New system tripping on low pressure??

[DTLarca]

I consider delta-T to be the best indicator of airflow. In this case 36C-20C=16K dT. I would consider this (borderline) sufficient.

The cond TD would be 33.5C-20C=13.5K TD, which would indicate a normal load, given sufficient airflow.

As you point out, the SCT should be higher than leaving air. This could be due to the timing of the measurements or instrument error.

In any case, if there is insufficient heat load picked up on the low side, we should expect insufficient heat load to be rejected on the high side (low TD).

At the moment since the condenser's air is leaving at about the same temperature as the saturated condensing temperature we would not know how much under normal the volume flow is. That's m³/kW norms versus this system.

Much of the confusion here comes about because this system is for a fresh air application and is a heat pump.

I have attached an image of the sort of configuration we normally use for 100% fresh air systems. The condenser is located in the exhaust air stream. There is also an adiabatic heat transfer plate heat exchanger...

Oops, suddenly I am unable to upload images from my computer? The facility has gone blank?

[mikeref]

Further tests, I've removed the ducting from the evap to eliminate that poissibility and it has made no difference either to the unit cycling on LP

This post from a few days ago, narrows down possibilities.

[DTLarca]

Bull shyte, temperatures do not change with altitude, what changes is gauge calibration, which is pressure. Alltitude also effects mass flow of given from a fan, and heat transfer coiefficients on heat transfer. There is no problem in this thread with refrigerant control!!!!

The temperatures do not change - the readings do. Subcool and superheat readings change.

A bourdon gauage calibrated to 0kpa at sea level will still show 0kpa if taken to the moon. On the moon then the superheat readings will appear negative when in fact positive and the subcool readings will be fantastically high when in fact normal.

[mad fridgie]

This post from a few days ago, narrows down possibilities.

Missed that one Mikeref, well spotted!
Have a doughnut!

[Gary]

In a real world Gary, the pressure is on when some one has paid for a system and it isn't working as specified. They don't care about theories they want results, now , not whenever.

I was a hands-on service tech for 40 years. I know all about the real world. In the real world, the more you learn, the better you serve your customers in the future.

[DTLarca]

Well perhaps you're right ... but this guy is on a commercial contract ... he has to get a system running, keep client happy and a reputation to maintain. If he fixes this problem, by any means, the client will be pleased. If on the other hand he pisses about fannying over theoretical issues of flow and bypass factors etc, the client ain't going to be very happy.

In a real world Gary, the pressure is on when some one has paid for a system and it isn't working as specified. They don't care about theories they want results, now , not whenever.

I feel for this guy, he needs some help to get him or the system out of the mire. He'll be reading all of these posts and be getting lost with the runaround, he needs to be offered sound practical advise that he can act on. If he were in an R & D shop then it would be different I'm sure.

This is basic commissioning knowledge. If you want to get into the design then we can go there but I think that would be unnecessary considering the scope of practicalities here.

Gary asked about the bypass factor principles I mentioned - not the original poster. So the answers were not addressed to the original poster - only to Gary. The comments about this being a fresh air application possibly being asked to operate out of range were aimed at the original poster and all other posters in general.

[mad fridgie]

The temperatures do not change - the readings do. Subcool and superheat readings change.

A bourdon gauage calibrated to 0kpa at sea level will still show 0kpa if taken to the moon. On the moon then the superheat readings will appear negative when in fact positive and the subcool readings will be fantastically high when in fact normal.

Guage surrounding pressure plus gauge mechanical pressure equal calibriation point, change surrounding pressure or mechanical pressure, calibriation point changes. When certifying gauges you test in a fixed pressure test chamber, a pressure over absolute vacuum, not a pressure over atmospheric conditions.
Why the surrounding pressure changes the stesses on the mechincal parts of a gauge

[mad fridgie]

I think we should move on and just help the OP with his problem
I think to make thing clear, we all need to know what excactly is his system.
Conditioned Air Flow
this comes from outside, inside or a mix,
it passes through a filter, heatexchanger, fan widget or what ever.
just a true description of what the process is. (as it would seem that we all have different ideas of what we are talking about, thus are giving different solutions based upon what we think we are dealing with) I think once this done we should all be singing on the same song sheet!

