please settle this debate

me and my friend were having a debate.
my statement:
during start up, the high side pressure in the system does not exceed its normal operating high side pressure.
you can find my explanation to him here.
his statement:
during start up, the high side pressure gets extremely high, up to twice as much; and then comes back down to normal operating pressure. he couldn't really provide me any sort of evidence for his argument, and i couldn't think of any myself. if you can, let me know.
please vote in the poll, and any comments on either side are much appreciated.
thanks for your time.

During a startup at a high load condition, one would expect head pressures to be slightly to somewhat above normal, and this would be due simply to the increased refrigerant flow rate thru the condenser at these conditions. But if condensing pressures are doubling during a startup, something is amiss. An overcharge, or a very small or dirty condenser.

Pressure should not skyrocket, but would be higher than "normal"

thanks for responding :)
Prof Sporlan, there is a couple of things i am still not very clear on...

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Originally posted by Prof Sporlan
During a startup at a high load condition

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do you mean high heat load on the evaporator? or something else?

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Originally posted by Prof Sporlan
increased refrigerant flow rate thru the condenser at these conditions.

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i would think that the pressure difference between the high and low side is inversely related to the flow through the compressor/condenser. in other words: in the beginning, the pressure difference is non-existant, therefore the flow is very high. but by the time the high side pressure will reach its normal operating value, the pressure difference between the high and the low side will be quite high, hence the flow will be much lower than in the very beginning. the flow now would be the same as the flow under normal working conditions. system is at equilibruim, and it didn't go over the normal operating high side pressure.

i am probably missing something...enlighten me! :D

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do you mean high heat load on the evaporator?

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This is correct...

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in the beginning, the pressure difference is non-existant, therefore the flow is very high.

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This is correct. A low pressure ratio (discharge pressure divided by suction pressure in absolute units) and a high suction pressure would allow the compressor to pump at high flow rates

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but by the time the high side pressure will reach its normal operating value, the pressure difference between the high and the low side will be quite high, hence the flow will be much lower than in the very beginning.

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Yes, but keep in mind that both pressure ratio and suction pressure influence compressor flow rate. A compressor will pump more refrigerant at a higher suction pressure, where you will have the high heat load on the evaporator. This is due to the fact the refrigerant vapor will have greater density than at lower suction pressures, assuming superheat remain reasonable, which allows the compressor to push more refrigerant mass

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Originally posted by Prof Sporlan
pressure ratio (discharge pressure divided by suction pressure in absolute units)

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i knew there must have been a term for that! :)

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Originally posted by Prof Sporlan
Yes, but keep in mind that both pressure ratio and suction pressure influence compressor flow rate. A compressor will pump more refrigerant at a higher suction pressure, where you will have the high heat load on the evaporator. This is due to the fact the refrigerant vapor will have greater density than at lower suction pressures, assuming superheat remain reasonable, which allows the compressor to push more refrigerant mass

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ahh, i see what you are saying.

that would be the case in an AC system, for example. where we have the heat load before we turn on the compressor.

now imagine a hypothetical system there the heat load is applied to the evaporator only AFTER it stabilizes. such as cooling electronics. you turn on the compressor, let it sit there for appropriate amount of time, and only then give the current to your chip (i.e. activate the heat source). in this case, we will NOT have a discharge pressure on the startup higher than the normal operating discharge pressure. is that correct?

thanks Marc :)
its not a specific problem i'm trying to pinpoint;
i'm going to build a custom phase change system soon,
so for now i'm just digging through theory...
i am pretty sure i've got this issue figured out, i just need a "yes" or a "no" answer to this question:

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Originally posted by dmitriyaz
now imagine a hypothetical system there the heat load is applied to the evaporator only AFTER it stabilizes. such as cooling electronics. you turn on the compressor, let it sit there for appropriate amount of time, and only then give the current to your chip (i.e. activate the heat source). in this case, we will NOT have a discharge pressure on the startup higher than the normal operating discharge pressure. is that correct?

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Prof Sporlan, where are you? ;)

dmitriyaz, I would say that you are the more correct, but not neccesarily absolutely right, as some of the answers have shown.
If you were normally condensing R22 @ 15Bar, and your sytem tried to double the head pressure, you'd trip the H.P. switch (typically set for around 24-26 Bar on our kit), and possibly even blow the PRV.
However, as the Prof says, if the load at startup is high enough, then the discharge pressure will go higher that normal operating conditions, before (in my company's equipment) the head pressure control system brings it under control, by modulating a variable speed condenser fan, or staging in extra fans.
So I vote for you.

