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detane
26-04-2009, 02:24 AM
Hi guys,this would would be my first post to RE and i would need the RE top guns to help me decide what to do for the case that i will submit to you:
-here is a cold store for fruits with a capacity for 700 tons of apples divided in 5 equal rooms(600 m3 each).
-the glycol coils for each room is a 55 kw ahu from hk refrigeration
-the central chiller is a friga bohn of 376 kw cooling capacity ,R 404 a ,with 3 bitzer twin screw compressor ((120 kw each),flooded evaporator (-8°c departing, -4°c return)
-air condenser friga bohn (8 fans 600 kw)
-glycol buffer tank of 4000 l
-120 mm dagard insulation wall and ceiling, no floor insulation.
-primary and secondary pumps are grunfoss 55 m3/h each
-The distribution pipe diameter is 110 mm in a 3 way valve bypass mode with on/off mode from the secondary pump.
the current objectif i have is two fold .
first i would like to add a high humidity tunnel precooler for up to 6 tons of grapes for long term storage(so2 fumigated) as well as 5 holding room (486 m3 each at -1°C)
what do you think would be the best , a high humidity airsprayer system (humidicoil from thermal engineering systems,or central aspiration system from frimetal sa)+ ultrasonic humidification and what would be the rating for the coil?
second objectif would be to reduce power consumption of this obviously oversized system to make it power efficient after the extension.?
.Any inputs would be greatly appreciated.

chemi-cool
26-04-2009, 10:26 AM
Before I get into suggestions, I would like you to read a few articles:

http://www.cababstractsplus.org/abstracts/Abstract.aspx?AcNo=20043014186

http://www.munters.us/upload/Case%20studies/CaseStudy_TableGrape.ENGLISH.pdf

http://www.trj-inc.com/forcedAirCool.html

I have used precooling mainly for melons but the system is the same.
I have never seen high air flow precoolers without direct expansion but this can be sorted if you keep the solution temp above 0°C while precooling.

Sandro Baptista
26-04-2009, 12:30 PM
first i would like to add a high humidity tunnel precooler for up to 6 tons of grapes for long term storage(so2 fumigated) as well as 5 holding room (486 m3 each at -1°C)
what do you think would be the best , a high humidity airsprayer system (humidicoil from thermal engineering systems,or central aspiration system from frimetal sa)+ ultrasonic humidification and what would be the rating for the coil?
second objectif would be to reduce power consumption of this obviously oversized system to make it power efficient after the extension.?
.Any inputs would be greatly appreciated.

Hi detane,

First let me tell you I'm not TOP GUN in the grapes area :D.

1) About the humidification I not familiar with frimetal sa system so I can't tell what the advantages/disadvantages of it compared with the ultrasonic system or others.
I can said by part that, except for certain cases, I like those that I call "of entalpy constant" (name invented by me :D) for systems that the liquid water passes to the vapor state in the air atmospher of the cold store by the evaporating of the water . They are those that we don't need to supply heat to produce water vapor, like for example good water injectors (sprinkle type), without let water droplets to the floor. Entalpy constant is good because the total heat load it remains the same.
At contray for the "no entalpy constant" types you have to addict more extra cooling capacity.

One of "no entalpy constant" that I know are those that use the principle of electric arcs (electrical induced with 2 electrods). I'm confused/forgetted (must re-read documentation) if the ultrasonic is "of entalpy constant". I think is it, but uses a litte electrical supply for guarantee by ultrasound that the water is well atomatised (very very low diameter droplets). I'm sure you better know this equipments that me.

:off topic: The pic you have send with an aircooler it looks it have a humidifier at right side of the aircooler, right?. It is the humifier of the ultrasonic type, right?

2) Aboutreducing power consumption...If you have always the compressors at no partial load but rather at 100% then I think you can't do it any miracle.

You have a good inertia thermal store (4000 liters glycol vessel) and also the pipes have much internal volume which increase internal thermal store (if the pipes are very length it's good » more internal volume). So maybe you can operate with the compressors with no partial load but only 0-100%, for each compressor. You control the operating by the glycol temperature in the glycol vessel (maybe you already have it controled by temperature).

How do you have the management of the compressors control?

Regards

icecube51
26-04-2009, 07:21 PM
maybe you could considder adiabatic cooling whit HO²....???

ice

detane
27-04-2009, 01:04 AM
Thank you guys for sharing your toughts , the idea being to choose between two precooler types for an already existing chiller and if i understood, it is useless to get an airspray system since it is only used with DX .
Can a 30kw dual flow coil do the trick(photo)?
As for compressor control it is done by on/off mode ,thermo regulated by control on the flooded evaporator leaving glycol (-8°c off / -6°c on ,DT 2 °C).No need to tell you that at part load the energy consumption is ridiculously high( we are at part load 90 % of the time, typical for a fruit cold store)
Would that be interesting and possible to remove one of the compressor and replace it with a smaller vfd driven piston compressor dedicated to part load , on a screw compressor pack.
thank you in advance

Sandro Baptista
27-04-2009, 09:23 AM
No need to tell you that at part load the energy consumption is ridiculously high( we are at part load 90 % of the time, typical for a fruit cold store)


Yes I know and that's why I was talking about working with each compressor at 0% or 100% (ON/OFF).
You mentioned you were at parte load at 90% of time. But that meant what? 2 compressors at 100% and the third compressor most of the time at 100% of capacity?

detane
27-04-2009, 12:32 PM
nope , its only one compressor out of 3 that works at part load (bitzer twin screw hsn 7461-80) Q= 120 kw at -10 °c evaporating.Typical result of this oversized compressor is short cycling (6 starts per hour, working 3 to 7 minutes each start

Sandro Baptista
27-04-2009, 02:17 PM
As I understood you have 2 compressors at 100% and the third with short cycle, right?