[nevgee]

This is basic commissioning knowledge. If you want to get into the design then we can go there but I think that would be unnecessary considering the scope of practicalities here.

Gary asked about the bypass factor principles I mentioned - not the original poster. So the answers were not addressed to the original poster - only to Gary. The comments about this being a fresh air application possibly being asked to operate out of range were aimed at the original poster and all other posters in general.

Going into the design principles would be ok for me and probably be more helpful, hence my reasoning over the style and build of the coil block. I couldn't see how bypass factors could have anything to do with basic commissioning! They are elements of design not commissioning. I doubt there are many peeps on here who would have the experience of designing air coils from basics, without a pc at hand.

I wasn't having a go at anyone ... just pointing out the main thread.

[DTLarca]

Guage surrounding pressure plus gauge mechanical pressure equal calibriation point, change surrounding pressure or mechanical pressure, calibriation point changes. When certifying gauges you test in a fixed pressure test chamber, a pressure over absolute vacuum, not a pressure over atmospheric conditions.
Why the surrounding pressure changes the stesses on the mechincal parts of a gauge

In the field a digital micron gauge can tell you your altitude.

It can tell you also if your vac pump is good. If it is good then you can use it to calibrate your service gauges for altitude. Pull a vac on the gauges to the best your pump can get then set your gauges to show absolute vacuum. When disconnected they will rise to give you a better indication of the actual atmospheric pressure. Reconnect them to any system you were analysing and suddenly you'll see the actual superheat is higher than you read before and the actual subcool is lower than you read before.

Do this to systems in Mexico or Johannesburg and adjust the superheat to a true design superheat and you will gain perhaps 10% capacity on the system.

[DTLarca]

Going into the design principles would be ok for me and probably be more helpful, hence my reasoning over the style and build of the coil block. I couldn't see how bypass factors could have anything to do with basic commissioning! They are elements of design not commissioning. I doubt there are many peeps on here who would have the experience of designing air coils from basics, without a pc at hand.

I wasn't having a go at anyone ... just pointing out the main thread.

No worries - but commissioning engineers must get the right SHR at the right total duty. For this they must understand the relationship between ADP and BPF. This is for custom build AHU's. For catalog AHU's only external static pressure and distribution balance is of concern. Standard AHU's have fans, motors and pulleys closely selected without much tolerance. Custom build AHU's these days have motors oversized by perhaps 40% with VFD's for RPM fine tuning according to ADP, SHR and BPF.

[mikeref]

Going over this thread from the start, there has been reference to the distributor piping, but i don't recall temperature readings at the individual circuit outlets to the collector. Would there be a chance the spider is incorrectly piped or some of the circuits have been shorted, or if the distributer is biased to only a few circuits? What about the distance between expansion valve and distributer?

[nevgee]

No worries - but commissioning engineers must get the right SHR at the right total duty. For this they must understand the relationship between ADP and BPF. This is for custom build AHU's. For catalog AHU's only external static pressure and distribution balance is of concern. Standard AHU's have fans, motors and pulleys closely selected without much tolerance. Custom build AHU's these days have motors oversized by perhaps 40% with VFD's for RPM fine tuning according to ADP, SHR and BPF.

Sensible Heat Ratio is preset at design given specific design parameters. Aparatus Dew Point and ByPass Factors are also predefined at design. I can't see how a commissioning Engineer is able to make any corrections to these factors when he is most like never to achieve designg conditions at all. I would find it very unusual that any one would have the luxury and satisfaction of designing an AHU system with ducting etc and for it to drop into a design performance for commissioning purposes. In a real world it never happens. There are many peeps who sit at their drwng brds believing that the world is perfectly round ...well it surely isn't and never will be. There is a great difference between design practice and the realms of commissioning and the reality of having to instal a system that wasn't thought thru properly.

[DTLarca]

Sensible Heat Ratio is preset at design given specific design parameters. Aparatus Dew Point and ByPass Factors are also predefined at design. I can't see how a commissioning Engineer is able to make any corrections to these factors when he is most like never to achieve designg conditions at all. I would find it very unusual that any one would have the luxury and satisfaction of designing an AHU system with ducting etc and for it to drop into a design performance for commissioning purposes. In a real world it never happens. There are many peeps who sit at their drwng brds believing that the world is perfectly round ...well it surely isn't and never will be. There is a great difference between design practice and the realms of commissioning and the reality of having to instal a system that wasn't thought thru properly.