I hate to disagree with Marc, but head pressure will generaly decrease with a restriction until the condenser gets full. As the condenser fills the pressure will rise. As the pressure rises, it is easier to condense the ***** in the condenser. More ***** condenses, that leaves more room for the incoming gas. More ***** stays in the condenser as a liquid so SC and SH increases but evap pressure drops. Now the compressor can't pump as much gas into the condenser.
I feel that restrictions are accused of increasing head pressure because somebody saw only low evap pressure and thought the system was undercharged. If the restriction is in the hot gas line it is a different story.

On start-up with an equalized system, the compressor is going to pump more *****. The valve will let less ***** out of the condenser though. Head pressure will start to rise and suction pressure will start to fall. Under normal loads the head pressure will not rise more than normal. If the evap is under a high load, the evap pressure stays high. The compressor can pump more ***** and the valve holds more ***** in the condenser. Condenser pressure will rise above normal. If the system is too small to pull the temp down, the compresser will not be happy. If the evap load drops in a few hours, the compressor will get a good work-out but will survive to do it again another day.

On this hypothetical system with a low evap load on start-up, high head means there is too much ***** in an unbalanced system.
When you are building your system, balance the compressor with the expansion valve and size your coils far a decent split ,DT. If everything else looks right, your condenser is too small.

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now imagine a hypothetical system there the heat load is applied to the evaporator only AFTER it stabilizes. such as cooling electronics. you turn on the compressor, let it sit there for appropriate amount of time, and only then give the current to your chip (i.e. activate the heat source). in this case, we will NOT have a discharge pressure on the startup higher than the normal operating discharge pressure. is that correct?

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Perhaps it would be better to look at this in the following manner: normal load on the evaporator should give you a normal operating discharge pressure. A low load on the evaporator will lower discharge pressures, and a high load on the evaporator will raise discharge pressures. And to some extent, "normal" is what the system designer chooses to be normal. One cannot operate a system at a no load condition unless there is provisions for capacity control, e.g., hot gas bypass. It would seem your hypothetical system would require capacity control for startup. Then when your evaporator sees its design heat load, all should operate normally.

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The Prof will remember these threads :)

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Aaaahhhhh, the good old days of alt.hvac, when it was the only viable hvac/r online forum around... :)

thanks for all your input guys,
i think i've got it figured out :)
by the way, this is a great forum!
i will hopefully contunue leeching off your knowledge for some time :D

Why are all these non-HVAC type trying to design cooling systems for their expensive electronics? This is a job many good people that have been working with this stuff for years can't do well. Then they give us scant details on their project and expect help. I think they would have better luck paying someone to look their system over.

I would suspect the system currently being tested does not have matching compressor and expansion valve. Or the evap is too small and they overcharged it trying to get the superheat down.

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Originally posted by superheat
Why are all these non-HVAC type trying to design cooling systems for their expensive electronics?

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superheat, i assume by "non-HVAC type" you are referring to myself. the answer to your question is: because i enjoy learning. i would be (almost) as well off by just slapping a heatsink and a fan on the parts i am planning to cool with phase change. thats obviously the reason i don't hire someone to do it for me.
secondly, there is no project yet. i dont have a system that needs troubleshooting. i am merely trying to expand my theoretical knowledge of how things work.

is it a problem that i ask questions here? :confused:

Yeah not only that but most "HVAC types" only know how to replace parts according to what the manufacturer tells them.I would like to see one average joe a/c tech make a system from scratch.

Refrigeration systems for computers are usually made by mismatched parts that you find for free/cheap.

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This is a job many good people that have been working with this stuff for years can't do well.

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Yeah like Chip-con, Asetek and Maxxxpert.Yet when someone that makes his own system from a refrigerator compressor and condensers they find laying around, they get much much lower temps and make it for alot cheaper.Why would you pay out the nose for higher temps?Not only that but you wouldn't get to enjoy building it yourself and get a wonderful gift of knowledge. :)

I think some of you have gotten the wrong idea of dmitriyaz's question.He was just asking if a basic systems (capillary not TXV) high side pressure rises above the normal operating pressure on startup.

No problem with questions. The forum allows many people to learn many things. It is just an observation, There are many of you out there on the different forums on the internet. Many of them do not ask good questions. There are rarely any controls questions.

I personally would never design a system with a cap tube. It is not real difficult to decrease the cap tube length, but does take time. TEV would be much simpler. Constant load, AEV would work nicely.

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is it a problem that i ask questions here?

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Not at all. Such questions often initiate interesting discussions. I, for one, welcome them. :)

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Yeah not only that but most "HVAC types" only know how to replace parts according to what the manufacturer tells them.

