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bersaga
04-09-2003, 08:17 AM
Being from a tropical environment , there are certain refrigeration practices in the colder climates that I'm new to.

For instance, I'm told that in winter , outdoor air-cooled Condensing Units are enclosed in a box-like structure to recycle the heat and for starting purposes. Is this true ?

Argus
04-09-2003, 10:34 AM
Low ambient conditions are a problem for some cooling applications in cold climates. Particularly for so-called 'comfort' air conditioning.

Our normal outdoor rating condition (UK) is 27 deg. designers may design to 30 or 32. This month we had temperatures in the high 30's for a week or two and the place came to a halt. These temperatures are exceptional. Frequently cooling takes place in winter ambients below 10 deg. (Why is a completely different matter)

There are many methods of maintaining a sufficiently high condensing condition. Mostly it involves modulating the speed of the condenser fans, although there are other methods involving refrigerant stacking, though this is not common. Most fan speed applications link to the discharge pressures, though some devices sense condenser temperatures.

I have not seen dampers, boxes or baffles to recirculate air employed in the UK, other than for weather protection, though sometimes acoustic enclosures around condensers unintentionally have the effect you describe
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bersaga
04-09-2003, 12:04 PM
Thanks Argus !

Dan
04-09-2003, 11:13 PM
For instance, I'm told that in winter , outdoor air-cooled Condensing Units are enclosed in a box-like structure to recycle the heat and for starting purposes. Is this true ?

I just want to be sure about what you are referring to, bersaga. Argus's response is a good response, but I am wondering if in Maylasia you are used to seeing exposed condensing units outdoors. By exposed condensing units, I mean with no sheet metal housing attached to them.

Condensing units designed for outdoor appications should have housings to protect components from the weather, whether it is warm or cold. In Flofida, I see a lot of "indoor" units set under a roof, but with cold weather devices such as Argus refers to added to the equipment. On the other hand, I have seen condensing units with weather protection and no cold weather operation options installed on them.

Weather protection and cold weather operation are separate considerations. Cold weather operation devices are usually add-on options that you can apply to both or neither condensing units.

bersaga
05-09-2003, 02:21 AM
Dan,

We have both outdoor and indoor types in Malaysia.

The outdoor types have sheet metal casing around them but NOT designed to recirculate hot air to heat -up condensor coils (of course not in this climate!) But how about cold weather start-ups? (that was my question)

Thanks

Argus
05-09-2003, 10:18 AM
Bersaga,

Odd as it sounds, it is customary to operate a/c units in cooling mode with ambients to zero (Celsius) in the UK. That?s why the fan speed controllers are fitted to compensate for the over condensing effect of low ambient conditions.

Recirculation of condenser air is generally a big problem associated with poor siting of condenser units in this part of the world.

I haven?t come across any special arrangements for cold starting of units ? they just get up and go.

However there is a problem at start up in cold weather connected with migration of refrigerant to the outdoor unit that you may be referring to.

Ina typical winter scenario in London, the indoor temperature is generally in the lower 20s the outdoor could be anywhere between zero and 10, in other words a large TD between the two coils, indoor and outdoor. Refrigerant will migrate in time to the coldest part of the system, usually the condenser coil. When the unit starts it can easily evacuate the evaporator, reduce the suction pressure rapidly to a point below the cut out setting of the low pressure switch, thereby inducing a nuisance trip. To overcome this, a timer is sometimes fitted to short out the LP switch for a few minutes until the system pressures have stabilised. Many a/c manufacturers fit this as a factory standard.
Solenoid valves designed to drop when the compressor stops can help, but in my experience refrigerant will always find a way to migrate.
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Dan
06-09-2003, 03:25 AM
I think Argus has addressed the point rather well. In the states we call it a "boot" start, where you bypass the low pressure control just to get things going.

It is an irony. We are using a refrigeration device to maintain a controlled temperature in a refrigerator that is warmer than it is outdoors.

Argus points toward air conditioning applications. In refrigeration applications the problem is a larger issue, but the same principles apply.

But back to your original question... a box perhaps outside a weather housing, if I am following you.

What do you suppose it is accomplishing compared to what Argus's suggestions appear to answer?

Are we simply attempting to re-create an indoor atmosphere within an outside condensing unit? Refrigeration units have sufficient controlling options to permit them to maintain refrigeration of 30 deg F even when exposed to -10 deg F ambient conditions.

It's nutty. But boxing it up all over again ispires guestions that could never end. You know. Those questions beginning with the word "Why?"

bersaga
06-09-2003, 06:40 AM
My 1st question came about because I was told that this is a common practice in Ireland ( I think!) - really , I was just curious - I didn't see the logic and so posed the question here !