Nevgee, if you and I were standing in front of an ahu that was not achieving the correct SHR, say the SHR was too high, what would be on your short list of things to check or adjust if you were about to turn around and give me suggestions?

[Gary]

Nevgee, if you and I were standing in front of an ahu that was not achieving the correct SHR, say the SHR was too high, what would be on your short list of things to check or adjust if you were about to turn around and give me suggestions?

Is there a neon sign on the side of the AHU that says, "My SHR is too high"?

[DTLarca]

Is there a neon sign on the side of the AHU that says, "My SHR is too high"?

Pretty much though actually a mini LCD indication

[DTLarca]

Is there a neon sign on the side of the AHU that says, "My SHR is too high"?

Consider it this way, Gary.

You are standing between two AHU's. One has a return of 22 and a supply of 12 while the other a return of 24 and a supply of 21.

For a given deviation in leaving air temperature from design by say 0.2K which one is further off the SHR mark? The answer comes to mind instantaneously - yes?

[Gary]

Pretty much though actually a mini LCD indication

That's what I get for retiring... they go and change everything.

I would take that to mean the humidity was too high, make sure everything is operating properly, then drop the air off temp (reduce airflow) just enough to get the RH where I want it.

In 40 years, I never once checked the SHR... or even cared about the SHR.

[nevgee]

Ok so I go thru the trouble of measuring everything and discover the SHR is wrong ... by how much has it to be out before I care? The ambient conditions are likely to change before I get the chance to correct my offsets. If its for comfort cooling why is there a problem if its not 0.7 or 0.75 or 0.8 ... who would bother?
Who would notice that the air is being treated in a slightly different way. the paper in the photo copier may not pass thru as smoothly .. some one may notice a little static or not ... I can't see why you are so "focussed" on this subject.

I've been in this industry a few yrs and have never heard anyone show so much passion for SHR ..... I am foncused.

[nevgee]

That's what I get for retiring... they go and change everything.


In 40 years, I never once checked the SHR... or even cared about the SHR.

Gary, I couldn't agree more with you on this one.

[Gary]

From a design/component sizing point of view, the SHR is important, but irrelevant to comissioning/servicing.

To my mind SHR is beside the point, RH is the point.

[mad fridgie]

SHR, has only reference to performance, if you have the design reference point to start with. I think someone has only ever read design books. If your air is vey dry, then the SHR will be high, if the air is humid, then then it will be low, what is important is the quality of the air leaving the ahu, the SHR% has no relevence to anything. It just a calculated number.

[nevgee]

Nevgee, if you and I were standing in front of an ahu that was not achieving the correct SHR, say the SHR was too high, what would be on your short list of things to check or adjust if you were about to turn around and give me suggestions?

You buying coffee or me?

[DTLarca]

Ok so I go thru the trouble of measuring everything and discover the SHR is wrong ... by how much has it to be out before I care? The ambient conditions are likely to change before I get the chance to correct my offsets. If its for comfort cooling why is there a problem if its not 0.7 or 0.75 or 0.8 ... who would bother?
Who would notice that the air is being treated in a slightly different way. the paper in the photo copier may not pass thru as smoothly .. some one may notice a little static or not ... I can't see why you are so "focussed" on this subject.

I've been in this industry a few yrs and have never heard anyone show so much passion for SHR ..... I am foncused.

nevgee, human comfort is first of all mostly effected by radiant temperatures and in particular radiant temperature asymmetries, secondly by dry bulb temperatures, thirdly by drafts, fourthly by discrepancies between neck and ankles greater than 3K and down the list we eventually come to humidity. this tells us SHR is actually not so important with everyday comfort cooling as compared to say the processing of drug powders or preventing the ink on the labels on Darwins specimen bottles at the Natural History Museum from running.

However, particularly for schools, where 3 litres per second per person is considered satisfactory in the UK - compare that to 10 l/s for office staff - at least it is over the 1 litre per second required for CO2 management, there is the tendency for humidities to hover up around the higher boundaries of the comfort envelope at 70%, from time to time and with a custom built AHU providing as high as possible a SHR for energy savings you are likely to depart from the limits ensuring comfort and health.

