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.
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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.
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, 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.
Mmmmm... did someone say "Coffee"?
:D coffee? ...I'm with you Gary the room is getting stuffy we need a break ;)
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 :confused:
Coffee? double shot please!
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.
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
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
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 :)
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.
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.
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)
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)
Correct :)Quote:
From this we can design the coil or if already existing the required SST to achieve the desired results.
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.!
Appologies to the OP, we seem to have highjacked your thread!
Give use some pics mate! Then we can get back on track!
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!
I'm still waiting to hear how SHR is relevant to service.
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.
Very relevant, and the fact that overcharging or undercharging effects subcooling, it also effects the performance of the evaporator, which in turn effects the evaporating temperature, and in turn increases or reduces the latent heat removal.
But as DTLarca says:
Very few refrigeration engineers know their stuff. It's like working in a tyre service center and not knowing what camber or caster is just because he only happens to repair punctures.
DTLarca made a very good point there.
But when working conditions change, SHR changes.
Failing to recognise that, makes a epic failure of your job.
SHR is not a constant figure along the operational envelope.
In need of some examples of refrigeration engineers or installers failures?
NNN, On site SHR, is a result of the plant and conditions, Yes the specifics of the plant, SCT, SST, sub cooling superheat all effect the result, but a pre determined SHR% is not for the service guys, it is the required end result that he is aiming for.
For this reason SHR is not a service tool or term, even if indirectly it is what they are actually measuring.
And when a butcher complains that his meat is too "soggy" - he can't slice through it - when he tries to cut it the meat just rolls with his knife. Simple solution - isolate one of the fans, blank that fans discharge - SHR reduces and meat is now cuttable with negligable loss in weight :)
The length of a piece of string - it depends on how how many angels you can stand on a pin head.
Now you are being wonderfully specific about those "previously mysterious plant specifics you were waffling on about" :)
Remember - we are not so much talking about service engineers here as we are about commissioning engineer. Though I for one expect my service engineers to also be commissioning engineers.
Regarding SHR control...
Reheat - so often made to work but horribly inefficiently on account of poor understanding of SHR and energy consumptions.
Face and Bypass - not very effective with chilled water systems.
variable air flow - not very effective with chilled water systems.
Variable refrigeration system capacity - ideal when both SH capacities are insufficient and SHR's are too high - not of any help if only the SHR is too high.
So it seems you do know a little more about SHR than the average person in the industry - perhaps too much to realise how important it is knowing it - you have forgotten what it means to not understand it but to be aware of it :)
Sample Level 3 Exam Question...
One of the most important factors governing humidity in a refrigerated space is?...
- The evaporator to air temperature difference?
- Compressor superheat?
- Condenser to air temperature difference?
- Suction superheat?
Another example Level 3 Exam Question - image attached.
Correct I know nothing about refrigeration, just I just make it up i as I go along! Luckly for me people had paid I fortune for my lack knowledge and for a number patented systems that are now used around the world, i just a con man! Especially around the world on industrial drying, somehow by luck again I reduced input energy cost down by in the best case by 81%, i just do not know how i achieved these results without using the Term SHC, i do remember, i think we a thing called enthalpy of an, in and out stream of air, I also think that vapour pressure was brought into the calculation, no i am wrong i wet my finger flung it around, got my design.
You asked a question about a plant i asked what was in the plant, giving some examples. How can I resolve a problem if i am not aware of the practical limitations of the plant, as well as know nothing I am neither a mind reader.
Lets see what you really know. Why does you ink run!, because it is wet does not count, why is it wet!
I don't doubt you manage with your work and I don't doubt that your patents are valid and sound.
Many people have enormous success at varying things but strangely all along never know as much as we expected them to know about their field of expertise.
You must remember that I did ask you two pretty specific questions and in both cases your replies amounted to "Well it depends" and "How long is a piece of string" when in fact there is only one fundamental way to reduce the SHR on a cooling coil. There are many ways to adjust the SHR of an overall system by adding additional apparatus to it but there is only one fundamental way to reduce the SHR on a cooling coil and if you have a mastery of SHR control you would immediately start with this one fundamental in your explanations. You did do this but half heartedly when you suggested reducing SST.
And there are ways to determine a rooms SHR by simply introducing a transitional instability and monitoring the reaction - you do not have to do all the calculations if the system already exists - but you do if the system is in design.
As Newton said "I do not know why the planets go the way they do - I can only use math to describe the way they go".
Why is there capillary action? Why are there weak intermolecular forces i.e. why is there capillary action with water and not with murcury? I don't know - I don't know why - I can only explain the way it happens.