compressor cycling patterns (there's a temperature recording device in the room).



However, when outdoor temps drop, even a few degrees, the system is no longer able to maintain the temp and it appears the compressor is now constantly running; or at least is no longer cycling with any regularity/apparent pattern.



When outside ambient returns to 70F and above, operation returns to normal.



Is this the expected operation when using a fan cycling control to maintain head pressure?

Techman

Hay Christoph

I think ur "low ambient" got lost .Repost on a new thread

g4ross

Mini Split and Low Ambient

I have a wine room with a Samsung/Carrier mini split.  When hot outside the room is a perfect 55 degrees.  When below sat 55 it goes to 64.  What can I do to regulate the temperature. Does a low ambient kit work or some other alternative??

Techman

Well said guys,

Mike T said it plain and  to the point. You are fooling the system into thinking it is operating in a 60-70* ambient even if it only a 40* day.