Operating SHR is, among a few other things amounting to mere synonyms, the line between return air dry and wet coordinates and ADP. ADP is discovered by extending a line from there on through leaving air dry and wet coordinates up to the saturation line. For every set of return air coordinates there is one and only one ADP that will give you the desired SHR. Control ADP and you control SHR.

[Gary]

Mmmmm... did someone say "Coffee"?

[nevgee]

coffee? ...I'm with you Gary the room is getting stuffy we need a break

[nevgee]

Or we need to adjust the Apparatus Dew Point ... or maybe change the BPF by allowing some dirt into the heat exchanger ... or letting someone open a few doors ... or windows ... I don't know ... perhaps some human factor to just mess it all up anyway.

As I said the world isn't perfectly round. But the book may say different. Who knows

Coffee? double shot please!

[DTLarca]

From a design/component sizing point of view, the SHR is important, but irrelevant to comissioning/servicing.

To my mind SHR is beside the point, RH is the point.

It's like an engineer I recently spoke to regarding a walk-in he looked at for us. I asked him what the subcool and superheat was. He said he did all that superheat and subcool stuff at college 20 years back but he no longer remembers what that all is. He said he has had success repairing systems now for 20 years so none of that can be very important.

I told him he would be surprised how much it helps his daily life if he took the time to get familiar with it all. I told him I do not doubt he has been able to get the systems working all these years - that was not in question otherwise we would not have subbed work to him.

You guys don't care much for BPF and ADP or SHR etc - well cool - i don't doubt you get by without caring much about it. But I know you are wrong when you imply that because of that it is therefore a load of trivia.

But if you don't know much about it then how can you be so sure you were always getting stuff right. Just because the customers have been happy all this time it doesn't necessarily mean you got it right every time.

On a 1000kW system it can mean £40,000.00 savings in electricity every year - at least.

[DTLarca]

Or we need to adjust the Apparatus Dew Point ... or maybe change the BPF by allowing some dirt into the heat exchanger ... or letting someone open a few doors ... or windows ... I don't know ... perhaps some human factor to just mess it all up anyway.

As I said the world isn't perfectly round. But the book may say different. Who knows

Coffee? double shot please!

Oops - the chilled ceilings are dripping water - quick - what are we going to do about the high effective SHR?

[nevgee]

nevgee, human comfort is first of all mostly effected by radiant temperatures and in particular radiant temperature asymmetries, secondly by dry bulb temperatures, thirdly by drafts, fourthly by discrepancies between neck and ankles greater than 3K and down the list we eventually come to humidity..

You failed to point out that everyone has a differing level of perceived comfort. Women greater than men. Then of course the children and the aged and infirm, those who may be at rest or active ... .so many variables and you didn't mention them ... I think you're getting tired, Ah, another physiological reason for temperature perception.

Is it so important? Nadda ... no ... niche ... La ... nien

[nevgee]

Ah Subcooling and superheat .... I can agree with you there .... I find it very suprising how many guys don't appreciate the value of those values

[DTLarca]

You failed to point out that everyone has a differing level of perceived comfort. Women greater than men. Then of course the children and the aged and infirm, those who may be at rest or active ... .so many variables and you didn't mention them ... I think you're getting tired, Ah, another physiological reason for temperature perseption.

Is it so important? Nadda ... no ... niche ... La ... nien

Yup - we can take that approach with subcool and superheat too

Beer can cold - badda badda bish bang bing bong - job done - customers happy - Is subcool and superheat so important? Nadda nix - load of tosh all that design crap!!

[nevgee]

But if you don't know much about it then how can you be so sure you were always getting stuff right.

You presume too much I think

[DTLarca]

You presume too much I think

Presume too mcuh - what - about the subby who told me he doesn't need to know about subcool and superheat - that it is not important?

I told him also that there were a lot more problems out there possibly still waiting remedy that were staring him in the face but because he did not understand subcool and superheat he simply had no idea they existed. He shook his head in disbelief. I didn't tell him that a few weeks prior our top engineer who knows his subcool and superheat pretty well followed him up on a site to repair the leak he reported. Turned out the condensing unit was dirty and the evaporator was dirty (cold room) and after both were cleaned the room temp came down and the sight glass cleared. He may think I presumed too much but then that is how irony works - doesn't it

[mad fridgie]

It's like an engineer I recently spoke to regarding a walk-in he looked at for us. I asked him what the subcool and superheat was. He said he did all that superheat and subcool stuff at college 20 years back but he no longer remembers what that all is. He said he has had success repairing systems now for 20 years so none of that can be very important.

