A technical question

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A technical question.
Using R22 I want to know what is the idea range of head pressure to get the maximum heat out if the system with the least power input, IE the most efficient head pressure on heating.
Please no guesses, I do not want to run the system at 400 PSI and chew up heaps of power.
I have ways of controlling the suction pressure thus controlling the head pressure within some limits. The condenser (Outdoor coil) is somewhat oversized so that again allows me a range of suction pressures while the unit is running on heating. Both the heating and cooling modes have TX valves for refrigerant control and there is a receiver in the system.
No worries about low temperature IE freezing the outdoor coil as it does not get all that cold here.
As I said the main aim is to get a good heat result with a minimum of power usage.
So what PSI should I aim at from a design point of view on heating and economy?
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Before a first year apprentice starts to tell me what the answer is, I have been involved in air conditioning for many decades, so am knowable on it. My design experience is mainly on straight cooling systems.
I designed a 175 ton plant (the biggest I have designed) and started up one system that had 9 ton of refrigerant in it. Plus I commissioned a few plants that had 3,300 volt motors driving the compressors.
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Not that I had anything to do with it but I saw one interesting reverse cycle plant which had multiple large fan coil units and had 6 x Carrier 5H120 compressors on it all coupled together.
For those not familiar with the Carrier 5H120 compressors they have usually something like a 100-125 HP motor driving each of them and are an unloading 12 cylinder compressor.
Nominal tonnage is 120 tons. The system beside having evaporators and a condenser had balancing coils to take up any imbalance in the system, as any of the fan coils could heat or cool simultaneously if required as there were both heating and cooling coils in each fan coil unit.
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On Mon, 7 Nov 2011 07:46:01 +1000, < snipped-for-privacy@truthonly.com.Sword of Baal> wrote:

maximum heat out if the system with the least power input, IE the most efficient head pressure on heating.

pressures while the unit is running on heating. Both the heating and cooling modes have TX valves for refrigerant control and there is a receiver in the system.

    Wrong question. Ask 'How do I get the most efficient operation ?', as you did at first.
    Answer : at the lowest input power level needed to exactly meet the current load, while running 24 / 7. Head pressure is only one part of that equation. Your head pressure will tend to be at the lowest point it can be to acheive that, but how you handle the fans, etc is all part of the picture.
And no, you don't run R22 at 400 PSI.

been involved in air conditioning for many decades, so am knowable on it. My design experience is mainly on straight cooling systems.

system that had 9 ton of refrigerant in it. Plus I commissioned a few plants that had 3,300 volt motors driving the compressors.

plant which had multiple large fan coil units and had 6 x Carrier 5H120 compressors on it all coupled together.

something like a 100-125 HP motor driving each of them and are an unloading 12 cylinder compressor.

condenser had balancing coils to take up any imbalance in the system, as any of the fan coils could heat or cool simultaneously if required as there were both heating and cooling coils in each fan coil unit.

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Refrigerant pressures are only a very small part of the picture... once you have verified 400cfm airflow per ton in a correctly matched system, the correct way to charge a heat pump is using superheat, and subcooling according to the manufacturers specifications.... PERIOD.

what is the cold ambient and the SST in the coil??

Correctly sized, proper airflow, and charge correctly balanced.

Look at a couple of charging charts

Resi systems are not for the industrial hammer mechanic, its more like working on swiss watches by comparison.
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On 11/6/2011 8:05 PM, Steve wrote:

I know a lot of HVAC guys who can't work on refrigeration systems. ^_^
TDD
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"The Daring Dufas" wrote in message

I have met a lot of HVAC people who should not be allowed to work on HVAC systems, but I asked a simple question and it seems no on has the answer.
IE what is the best head pressure for the most economical operation in heating mode using R22.
I have worked on domestic fridges, domestic air conditioning systems, bar refrigeration systems (Temprites and bottle cabinets), commercial and industrial refrigeration and also huge systems like cascade systems with screw compressors, as well as commercial and industrial air conditioning systems.
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Your not listening to what your being told. There is *NO* "best head pressure" for any specific refrigerant. the head pressure will vary depending on temperature and load. You say the system has TXVs on it... ok... charge the system by subcooling, and superheat.
You wouldn't be an EE by any chance, would you??
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"Steve" wrote in message

You cannot read, I am talking about head pressure, not the simple act of charging a system.
You have already shown your lack of knowledge by talking about a 'a balanced charge' in a system that has a receiver and TX valves.

You would not be a first year fridge mechanic's assistant would you?
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Your ignorance is showing.... there is no "best" pressure... LLP, LLT, SP, ST, SH, and SC are dependent on ambient temps, and RH for both the condenser and evap..... especially with TXVs.
Your not going to have the same pressures and temps at 30F OD ambient as what you would have at 60F OD ambient. Maybe you should do a little homework on refrigerant pressure/temperature relationships.
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"Steve" wrote in message

True
But of course you jump in without knowing anything of the conditions here, for instance, the lowest temperature recorded here was 7.3 c (45.14 f) many years ago, in the last 10 years it has dropped to 9 c (48.2) twice early in the morning, other years we see it drop to 11, (51.8 f) two or 3 times a year again early in the morning. During the cooler weather it usually gets up to 27 c (80 f) during the day so the houses do not get icy cold and require little heating, mainly early in the morning.
Hence the size of the outdoor coil and the amount of air going over it can control the suction pressure considerably, this will have a big effect on the head pressure. IE a large outdoor coil and high fan speed will keep the suction pressure up considerably.

