dehumidifier design,pls help to review

[Lc_shi]

Hi sirs,
I've just finished the primary system design for a dehumidifier.Brief as below:
#copland normal compressor
# evaporative condenser(manufactued by ourselves)
#dehumid coil --Dx type（8 rows --4 rows are NO,other 2 by EMV control to meet the load variation--activated by dehumid coil outlet air temp)
#reheating coil parellel to evap condenser --about 10~15% of total condensing heat --two circuits to control the rehating capacity--by rehating outlet air temp
#compressor capacity is controlled by Hot gas bypass valve

I'm not sure if i make it clear.
pls help to have a review. Your input is welcome.-

best regards
LC

[frank]

Hi Ic Shi

What will happen to the check valves on the condenser outlet and reheat coil outlet if you have unbalanced liquid line pressures?

[Lc_shi]

very professional comments! many thanks for your input.I'll have a deep study on it.

best regards
LC-

[star882]

Here's a GIF image for those who fear macro viruses:
http://tinypic.com/kdle0l.gif
Why is there an evaporative condenser?
And exactly what are the design requirements?

[Lc_shi]

Hi Frank & Marc,
would u pls help to confirm if there is potential conflict at part load condition: evap area reduce by cutoff some circuits and hot gas bypass to the evap(will lift evap temp and pressure) happen at same time; I mean if it feasible to use fixed area evap? thanks!


beat regards
LC-

[frank]

With hot gas bypass capacity control I always like to see an accumulator and CPR valve in place.

If you have part load and close 2 of the 3 circuits in the evap you will have an imbalanced system. The large amount of hot gas returning to the compressor will result in hot suction pressures, increased oil temperatures and reduced compressor life.

What size of system are we talking about here?

[Lc_shi]

You're very correct.I'll consider to use VFD compressor to solve it.
The system is for a prototype test. We'll build a 11kw cooling capacity ( tc=40C,te=2C,air volume 650m3/h)at first.

thanks for your professional comments

regards
LC

[Peter_1]

Haven't read through all the messages of this thread, there are the last weeks lot of new messages each day.

We made in the past some wood dryers (6 or 8) for oak wood drying.
The units were equipped with +/-7.5 Hp Bitzer and 10 HP Bitzer and they stood in the room itselves where it was +/- 55°C to 60°C.

Condensors mounted after the evaporators - both OEM made - and also electrcial heating for preheating the room.
Ran on R114a and we had to epoxycoat the U-truns on the evaporator due to the released chemicals in the air.

Casings were in aluminium.

It took +/- 3 weeks to dry the stacked wood (roomsize +/- 8 x 6 x 4 m)

[Lc_shi]

Hi Marc,
You mean to put the condensers in series,the reheating coil is after the evap cond and subcool the liquid. By adjusting fans to add reheating capacity,it's a good idea. There are 2 things need to confirm:
1). the reheating coil need to size bigger due to no-phase change inside?
2). if in parellel,what the worst case?

Hi peter,
I'm not familiar with r114a,why use it? How to reclaim condending heat in your unit? pls give some more information.

I'm grateful to you all sharing your time for helping me.

regards
LC-

[US Iceman]

LC,

R-114 was a low pressure refrigerant that was sometimes used for high temperature heat pumps. With this refrigerant you could obtain high temperatures for heating from the condenser.

Unfortunately, R-114 was removed from availability due to the ozone mess.

[Peter_1]

In addition to USIceman's answer...I think there was then no other gas available which gave acceptable low high pressures when the unit was running at ambient of 55°C, sometimes 60°C.

Your second question: if we install the condensor direct behind teh evepaorator, all the heat is then reclaimed, inlcuded the power that the compressor took from teh net. But this you knew already.

I notice many times in your posts that you have a very interesting job, you're always busy with not standard things. It seems that they give you the time to experiment.
Are these 'machines' you're making intended for mass production?

[Lc_shi]

It's a hard job for test new product one by one. I'm in the R&D division which decide my responsibility. For me,all the work is same(you live by it).the 2nd question is where to put the reclaim coil \parellel with condenser or in series. parellel condensers deal with the refrigrant flow distribution issue.Need go further to make it clear. This machine is intended to go mass volume production,however it depends on the market situation.

[Lc_shi]

Marc，
I don't want to put the condensers in series because it need to adjust the water flow of the condenser(PHE i use).it's not fit for a small capacity system. How do you think the main concerns of the parellel condenser? pls advise. thx

regards
LC

[Rob S]

R236fa is the replacement for R114
http://refrigerants.dupont.com/Suva/...suva236fa.html

[frank]

Hi Ic_shi

I found this helpful bulletin about hot gas bypass design, thought it might be of interest to you http://www.trane.com/commercial/libr...07_EN_0503.pdf

[Lc_shi]

Hi Frank
Thanks. I keep it.
The sample machine is on testing. The system is feasible but the design need to be improved.

best regards
LC

[coolman]

Hello Ic shi

I did work a long time with a company who did produce dehumidification equipment.
So I do have a little experience on this kind of equipment.