Typical result of this oversized compressor is short cycling (6 starts per hour, working 3 to 7 minutes each start

Let see: 6 starts per hour, considering working 4 minutes during start-stop and assuming that you have an intentional timer delay of 5 min in stop-start it results about 1 hour...just like you said. Hmm...it's a little strange a great oscillation of about 120 kW considering you a great inertia termal reserve...maybe there is great variations on the cooling demand...

I suggest if you want to decrease the number of start-sart to increase the differential of the glycol temperature setpoint to about 5 K. For instead stop the compressor when the glycol in the tank is at -11ºC and start again when the Pt-100 read -6ºC.

What is your current setpoint temperature and differential setpoint?

detane
27-04-2009, 07:46 PM
Thanx sandro to bare with me as this might be a bit more complicated than you may have thougt.we were sold a hugely oversized chiller for an actual load of only 700 tons apples(1200 tons after extension) .it was an intentional hold up from the local installing company, because we were stupid enough not to get a consultant beforehand for such a serious investment, but as the saying goes :fool me once shame on you, fool me twice shame on me.
Anyway to explain clearly:
from the start to the end of the apple storage season we only work with compressor (1) at 100 %and compressor (2)and (3) at 0 % (you have to admit that it is a very very expensive redundancy at 25000 euros apiece).
by the way the flooded evaporator has an operating limit of -10°c( which is weird). Increasing to this limit might have some risk(bundle freezing) and too much fluctuation in the room
especially long term storage,we have tried it to relieve the compressor ,the result on the apples was not good,as stability is the key to their storage.
My point being ,knowingall these limitations that i have,would not that be wise to downsize compressor (2) or (3) (both of them are not used as remainder) and replace with a piston compressor with part load capability such as vfd bitzer octagon or vfd bock for example ,and use this compressor to adapt to temperature fluctuation (never more than 2°c) of the load.I hope i make some sense and sorry for not using the layman terms.does this operation achieve any energy efficiency or it is just another gimmick

Sandro Baptista
28-04-2009, 01:26 PM
Okay, I accept that glycol to cold can result in more dehydration of the cold store (however you have humidifiers!) and very frost formation. What's your defrost system for the aircoolers, anyway?
The suggest that I made was in an attempting of mantain the compressor working continouos during more time and more time continuos stoped.

So what about stop the compressor at -8ºC and start again when the Pt-100 read -3ºC. In this case the problem that can result is the eventual lack of capacity in some cold stores...but even so it may happens that they work more hours per day without have bigger spreads of temperature. If the cold store is already load and with the product already cold then it should't be a problem.

I continue to think that you can do improvements because by the data you gave me:

- only one compressor working and that gives 120 kW and works during 1 hour around 30 minutes. So you have a cooling need of about 60 kW.h

- You have a good inertia thermal storage. Let consider only the tank 4.000 liters and forget the internal volume of the water main pipes of the plant.

- If you have 4.000 liters of storage capacity then with glycol water (cp = ?, depends of the type of glycol and concentration). Going to assume 3,8 kJ/kgK. Aceepting a variation of the glycol inside the tank of 5 K (-8ºC » -3ºC for instead) then the tank can delivery a refrigeration energy of 76.000 kJ.

- 76.000 kJ is 21 kWh. So it means that the compresor in conditions above it could mantain running continouos during 20 minutes. And is less cooling demand it would be more than 20 minutes. Don't forget that I didn't count with the internal volume of the pipes.

- Another solution it would be to you apply one inverter (VSD) to the first compressor. I know that even for semihermetic compressors it's possible (if your compressor motor isn't in the termal limit it wouldn't have problem) nevertheless consult BITZER. I think the minimum speed drive would be about 1.000 rpm and so you would have a modulating range at 100% part load capacity from 1.000 to 3.000 rpm/50 Hz or 3.600 rpm/60 Hz and it "soft start" included in the inverter.

A smaller piston compressor like you said I think is also a good idea.


Regards.

bananajim
26-08-2010, 03:16 PM
Interesting reading. I normally use a "poor mans rack", dual 20's parallel-piped, lead with unloaders. Better yet, use dual 40's and connect 2 tunnels/rooms to them. Stagger loading so the first tunnel is finishing, when you load the 2nd, this will give you nearly 70HP for the hot load coming in.

Ultra high humidity precoolers, has to be Humifresh, they perfected the Filacell wet cooler for forced air precooling in the mid-1960s. You cannot do better than 250gpm flow in your cooling coil, and saturated air at discharge.

NE-Postharvest.com/precoolers.htm

NoNickName
26-08-2010, 03:43 PM
I would also suggest to install a VFD on one of the compressors (the first in the FIFO) and stop the lead/lag rotation.

As for the humidification, I would suggest to use electrode steam humidifier, because it would induce sterile steam at the price of some latent heat added. Any other means of humidification may cause airborne contaminants to be sprayed over the fruits (bacteria, viruses, silicates and chemicals contained in the tap water).
Electrode steam humidifiers, on the other hand, require more maintenance (replacement of boiler bottles), but would ensure higher sterility and quality to the product.