I told him he would be surprised how much it helps his daily life if he took the time to get familiar with it all. I told him I do not doubt he has been able to get the systems working all these years - that was not in question otherwise we would not have subbed work to him.

You guys don't care much for BPF and ADP or SHR etc - well cool - i don't doubt you get by without caring much about it. But I know you are wrong when you imply that because of that it is therefore a load of trivia.

But if you don't know much about it then how can you be so sure you were always getting stuff right. Just because the customers have been happy all this time it doesn't necessarily mean you got it right every time.

On a 1000kW system it can mean £40,000.00 savings in electricity every year - at least.

I think you are mis understanding SHR, is simply a calculation of the process Energy in, Energy out split between sensible and latent. When designing you calc on a steady state, or I should say Ideal, then design should allow for variations from steady state.
Many but not all, may use different methods to calculate load profiles, relying on various test equipment, flow, preesure temp humidity. You can only have a set of ideal SHR conditions under a set of fixed process conditions, the process conditions change then so must the ideal SHR conditions change. So what i am saying is actual SHR is irrelevent, unless the you have the ideal process SHR for that particular plant aand the different conditions, of which the average engineer on site is not going to have at hand.

[DTLarca]

I think you are mis understanding SHR, is simply a calculation of the process Energy in, Energy out split between sensible and latent. When designing you calc on a steady state, or I should say Ideal, then design should allow for variations from steady state.

What makes you think I misunderstand it?

[mad fridgie]

What makes you think I misunderstand it?

I applogise for how that read, (not my intention), my fingers were typing slower than the brain was thinking. I think you misunderstand how SHR is applied out in the field, and how many of us are aware of it as a load profile and how irrelevent the SHR% is, unless you know the particular process variables. You may SHR as your control variable within your software, but surely must have a floating set point to meet the process variables, there are many methods of control, how to reach optimum performance ( meeting process requirments and energy efficiency) is very plant specific

[DTLarca]

I applogise for how that read, (not my intention), my fingers were typing slower than the brain was thinking. I think you misunderstand how SHR is applied out in the field, and how many of us are aware of it as a load profile and how irrelevent the SHR% is, unless you know the particular process variables. You may SHR as your control variable within your software, but surely must have a floating set point to meet the process variables, there are many methods of control, how to reach optimum performance ( meeting process requirments and energy efficiency) is very plant specific

In the world of philosophical logic we say that a theory that explains everything explains nothing.

Global generalisation like "Plant Specific" sound precise but in fact are so vague they are meaningless. Propositions are either analytic or synthetic and synthetic statements while being contingent or a posteriori are meaningless if they in principle at least cannot be varified and no option is left open for them to be falsified.

The problem with such statements is that they are so easily converted from synethetic to analytic. The no true Scotsman Fallacy is a good example:
http://en.wikipedia.org/wiki/No_true_Scotsman

Anyway - say it was you instead of me who was called to the Natural History Museum to try solve the problem of Darwin's specimen bottle label's ink running.

Simply by manipulating the operation of the plant how do you first estimate the space SHR and then the plants effective SHR? What common factors would you look at to see where things might have gone wrong.

[mad fridgie]

Firstly lest clarify SHR
Semsible Heat Ratio = the amount of energy remove by an evaporator split into the two types of energy remove sensible and latent. (drop in temp and moisture removed)

[mad fridgie]

In the world of philosophical logic we say that a theory that explains everything explains nothing.

Global generalisation like "Plant Specific" sound precise but in fact are so vague they are meaningless. Propositions are either analytic or synthetic and synthetic statements while being contingent or a posteriori are meaningless if they in principle at least cannot be varified and no option is left open for them to be falsified.

The problem with such statements is that they are so easily converted from synethetic to analytic. The no true Scotsman Fallacy is a good example:
http://en.wikipedia.org/wiki/No_true_Scotsman

Anyway - say it was you instead of me who was called to the Natural History Museum to try solve the problem of Darwin's specimen bottle label's ink running.

