Hi,
Does anybody asset SHF in cold rooms?
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Hi,
Does anybody asset SHF in cold rooms?
No, because the air conditions are not as important as the product conditions!
RH is more important for product quality.
Sorry I didn't understood you.Quote:
Originally Posted by dvd365
I think you do know is significance SHR (SENSIBLE HEAT RATIO). The ratio between the sensible and the total heat. Total heat is compound by: sensible heat and latent heat.
This is influenced by the relative humidity and the working conditions of the air coolers.
What do you need to know?
Sandro, he wants to know if you use SHR in your coldrooms for anything.
RH and SHR are independent variables, this means that you can have the same RH with different SHR and conversely the same SHR at different RH, so to get an "influence" of these variables you need something to relate them like an evaporator but also water vapour contribution by infiltration and from the product.
No of ones is wrong. you are also right but as I said SHR is influenced by the relative humidity and the working conditions of the air coolers. For example If the same cold store now receives only wrapped products the RH will decreases so SHR increases. If the evaporators are changed for ones with smaller DTM then the RH increases.Quote:
Originally Posted by aramis
You're absolutely right Sandro, sorry!
This happens when I read fast and not completely, but you did not answer dvd365 question: do you use SHR for any calculation (or other purpose) in your coldroms.
I could'nt think of any use, only for cooling towers but not for coldrooms.
Quote:
Originally Posted by aramis
Well, on a chilling tunnel for bovine or pork carcases they enter a t very high temperatures so the humidity will be very high and release a lot of water vapour. The design of the air coolers shall have this in consideration, I mean low SHR (maybe about 0,6?!?).
Sorry if i did not give much details at the begining.
Well, in many refrigeration handbooks is said you have to take in account SHF or SHR, sensible heat factor/ratio, in order to make a good evaporator selection or at least you take care about evaporator ratings in manufacturer brochures.
I am trying to understand what I have to do with a SHR after I got it. For instance, I have a store cold room for frozen vegetables with a SHR=0,86, due to infiltrations.
I have cold room conditions, -20ºC 90%RH, 80Tn/24h. So I ask myself if I have to operate as in air condition selection in order to asses the evaporator air flow, aparatus dew point and so on. How do you operate in this cases?
I think the SHR I have is the room SHR, and I need to obtain the effective SHR, but i do not have very clear what is the path I have to follow.
In P.C.Koelet handbook is indicated aparatus dew point is evaporation temperature. If I have room conditions and aparatus dew point, let´s say -25ºC, we have the process line, and I can assume a by-pass factor too. But if i assume all this, what is SHR for?.
Iam trying to follow an air condition handbook, but I do not understand how to do it yet.
Maybe an evaporator designer will worry about SHR because different designs may condense more water than others, but from a coldroom designer point of view you only need to worry on the RH level of the coldroom.Quote:
Well, in many refrigeration handbooks is said you have to take in account SHF or SHR, sensible heat factor/ratio, in order to make a good evaporator selection or at least you take care about evaporator ratings in manufacturer brochures.
Many posters in RE will tell you that there is a relation between RH and evaporator differential but these are only “Thumb Rules”.
Relative humidity is a result of dry bulb temperature and the water content in the coldroom. Water content is the result of a mass equilibrium between the water vapour coming in through infiltration and diffusion (through walls, ceiling and especially floor) plus water loss from the product and people (mainly respiration) and free water coming in with the product and water taken out by the evaporator .
Some industry statistics have been around for many years relating RH with Dt (temperature differential) and to be more precise Dt1 (inlet evaporator temperature minus saturation temperature measured at evaporator exit)
These “Thumb Rules” are good for many unknown situations that are very difficult to predict without reliable data, but it is not a guarantee they will apply to your particular case and you may need humidifiers.
Many have been posted, here is one very old, from Prestcold’s “Refrigeration Engineer Data” in imperial units for forced convection (you posted °C so please note: °F below):
RH (%) TD, °F These are based on Log Mean TD=(Dt1-Dt2)/log(base e)(Dt1/Dt2)
95-91 08-10
90-86 10-12
85-81 12-14
80-76 14-16
75-70 16-18
How did you calculate this? You need the information of the mass balance mentioned above!Quote:
I am trying to understand what I have to do with a SHR after I got it. For instance, I have a store cold room for frozen vegetables with a SHR=0,86, due to infiltrations.
You can practice your trigonometry SHR is the cosine while the ratio (enthalpy difference)/(Humidity Ratio or DW) is the tangent of the same right angle triangle.
The air transformation in the evaporator may be plotted in a psychrometric chart if your evaporator SHR is 0.86 then it is almost a horizontal line and either the temperature difference TD1 is very little or you would produce oversaturated conditions air on the evaporator.