Can You give some more information on the use of your design I.E existing relative humidity required humidity temperature range where the unit will be operating, required amount of extraction etc, and if you can and will the use of the machine.

Regards Victor

[Lc_shi]

Hi Coolman
Thanks for your goodwill
The design requirement is:
#inlet air 29C/75%RH
#supply air 18C/53%RH
#Air volume 650m3/hr
#moisture detraction 9.5kg/hr

To use a parellel condenser to get heat for air reheating after dehumidifying.

best regards
LC

[Lc_shi]

The prototype is under testing. Initial result as follow:
#Air volume measured by anemoscope : 680CHM
#dehumidify coil out let air 8C/100%RH(inlet 29C/75%RH)
# moisture remove measured by measuring cup 6.9~7.4
#ZR47 copland compressor power 2.6KW
#discharge pressure : 14.2bar gauge
#average suction pressure:4.0bar gauge
# while the parellel recovery condenser is on ,discharge pressure drop to 12.5bar gauge, suction no obvious change
# dehumidifying coil :face velocity 2.3m/s,row depth 8

I wonder why the moisture is not meet the designed value but the air enthalpy and humidity difference by calculation is bigger than designed value.
it's due to measuring method or other issue?. Pls advise

regards
LC

[Peter_1]

Perhaps first some questions Lc-shi:

* Why have you installed 2 condensers ons this circuit, an air cooled and a water cooled one?
* Why not 1 air cooled condenser just behind the evaporator with it's inlet conditions= outlet evaporator.
* What is the product that this dryer has to be dried.

We rented in the past dehumidifiers (small building dryers) and I still have some in my shop. Should it help if I take some pictures but theses things are really basic, nothing compared to what you have made.

How have you calculated the outlet values?

The real outlet conditions, are those measured at the outlet of the evaporator or at the outlet of the condenser-reheated air?

If you measure them after the evaporator, then it's normal that you have 100% RH.
In the i/x diagram (or Carrier chart, psychrometric chart whatever you name it) cooling line goes straight down till the dewpoint line of 100 % is reached.

Then the moisture condens on the coil but don't forget, you still have the bypass factor of the coil (I don't know if you use this expression) where air isn't dried and passes through the fins without interacting with them.

But the air coming out the evaporator is always 100%RH and that's also the meaning of your machine.
It's colder then calculated but it is very difficult to calculate it exactly.
With the experience now gained, you can now recalculate backwards and make an evaporator which is a litlle big larger. (if you thought your drier didn't dry enough)

You must now take this point of 8°C and 100% RH and draw a line back down wards (reheating it back to the initial inlet temperature with the condenser) which gives you an outlet of +/-7% RH which isn't that bad.

If you reheat it to 18°C, then you become +/-17%, so much drier then needed.
So if you think it's to dry, then you need a smaller compressor so that the air isn't cooled that much.

This of course if you measured the evaporator outlet conditions.
If you don't reheat it, this air f 8°C/100% is rejected back in the air I suppose where it will reach a new equilibrium at a lower RH.

[Lc_shi]

Hi Peter
Thanks for your spending time to think it so deep.
i'll wrap up the test data and confirm the result within next week.

best regards
LC

[nimamu]

Hi LC

You are proposing to use only 15% of the condenser heat for reheat. If this is the case your unit would dehumidify as well as cool the air. is this what you require?

Also it may be better to have your condensers in series, than in parallel.

Regards

Nimamu

[Lc_shi]

Yes， 15%~20% condensing heat for dehumified air reheat. Why you think it's better to have condensers in series than in parellel?

best regards
LC

[Renato RR]

Condensers in series?What?Why?

Renato

[Lc_shi]

one condenser is for reheat dehumidified air--in other word ,it's heat recovery

rgds
LC

[Peter_1]

If you completely needs to reheat to the same inlet temperature, then you must add a condensor of +/-80 %of the total capacity.
Sounds confusing for me.

[Lc_shi]

no,only need to reheat to 18C (only sensible) to air conditioned space. No need to same as inlet,it's for comfort ac

rgds
LC

[nimamu]

Hi LC

I have been out of station, therefore not read my e-mail during that period. To assist you further in the dehumidifier design, please provide the following information.

1). Air flow throught the dehumidifier (cfm or l/s).
2). Condition of air entering dehumidifier (dry bulb temperature, wet bulb temperature or relative humidity).
3). Condition of air required for leaving dehumidifier (dry bulb temperature, wet bulb temperature or relative humidity).

Thanks and regards

nimamu

[Lc_shi]

Hi Dear nimamu
What you need is mentioned in previous content of this thread
I'm grateful for you help and like to hear your ideas.
thanks

best regards
LC