However the answers have led me to ask another question : why wouldn't you use heat pumps and reverse the cycle in winter when indoors need to be warmer than ambient ?

rbartlett
06-09-2003, 07:06 AM
heatpumps..

if you are talking a/c -we do..

if you are talking fridge-we don't, we just leave it off on the stat;-)

cheers

richard

cooltrain
06-09-2003, 07:28 AM
Hi!

We have a slight similar application on equipment cooling. Here in Saudi, most of the computer rooms, electrical substations and tele-substations are airconditioned all-year round. During low ambient temp., usually, from 22C to 9C in some areas, the a/c system runs on fancycling sequence.
Electrically, the compressors are packed with potential relay and starting capacitor. Auto restart is also included. Crankcase heater is energized continously and optional outdoor coil heater that help to sustain the desired discharge pressure by heating the coil.
Outdoor air cannot be utilize indoor due to dusty environment (sand storm unpredictable).

Heatpump system and fresh air economizer are usually used in comfort cooling with extra filtration.

Best Regards

Argus
06-09-2003, 11:03 AM
We do!

It just happens that some parts of some buildings always have a cooling load ? year round.

Heat pumps make up the largest part of the UK?s unitary split market. In that category are all sorts of split systems and VRF in all their various disguises.

The reason why there is a large amount of cooling applications working in the winter months revolves around the design of building stock in the UK?s main cities which are largely built by speculators for office rental, and working practices that go on in some of them. I daresay that it is similar in other countries, too. In short, the cores of most building are net producers of heat, comprising of computers, terminals and people. It is for this reason that heat reclaim VRF systems are big sellers here.

The next question must be why building are not built more efficiently to minimise heat losses and wastage???.. I will leave the answer to that one to some of the architects involved.... Or others who may be more tactful than me.
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Dan
06-09-2003, 02:23 PM
We use reverse cycle defrost in low temperature refrigeration and it works quite well. Most of the problems stem from our lousy documentation and the unfamiliarity of the service technicians.

rbartlett
06-09-2003, 07:35 PM
i wondered what KSA ment -i thought it was an american thing..

anyway i've done some work in saudi on the big american airbase al harhj (spelling!!) thought s.a was okay..some funny ways but pretty good..

pretty dodgy for you there now adays isn't it??...

cheers

richard

i don't seem to remember ever seeing a reversing valve in a fridge system..hot gas and kool gas yes but no r.v..more info pls dan..

Dan
07-09-2003, 12:27 AM
Hussmann, and I assume other manufacturers have condensing units that use reverse cycle (heat pump) methods for defrosting.

They employ the same sort of valve as heat pump does to reverse the flow from discharge into the suction line. Accumulators and CPR valves are employed, as well.

The tricky part is achieving the proper charge for a 200-foot run. Oddly, there are circumstances where a proper charge for refrigeration is not sufficient for a proper defrost. It puzzles me.

benncool
17-11-2003, 11:00 PM
Up here in Vermont in the NE-USA we have a wide temperature swing throughout the year. It has been as low as -33*C (-35*F) and +32*C (+90*F)

So we have to be prepared for anything. Running in low ambient temperatures can be tricky. Computer room for example my need air conditioning cooling equipment all year long.

Refrigerant always wants to migrate to the coldest spot. If your compressor is out side this is where the refrigerant will end up during the off cycle. When the compressor starts the liquid refrigerant will wipe the oil out of the crankcase. So it is necessary to have crankcase heaters on the compressors to keep the liquid refrigerant from migrating to the oil.

Condenser fan cycling is common to keep the head pressure up in the high side. This is sometimes done with a thermostat sensing the outdoor temperature. But I much perfer using a reverse actioning pressure switch.

For centain applications it is necessary to use hot gas bypass valves. These let the discharge gas by pass the condenser. The hot gas will enter the receiver and drive the lazy liquid refrigerant out of the vessel and up to the expansion valve.

Lots of fun working in these kinds of temperatures in the winter. This is when we guys up north start to think about a vacation in Florida.