HINT: Nobody here selecting an evaporator will plot anything in a psychrometric chart! It may be useful in teaching the process.
In a previous post I mentioned that in coldrooms the product conditions are more important than the air conditions, this concept is hard to get for someone used to air conditioning calculations where your goal is comfort which many be confused with air property’s conditions.Quote:
I have cold room conditions, -20ºC 90%RH, 80Tn/24h. So I ask myself if I have to operate as in air condition selection in order to asses the evaporator air flow, aparatus dew point and so on. How do you operate in this cases?
Air conditions are everything BUT stable in a coldroom. The air is the transport media of the energy you want to transfer from the product so when it reaches the product it increases temperature and water content to release it in the evaporator.
The evaporator airflow must be calculated either with more “thumb rules” or you must estimate the velocity profile of the room to keep minimum air velocity around the product for it to cool in a given time.
In a storage room you need to stabilize the temperature of a product which may come lower than -20 in the surface but higher in its core. For this you need sufficient air velocity and more if the product is wrapped in insulating materials (which from the freezing time and energy use point of view a stupidity, it only makes sense if you see it from the global process point of view).
Air velocities are another set of “thumb rules” though backed up with years of experimentation published in books like Plank´s “Die Anwendung der Kälte in der Lebensmittelindustrie” or ASHRAE’s handbooks of course.
This is the assumption that is holding you back because it is wrong!Quote:
I think the SHR I have is the room SHR, and I need to obtain the effective SHR, but i do not have very clear what is the path I have to follow.
In a perfect world dew point is evaporation temperature, but a -20°C coldroom is closer to hell than a perfect world! The problem is that given the evaporator temperature is at dew point it will freeze solid on the evaporator, making a perfect heat insulating sheet on the coil. So you need to go far below the theoretical minimum temperature to a real world temperature that can transfer the amount of heat you want even through this shroud.Quote:
In P.C.Koelet handbook is indicated aparatus dew point is evaporation temperature. If I have room conditions and aparatus dew point, let´s say -25ºC, we have the process line, and I can assume a by-pass factor too. But if i assume all this, what is SHR for?.
NOW I GET IT!!!! Please buy yourself a good refrigeration book, maybe ASHRAE’s Refrigeration Handbook, but I hope others will post better books to learn!Quote:
Iam trying to follow an air condition handbook, but I do not understand how to do it yet.
I calculated all load concepts (wall transmision, product load, infiltration loads, ventilator, machinery, people loads), so I had total heat.Quote:
Originally Posted by aramis
I assumed only infiltration loads had lantent load. So I divided infiltration load in sensible and latent. I calculated SHR for infiltration load. Total infiltration load: 21,377KW in acordance with almost 6 hour doors opening (80Tn/24h, 700Kg/pallet, 5 minutes to pick each pallet) and Goseney&Olama expresion (ASHRAE REFRIGERATION 2006, CHAPTER 13).
Sensible heat is Qs=1,006*(25-(-20)) Kj/Kg. Total heat is: H(25º, 49%)-H(-20º, 90%)=49,81-(-18.81) Kj/Kg. If you divide Qs/(H1-H2)=0,66, so latent infiltration load is 21.377*(1-0,66)=7,268Kw and sensible infiltration load 21.377*0,66=14,108Kw. So you get infiltration load in two parts.
Then I summed all sensible loads:
Transmision:12.218Kw
Product:11.975Kw
Infiltration:14.108Kw
Equipment load:5.111kW
People: 0.174Kw
Service load:2.419Kw
TOTAL SENSIBLE=46.584kW
TOTAL LATENT=7.268KW
TOTAL=53.852KW
SHR=46.584/53.852=0.86
I
Depending on the product and it’s packing all products give away a fraction of its weight, mostly in the form of latent load:Quote:
I assumed only infiltration loads had lantent load.
http://onlinelibrary.wiley.com/doi/1...50303/abstract or http://ntlinsulation.com/images/watervapor.pdf
But also: Respiration from workers, Combustion engines (like forklifts) that go in and out, and diffusion through walls and floor even if your vapor barrier is perfectly executed it is not 100% efficient, and the older it gets the higher the value.
Quote:
So I divided infiltration load in sensible and latent. I calculated SHR for infiltration load. Total infiltration load: 21,377KW in acordance with almost 6 hour doors opening (80Tn/24h, 700Kg/pallet, 5 minutes to pick each pallet) and Goseney&Olama expresion (ASHRAE REFRIGERATION 2006, CHAPTER 13).