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I enjoy helping people, but I'm not too fond of being insulted. Go figure.

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I would like to see one average joe a/c tech make a system from scratch.

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You seem to think that's the hard part.

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I think some of you have gotten the wrong idea of dmitriyaz's question.He was just asking if a basic systems (capillary not TXV) high side pressure rises above the normal operating pressure on startup.

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Why don't you enlighten us ignorant HVAC types? Your answer to the question is... ???

Gary:
I was not talking about you, or 99% of the people on this board.But most average beer cooler repairmen HVAC guys don't know anything about refrigeration but replacing parts.I am sorry you thought I was referring to you.You are more what I would refer to as a technician. :)

I know a guy that that works on refrigerators, and he thinks you can get -100c by using some kind of fast cooling condenser. :D
He has alot of interesting thoughts that is for sure. :)

I am sorry you took it the wrong way.

Oh yeah, I know repairing systems and actually diagnosing problems is much harder than building refrigeration systems from scratch.But I was referring to people like vic who don't really diagnose things.Just replace things till it works, there are alot of so-called "techs" that do that.They are in every field, computer repair, rv repair, a/c repair etc.

What makes you think you are an "average joe" HVAC repairman, Gary?

I think that anybody who can truely diagnos a system fault has engineered some repairs. That might not be design from scratch, but it is close to the same thing. You have to know how all the parts work and interact to figure out all but the basic repairs.

I am truely sorry that I got this thread going in a negative direction. I promise to try not to get this board going in the negative direction of others internet boards seem to like so much.

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I think that anybody who can truely diagnos a system fault has engineered some repairs. That might not be design from scratch, but it is close to the same thing. You have to know how all the parts work and interact to figure out all but the basic repairs.

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I agree with you 100%.

I also understand your problem with all these people trying to build refrigeration systems for their computers.I have seen a few people on here asking some really stupid questions about cooling their computer :)
If they want to get questions answered, they should at least learn as much as they can till they HAVE to ask a question.

Or ask better questions than: "which refrigerant is the coldest and best, and how can i make the best system that is quiet and small and only costs 5 dollars" :D
I can definately understand. :)

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Originally posted by dmitriyaz
me and my friend were having a debate.
my statement:
during start up, the high side pressure in the system does not exceed its normal operating high side pressure.
you can find my explanation to him here.
his statement:
during start up, the high side pressure gets extremely high, up to twice as much; and then comes back down to normal operating pressure. he couldn't really provide me any sort of evidence for his argument, and i couldn't think of any myself. if you can, let me know.
please vote in the poll, and any comments on either side are much appreciated.
thanks for your time.

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Theirs only one good way to prove this to a stubborn friend. If you have a car with a working A/C system, hook up a set of gauges, one on the high side and one on the low side. Turn your car on, and turn the ac on... roll down the windows, find the throttle linkage to the throttle body/carburator and rev the engine to 3000+ rpms and let your friend watch the pressures rise/fall.

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...and how can i make the best system that is quiet and small and only costs 5 dollars"...

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sometimes it is understandable for someone to ask that question if they have a shed-load of working spare parts. If you already have the parts, you wouldnt need to spend money :D

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Originally posted by abcdefg1675
Theirs only one good way to prove this to a stubborn friend. If you have a car with a working A/C system, hook up a set of gauges, one on the high side and one on the low side. Turn your car on, and turn the ac on... roll down the windows, find the throttle linkage to the throttle body/carburator and rev the engine to 3000+ rpms and let your friend watch the pressures rise/fall.

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:confused:
Who said anything about a car a/c system?They differ from the system he is talking about.
Try reading a bit next time. ;)

Just for the record I am his friend.The point I was trying to make was that there is a larger load on the system upon startup.And you really feel it if your condenser cant handle much more than average operating conditions.He is actually the stubborn one here. ;)
I will have to take a couple movies of the low and high side gauges upon startup for him I suppose.

When I told him that the pressures "get extremely high" I actually told him that they skyrocketed on one system I had.But he thought I meant for all systems.Our real argument was the increased load on startup.

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Originally posted by abcdefg1675
sometimes it is understandable for someone to ask that question if they have a shed-load of working spare parts. If you already have the parts, you wouldnt need to spend money :D

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You missed my point.I am talking about the average overclocker that knows nothing about refrigeration or even how to build anything, but he thinks you can explain everything to him in one sitting so he can go out and build it perfectly.