Simply by manipulating the operation of the plant how do you first estimate the space SHR and then the plants effective SHR? What common factors would you look at to see where things might have gone wrong.

Prior to making changes you must understand the processes.
I would presume that they have a desire dry bulb temp? (lets say yes, just for ease) Then we need to know at this temperature, at what humidity does the ink run (normally contact ink manufacture), So know we have point, we then can calculate the resperation rate of the plants. external load requirements, then we need to know the fresh air make up and under what conditions this enters. We then can calculate max and min enthalpy of the evap entering air, we know the required air outlet quality. From this we can design the coil or if already existing the required SST to achieve the desired results. the design is easy it is all about the process. Depending upon what is variable within the system, would determine how you would control for optimum efficiency. (and dare I say how much the client is willing to pay, which in many cases is the actual limiting factor)

[DTLarca]

Firstly lest clarify SHR
Specific Heat Ratio = the amount of energy remove by an evaporator split into the two types of energy remove sensible and latent. (drop in temp and moisture removed)

SHR = SH/(SH+LH)

Where SH+LH = Total heat or TH

SHR = SH/TH

The room SHR and the effective AHU SHR need to be similar but when fresh air is involved the coil SHR will usually be lower than the other two.

[DTLarca]

From this we can design the coil or if already existing the required SST to achieve the desired results.

Correct

Do how do we adjust the SST.

BTW - I did not ask how to calculate the room SHR - I asked how you would imply it as in by introducing an instability to the room to monitor the ensuing rates of change.

[mad fridgie]

Correct

Do how do we adjust the SST.

BTW - I did not ask how to calculate the room SHR - I asked how you would imply it as in by introducing an instability to the room to monitor the ensuing rates of change.

If I do not know the reqired enthalpy, then how do I know what to adjust the SST to. thus you do need to calculate!
How to do it so many ways and is plant specific, if you have excess refrigeration capacity and capacity control of the compressors, it may just be as simple as adjusting suction set point! This bit really comes down to what you have. I do not have problems only solutions waiting to be found.!

[mad fridgie]

Appologies to the OP, we seem to have highjacked your thread!
Give use some pics mate! Then we can get back on track!

[DTLarca]

If I do not know the reqired enthalpy, then how do I know what to adjust the SST to. thus you do need to calculate!
How to do it so many ways and is plant specific, if you have excess refrigeration capacity and capacity control of the compressors, it may just be as simple as adjusting suction set point! This bit really comes down to what you have. I do not have problems only solutions waiting to be found.!

You are not designing the system so you do not need specifics - ratios will do.

So for DX yes you lower the SST but by merely lowering the Suction Pressure Set Point you have also increased sensible cooling and thus you have just given the RH a cause to rise precisely when you wanted it to drop. So what would be a better solution?

Also - what would you do if it was a chilled water cooled AHU?

[mad fridgie]

You are not designing the system so you do not need specifics - ratios will do.

So for DX yes you lower the SST but by merely lowering the Suction Pressure Set Point you have also increased sensible cooling and thus you have just given the RH a cause to rise precisely when you wanted it to drop. So what would be a better solution?

Also - what would you do if it was a chilled water cooled AHU?

True scotsman, how long is a piece of string. Does the system have re-heat, by-pass dampers, variable airflow, variable refrigeration capacity and so on. The system will determine the fix, are there any money limitations.
Chilled water, is it a dedicated chiller or common, plus all of the above.
You gather information before you make your changes, or you pay for the best to do it for you. and I do cost!

[Gary]

I'm still waiting to hear how SHR is relevant to service.

[Gary]

Appologies to the OP, we seem to have highjacked your thread!
Give use some pics mate! Then we can get back on track!

We are just entertaining each other while we wait for the OP's return.

[mikeref]

We are just entertaining each other while we wait for the OP's return.

Damn, you boys still gassbagging? This latest debate has taken most of my saturday with intense interest, and i went out with the MRS. Mikeref for some ( boring) shopping during it all. I am waiting with anticipation for our host to return with pics or info on his dilemma.... mike.

[nevgee]

I'm still waiting to hear how SHR is relevant to service.

... and especially a kitchen fresh air system !

The potato croquettes might go soggy or perhaps the ink on the waiter's tickets might run

Bearing in mind we're not talking plant specifics here as that would be too generalised.