Sensible heat is Qs=1,006*(25-(-20)) Kj/Kg. Total heat is: H(25º, 49%)-H(-20º, 90%)=49,81-(-18.81) Kj/Kg. If you divide Qs/(H1-H2)=0,66, so latent infiltration load is 21.377*(1-0,66)=7,268Kw and sensible infiltration load 21.377*0,66=14,108Kw. So you get infiltration load in two parts.
Then I summed all sensible loads:
Transmision:12.218Kw
Product:11.975Kw
Infiltration:14.108Kw
Equipment load:5.111kW
People: 0.174Kw
Service load:2.419Kw
TOTAL SENSIBLE=46.584kW
TOTAL LATENT=7.268KW
TOTAL=53.852KW
SHR=46.584/53.852=0.86
People load includes latent load.
If you included forklifts under equipment it also should include latent load.
All this seems OK though the latent load may be underestimated, you should also follow ASHRAE’s recommended calculation of a safety factor page R13.7 this will compensate for the missing calculations.
Now you have to estimate the losses in your refrigeration system to estimate:
- Temperature differential the evaporator will work with. Take into account different manufacturer’s use different type of differentials. Probably using the “thumb rules”.
- saturated pressure this evaporator will work with.
- pressure differential applied to each expansion valve. At this point you will have to make assumptions on your condenser or make an assumption on the liquid line pressure at your evaporators.
For example: Assuming your manufacturer works with Dt1 and you want over 80% RH in your coldroom, then you need 5°C Dt1 for your evaporators and possibly the pressure drop equivalent to 4°C of saturated temperature drop in suction lines. Be careful 5°C is the least superheat mechanical expansion valves can be adjusted for.
Now search the manufacturer’s literature or better selection program to select the evaporator.
Check it is consistent with your equipment load calculations.
Get a manufacturer program to select equipment, If you can’t: Search compressor manufacturer’s literature taking special care in reading how they estimate their capacity, try to use tables with no subcooling. If the manufacturer’s suction values don’t fit try to estimate the % of change in specific volume from your conditions to their conditions and adjust capacity.
With compressor’s capacity and power input, estimate your condenser and with manufacturer’s program estimate if you have enough subcooling to reach your TEV with liquid refrigerant.
Now you have equipment selected you should check if all your assumptions are satisfied.
Is this right? … Wrong!!! This method is called selection for an operating condition, it is less wrong for coldrooms than for tunnels or precooling because you estimate either maximum or average loads but you really have to check at all possible working conditions.
To do this better you have to calculate with the same method your MAXIMUM and MINIMUM loads and see if you need capacity control.
But the correct procedure is in the literature and must take into account ambient fluctuations also. Or you need specialized software (not freely available) and training.
Hi,
I have read bit more about SHR and coil selection.(Koelet, Stoecker, Ricard Jimenez)
In accordance with several authors, you have to check room´s SHR and the manufacturer coil ratings to check coil´s performance mathcs room´s demand, so SHR affects convective heat transfer coefficients.
I have gone one step beyond, and I have estimated an ADP in an analiticaly way, not graphically, but at the moment I am checking for possible erros. I have obtained with a SHR=0,86 an ADP=-21,52ºC with an "excell" goal seek function to solve the equations.
Does somebody Know what is the meaning of the attachted image?. I usually find other definition for latent and sensible heat!!
Attachment 8881
No. Nobody in the refrigeration business will worry about SHR for several reasons. Reread these authors and check if they are talking of the process in general or for air conditioning.Quote:
In accordance with several authors, you have to check room´s SHR and the manufacturer coil ratings to check coil´s performance mathcs room´s demand, so SHR affects convective heat transfer coefficients.
A cooler working below dew point will always cool and remove water, and this is why any text covering the subject will show you a constant SHR line in the chart but for teaching purposes only.
Instantaneously you may consider any cooling process as constant SHR, but not in time because the conditions in coldrooms are variable and in the evaporator dynamic.
In air conditioning you try to keep the room conditions stable within comfort zone so the input conditions to the evaporator are relatively stable but the output conditions depend a lot on the latent load which is a random variable.
Below +5°C the absolute water content has dropped to less than 10% of minimum air conditioning applications and you need EXV to evaporate above °C and still keep these conditions. With mechanic valves this is not possible for room of +5°C and below you must evaporate under 0°C.
If you see the evaporator capacity curves from Küba from -10°C to -3°C (depending on Dt) they show a continuous curve but this is not so. What you get when you measure is a cloud of points around the curves they show. This is because latent load is completely unstable with the tubes covered with water one second and withoutr the next.
Below -10°C at any (normal) differential the heat transfer conditions are always getting worse in time (no constant SHR) because of the insulating effect of the ice on the coil and the higher air pressure drop. This also happens at higher temperatures, but depending on the differential (i.e. the air still being over 0°C within the coil).