Quote:

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Originally posted by dmitriyaz
me and my friend were having a debate.
my statement:
during start up, the high side pressure in the system does not exceed its normal operating high side pressure.
you can find my explanation to him here.
his statement:
during start up, the high side pressure gets extremely high, up to twice as much; and then comes back down to normal operating pressure. he couldn't really provide me any sort of evidence for his argument, and i couldn't think of any myself. if you can, let me know.
please vote in the poll, and any comments on either side are much appreciated.
thanks for your time.

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If the problem is really a high evaporator load and higer than normal room air temps, yes, the suction pressure will increase and some time drastically. Ways to control the excessive statr up suction pressure are use of a MP or pressure limiting expansion valve. An alternate method is to use a compressor suction pressure regulator or CPR valve (crankcase pressure regulator).

As for the restricted liquid line, if this is the case, you will tend to run high suction superheat and low suction pressure since you can not feed enough liquid to the coil. The compressor would be a PD (positive displacment) type and all it knows is to pump so many CFM, irreguardless of the load or liquid feed. If you do not feed sufficient liquid for the load, the load will evaporate all of the liquid available and the compressor sucks it out. If the line of expansion device is severely restricted, the incoming mass flow will be less than the CFM the compressor is pumping and the suction pressure drops.

I have designed low temp glycol chillers (two 700 HP screw units with flooded evaporators) which had to start up against a high temp load and pull down without operator attention. The last one had to start with 120 dF process liquid. Due to the total system volume and piping to pull down, these machines operated for close to 2 hours at elevated evaporator pressures before reaching the design pressure/temperature. The control was achieved by amp limiting the compressor, high discharge pressure load limiting, load ramp up control and a modulating suction pressure valve, to hold the high pressure off the compressor until it was within operating limits.

Ken

The head press will float up and down based on ambient temp and load. With ambient remaining constant and cond rejecting more heat head will go up. This is a normal condition.

Prof Sporlan is right if head press is abnormally high something is amiss.

I have seen non condensables in the system give the indications that were described above.

Fit high and low side gauges to a system , start the compressor, suddenly the refrigerant standing pressure changes , it gets lower on the low side and higher on the high. A good reason for the head to keep climbing and then drop to a steady level would be the activation of a condenser fan /pump / waterspray,

this is going to be short and sweet. The head pressure will be very high because the evap temp is high the evap pressure is high so the head pressure will be very high use a evap press reg or crankcase pressure regulator to avoid this . this is a very common siuation it happens every time a system starts up.

wesmax

Second comment. You can put all the formulas to paper you want the fact is undisputable the higher the suction the higher the head .

wesmax

pressure will be higher than working pressure... in my humble opinon

there is no better teacher than experence with hands on work.

wesmax

Hi All ,
Now heres a thought for you.Which is starting first - The condensor fan motor , OR the compressor.Simaltaneous start will give you slight rise before settling , Compressor on start delay with Condensor fan start on make of cooling call = cooling of standing refrigerant in condensor - pre compressor start for delay period usually 3- 5 minutes , very handy for our hot clmates like here in oz.

you may start the condensor fan first but we start the compressor first here in the winter to get enough heat in the condensor to move the gas around.
wesmax

higher suction at start up = higher head pressure.

you are rite sometimes depends on the condensor temp here in alberta in the winter the outdoor temp is -30 so air over the cond is -30 so head is not up. You realy do not give enough info in the question.
wesmax

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Originally Posted by mr cool

higher suction at start up = higher head pressure.

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This is pretty much it, when a unit starts it's going to be at it's highest suction pressure, which means the head pressure will be highest.
I was working on a upright freezer yesterday. Ambient was 39C, but it was hotter than that due to all the other condensing units.
After defrosting the coil (so the evap coil was warm), I couldn't get the damn thing running again. It kept on tripping out for some reason, while all the other freezers at -18 were cycling fine.
So I had to increase HP cut out from 2600Kpa to 2800kpa just to get it started.
edit: Nothing changed on the system, it was cycling fine, not tripping out the 2600kPa HP before I defrosted it.

I cannot vote . But High side pressure rise up and gradually drops down. Its a common phenomenon in the compressors. You can say this is an instantaneous rise due to back pressure in the condenser. If you start the compressor with throttled suction valve then you may notice high pressure side never exceed the set values. That's why when condition is bad suction is throttle and slowly open at start up. In new systems this part is looked after by the programs and related solenoid valves

Most systems that i work on have some sort of capacity control, be it converters/inverters on VRV/F's or unloading valves, slide mechanisms etc. on chillers, I am not suggesting that the design condensing temperature is achieved instantaneously but head pressure control is very precise in most modern systems.
Of the two answers I agree with you.