Gary Collins
24-01-2004, 03:47 AM
Recirculating the condenser air would require frequent adjustment as the ambient conditions during the winter can change dramatically. The most prevalent methods for head pressure control are:

a) cond fan cycling

b) variable speed control based on liquid sub-cooling

c) variable air flow via a refrigerant powered actuator that opens louvers to increase air flow as the pressure increases

d) Floodback head pressure control where a valve at the condenser drain closes and fills the condenser with liquid refrigerant to reduce the effective heat transfer area. A by-pass valve puts positive pressure on the receiver to maintain stable liquid line pressure at the flow control device. The flood back method requires a receiver that has the refrigerant volumetric capacity to flood the condenser and pump down the remaining system and be only 80% full. This method effective to -40C (F)

Frig Pig
27-01-2004, 05:18 AM
Kind of an old post but Iíll toss in my 2 cents.
Right now Jan 26 @10:30 p.m. it is MINUS -53 degrees Celsius w/ wind chill (slight breeze actu.) and hottest day in memory is around PLUS +40 degrees Celsius. thatís about a NINETY-degree swing.
Computer rooms, coolers, freezers, supermarkets all have to remain cool while surrounded by 70 degree air no matter what the temp is outside. Basically all these run off of a combination of reverse acting pressure controls for the condenser fans and some sort of Headmaster control. http://www.alcocontrols.com/aw200311/web/products/regulators/catalog/(HPC).pdf
Supermarkets used holdback valves and differential valves in combination with fan cycling to run year round.

Gary Collins
29-01-2004, 05:20 AM
Frig Pig, I'm not familiar with the term reverse acting pressure controls as it relates to head pressure control. Maybe my lingo is different for the same thing. Would you please explain.

Thank you

Gary C in So Ca

chemi-cool
29-01-2004, 03:28 PM
hi gary,
in AC I always add economizer to cut down electric bills and use cool air from outside when cooling is needed in the winter.
because we get many sand storms in this part of the world a good filtration of the air is required.

thanks god we dont have the problems of edmonton in canada, but the TD between sommer and winter is about 45C.

in recent years I have started to use larger condensers with less rows in depth. in this method, the amount of refrigerant is about 40% less and in the winter when its get cold -2>-5C, it heats the gas real quick and head pressure is kept using fan cycling via head pressure.

in sommer when tep. are over 40C it does'nt get too high from the same reason it gets hot in the winter.

I am interested in this method "floodback head pressure control". can you post a lines diagram of such system or send it to me by e-mail or direct me to a site where I can see how it works.

thanks

chemi

Peter_1
29-01-2004, 07:38 PM
Originally posted by chemi-cool
hi gary,
I am interested in this method "floodback head pressure control". can you post a lines diagram of such system or send it to me by e-mail or direct me to a site where I can see how it works.

thanks

chemi

I think this is what you're looking for Chemi

Gary Collins
30-01-2004, 04:54 AM
Hi Chemi

The attachment floodbackweb.jpg shows the relationship of the 2 valves required for floodback head pressure control. The valve identified in the diagram by KVR is set to a pressure dependant on the type of refrigerant. R-404A, R-22 would be set to open at 180 psig. R-134a or R-409a would be set at 80-100psig.
The condenser fans are running continuously so the valve will remain closed as liquid refrigerant accumulates in the condenser. As the refrigerant accumulates it reduces the amount of heat transfer area available which causes the discharge pressure to increase. The NRD valve in the diagram is a pressure differential valve. When the KVR valve is closed there is a drop in liquid line pressure. When the drop exceeds 20psid of the discharge line the NRD valve will open allowing the receiver and liquid line pressure to be maintained at 20psi below the compressor discharge. This will keep the expansion valve performance stable. The KVR will modulate to maintain a stable discharge and liquid line pressure. Its important to calculate the internal volume of the condenser to ensure that the receiver has sufficient refrigerant to flood the condenser and still keep a liquid seal in the receiver. Some conditions call for 95% flooding of the condenser. I hope this helps you. Sporlan has an excellent procedure for charging floodback systems

Peter_1
30-01-2004, 06:09 AM
Originally posted by Gary Collins
Hi Chemi



+/- The same principle

chemi-cool
30-01-2004, 01:19 PM
hi peter and gary,

thanks very much.
stupid me, we call it a different name so when I saw the picture you have sent peter, it came back to me. must be the age shows its signs!!!

we call it here simply "KVR & NRD" as they always go together.

must have lost that particular mamory cell in my brain. anyway it stored now in a new cell!!

thanks guys.

chemi

TXiceman
08-03-2005, 01:53 AM
Very seldom, do we see recirculated air for low ambient control/operation. For refrigeration or A/C we offer several methods and combonations.

We have 20 dF fan cycling, 0 dF fan cycling, condenser flood back, variable speed fans and heated and insulated receivers. Depending on the equipment, the application and where the equipment is headed, we determine the best method of head pressure control.

For evaporative condenser units,we have another set of design criteria to use and combine with the above designs.

Ken