Looks like someone needs to teach you basic refrigeration.
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Sure... as long as you can keep the SP above 59F @ 400cfm/ton evaporator airflow otherwise, your gonna have issues

Looks like if you laid *EVERYTHING* out in the first place instead of giving little dribs and drabs of information... My crystal ball is in the shop.
I figure either your a troll, or an EE
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"Steve" wrote in message

SP (Suction Pressure) is measured in PSI in imperial measurement, not degrees. Suction Temperature is measured in degrees.
I have seen many evaporators with a suction pressure of around 52 PSI (around 28 f) on R22 and they do not ice up.

As I said I did not need 1st year apprentices guessing at answers.
Goodbye Steve.
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Your right.... my bad.... brain going faster than fingers can type.
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On 11/07/2011 09:50 AM, snipped-for-privacy@truthonly.com.Sword of Baal wrote:

motor. So, getting the lowest temperature in the condenser and the highest temperature in the evaporator that you can stand and still accomplish the job is the setup that gives lowest energy input.
In many cases, changing refrigerant may give better results.
If you are heating a space with a heat pump, trying to get 130 F air is going to be a lot less efficient that some other scheme that heats the room at a lower temperature. So, a hot-air heat pump might be less efficient than maybe a radiant floor heat system.
Jon
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wrote:

    Wrong.
    You just contradicted yourself.
    'lowest head pressure...' = correct.
    'highest temp in the evap ....' requires the exact opposite of that.

    Correct. As I previously told him , the LOWEST head pressure that meets the load will be the most efficient ( blissfully pretending that that one factor determines SYSTEM efficiency all by itself, which of course it does not ).

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How so? In heat mode the evaporator coil is the outdoor coil.
With a heat pump the coils are called indoor and outdoor, not evaporator and condenser because the 2 can switch roles.
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When AC is in cooling mode inside is Evaporator !
When AC is in Heating mode then inside is be comes Condenser!
Regardless what mode unit maybe running at it always has,
Condenser and Evaporator and that is on any Refrigeration systems!!!
Word Coil can be anything including your scramble brain
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On 11/15/2011 05:23 PM, .p.jm.@see_my_sig_for_address.com wrote:

and low air flow (think auto AC) cool a space by supplying a small volume of very cold air. This is not efficient, as it absorbs heat at a much lower temperature than the cooled space is actually at.
For the specific heating situation, then a larger evaporator and ample airflow will be more efficient than the situation with a small evap and less air flow, again, you want the saturation temp of the evaporator to be as close to the air temp there as you can.
That's what I was TRYING to say. In other words, transferring heat with the minimum temperature difference between hot and cold would normally be the most efficient system, especially if the whole system is designed to operate at those temperatures.
Jon
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wrote:

    Yep.
    Pretty much what I said ~ 20 posts ago :-)
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Simple, HVAC/R is always a reverse cycle, it is a reverse Rankine cycle. In a Rankine cycle heat engine you use a difference in temperature (Delta T) to convert heat to mechanical work, and the larger the temp difference between hot and cold the more efficient the machine is, IE more work out VS heat transferred from hot to cold.
Since refrigeration is a reverse Rankine cycle the lower the temp difference between hot and cold the more heat is "pumped" instead of just being converted (more heat transferred compared to work in).
In your/any case for the highest COP (watt-hours transferred divided by watt-hours consumed/converted) in heating and the highest EER in cooling (BTUs transferred divided by watt-hours consumed) you want as little difference as possible between suction and discharge pressures and temps.
The absolute best you could theoretically do is to have the evaporating temp be the same as its surroundings (in this case outdoor ambient) and have the condensing temp also be exactly the same as its surroundings (in this case indoor ambient.) This is obviously impossible in the real world.
Attempting to control head pressure by turning off outdoor unit fans in heat mode only makes the evaporating temperature that much lower than the building temperature, hence a higher delta T, and more energy consumed (less efficiency).
You need to find a way to slow down or unload the compressor instead, and this doesn't include suction throttling.
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"Daniel who wants to know" wrote in message

Thank you for your input.
The controlling of the fans in this case are for simple control of the head pressure and although they will limit the amount of heat produced, it is necessary to stop the head pressure from going over the top.
It is not practical to unload the compressor as it is a hermetically sealed recip, and the manufactures do not like the idea of speed control on this compressor, maybe because lubrication problems at lower speed?
In general I am looking to make the unit heating efficient within reason, but as heating is not a big deal here (In the tropics) it is not used every day during our cooler season for heating, so in general the while plant needs to be designed as a cooling system primarily, I do want it to heat efficiently.
It seems most people here do not have any heating, as they say they do not need it, but as a person who hates the cold, I want it!!!!
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