The combined effects of varying amounts of water in the coil and ice affect the latent heat load of the coil (no constant SHR).
These conditions taken to the extreme cause water carryover and ice fog in the coldroom.
Sorry the dew point depends on two variables SHR is one but I’m missing the other.Quote:
I have gone one step beyond, and I have estimated an ADP in an analiticaly way, not graphically, but at the moment I am checking for possible erros. I have obtained with a SHR=0,86 an ADP=-21,52ºC with an "excell" goal seek function to solve the equations.
The drawing is sloppy because the middle inclined line should be the diagonal of the parallelogram formed by the dotted lines reflected over the process line. The other “constant enthalpy” lines should be parallel to that diagonal.Quote:
Does somebody Know what is the meaning of the attachted image?. I usually find other definition for latent and sensible heat!!
This dotted line is the right angle triangle I mentioned before though I always use the reflection of the one drawn.
Anyway the triangles defined by a diagonal and the sides of a parallelogram are congruent!
EDITED days after original post: Corrected the QUOTEs and I forgot to mention another sloppy detail in the drawing: the vertex of the right angle triangle does not (generally) lay on the saturation curve!
I am not agree with this point. Please, take an eye on this:Quote:
Originally Posted by aramis
http://www.colmaccoil.com/Whatsnew/P...ALBULLETIN.PDF
In handbooks I have read (Betwen others: "INDUSTRIAL REFRIGERATION:PRINCIPLES, DESING AND APPLICATIONS" (P.C.KOELET), "INDUSTRIAL REFRIGERATION" (W.STOECKER), "FRIO INDUSTRIAL" (RICARD GIMENEZ)) it is indicated it is important to assess SHR, in order to understand the eviroment in which the coil has to work and to make a good selection.
I agree with you, assessments are for a single working point, because as you said evaporator and cold room conditions are dynamic, but in the sizing process you need to select the equipment it suits better.Quote:
Originally Posted by aramis
And when I asked for if people takes care of SHR were because I was finding literature about it.
Do not you think you keep product conditions through air conditions?Quote:
Originally Posted by aramis
Are not all loads random?. When a load is calculate it is assumed for the worst case, so it is needed to know SHR in the worst case too, to check manufacturer coil ratings are suitable with the cold room demand, betwen others parameters.
We have SHR=0,86, and the cold room conditions, so we have a point and a slope. So we have a line, we have a function, humidity content depends on temperature. Taking apart temperature.Quote:
Originally Posted by aramis
We have temperature dew point function too (ASHRAE FUNDAMENTALS 2009 CH.1), so if there is a common point you find it solving the temperature equality for humidity content. Then sustitute in line ecuation and you get a temperature, that is supposed to be ADP, but I do not have this very clear yet.
Sorry I missed this for so long!Quote:
Originally Posted by dvd365
As I said when you design a coil you do use SHR to focus your design for a target air in condition. This means that you will optimize your design based on constant SHR, only because it is difficult to optimize anything if everything around you is moving (like in an earthquake!).
This affects FPI, distribution, air pressure drop, flow pattern, air face speed, etc.
SHR is also used in teaching about air-refrigerant HE for it gives insight in the process.
But you do not worry about SHR when you want to select a coil for a coldroom as you don’t have a target SHR in your objectives.
I can’t recall having had a coldroom with constant SHR.
I don’t write any SHR assumptions in my specs for any REFRIGERATION projects. Remember we were always talking below +5°C.
Maybe I am wrong! I am willing to change if anyone can prove to me that I need to start considering SHR in my selections.
Oh! and I don't recall reading manufacturer's literature that requiere SHR for coil selection! ... Though I must admit I know just a few manufacturers.
Yes you do! And this is why I mentioned RH is more important than SHR for it is RH that rules evaporation processes.Quote:
Originally Posted by dvd365
Yes! But not as random as the “air in” conditions of the coil. What is most random in a coldroom is the airflow, more than in air conditioning.Quote:
Are not all loads random?.
Well not just SHR, and not just maximum, you need minimum conditions also or you may run in trouble, though solving oversized systems is easier and less expensive than solving undersized installations.Quote:
When a load is calculate it is assumed for the worst case, so it is needed to know SHR in the worst case too, to check manufacturer coil ratings are suitable with the cold room demand, betwen others parameters.
The problem in the field is that you have to work with unreliable information and customers who are not willing to pay for models and simulations for problem solving, so what we do is apply proven “thumb rules” that you trust and your experience (and you succeed most of the time).
I have found another document it reinforces my thinkings about SHR. It belongs to "THE INSTITUTE OF REFRIGERATION" ("The Effect of Water Vapour on Food Refrigeration Systems") by D. J. Cleland (www.ior.org.ok).
I am still digesting the reading.