How to reduce capacity?

[chrishaji]

Hi
You must excuse my poor knowledge of air conditioning systems, (I am a software engineer), but I hope you can help
me with the following:-

I am trying to sort out problems with some mushroom growing rooms, which have been built in a very hot humid country in the Far East. (Max 40C 85%RH ambient)
These rooms have a simple recirc/fresh air damper and mixing box, followed by an evaporator, an electric heater and a variable speed fan.
The fan is then used supposed to blow the resultant air into an overhead ducting system at around 22C 90%RH.
The compressor is a Copeland scroll VR61KF-TFP-542 5HP 3 Phase, using a simple condenser. They also use a 8HP unit on one of the rooms.
The rooms are around 150m3 in volume and have a requirement for chilling between 1 or 2 KW at certain phases of the growing cycle and nearly 15KW at other times.
The reason for this is that it is necessary to flush out CO2 (produced by the mushrooms) to enable a good crop.
This means air changes of up to 7 per hour, i.e. around 150 x 7 = 1050m3 per hour of conditioned air from outside.
This obviously means that it is necessary for a system with a very high dynamic range.

The main problems are:
(a) Evaporator ice up at low fan revs during the night.
(b) Instability and swinging hot/cold
(c) Compressor breakdown

My questions are:
1. How can I avoid large transients in temperature during minimum demand?

2. how can I reduce the capacity of the system when necessary?
They have already been installed, so I really need a cheap solution.
I realise that a frequency controller could be used, but this is expensive.

3. It is sometimes necessary to reduce fan speed to avoid drying the mushrooms out too much. Would this be the cause of ice-up, or should the controller detect the temperature of the evaporator and avoid switching the compressor on? The trouble with this is that the temperature control would not be too good.

I attach a picture of the compressor below
Thanks for any suggestions.
Chris

[Gary]

3. It is sometimes necessary to reduce fan speed to avoid drying the mushrooms out too much.

Reducing fan speed makes the air drier.

[chemi-cool]

You can add a small fan in the room just for circulating the air and change the control circuit so that the evaporator fans will run only when the compressor is running.

[chrishaji]

Thanks - but the problem of a very large temperature change will still exist when the system is on full recirc. The problem is the capacity of the chiller - its far too great when the room does not need it, even when the compressor is only on for a short time.

[Gary]

These rooms have a simple recirc/fresh air damper and mixing box, followed by an evaporator, an electric heater and a variable speed fan.

The first thing I would do is disconnect that electric heater.

Next, I would install an evaporator pressure regulator (EPR) valve in the suction line. I would adjust the EPR such that the supply air temperature cannot drop below 22C.

If the EPR causes the suction pressure at the compressor inlet to drop into vacuum, then further unloading is needed... and given the wide load variation this seems very likely.

[The Viking]

Sorry but...

What you need is to call a refrigeration engineer out to redesign the system for you.
Just like the previous posters I could point you in the direction of possible solutions but without all the data and a hands on knowledge of the system(s), we can only throw food for thoughts at you.

It's a bit like software, if you got a program that needs tweaking, do you ask a fridge guy to come in and sort it for you?

Possible solutions includes; EPR (as previously mentioned) maybe in combination with hotgas bypass to rectify the problems caused by the EPR and liquid injection to overcome the issues caused by hotgas bypass... OR replacing the expansion device with an electronic one and an inverter to control the speed of the compressor.

But the only one who would know is someone that knows the units and have the knowledge of what solutions are available and how they affect system performance.

Sorry.

[Gary]

Sorry but...

What you need is to call a refrigeration engineer out to redesign the system for you.
Just like the previous posters I could point you in the direction of possible solutions but without all the data and a hands on knowledge of the system(s), we can only throw food for thoughts at you.

Being that chrishaji is in the UK and the system is in the far east, it can be assumed that we are throwing food for thought, by proxi, perhaps by interpreter, at a refrigeration engineer who is located in the far east.

[chrishaji]

Unfortunately, the original installer was sacked. There doesn't seem to be any people with in-depth fundamental knowledge available.
These rooms are in central China. This is not a normal application as you can see.
I will just have to learn more about refrigeration systems and become an expert myself! Customer wont pay for an on-site designer. He expects me to be jack of all trades!
I like the dogs on your website, by the way!
Regards Chris

[mad fridgie]

Typical mushroom problems, requires Rolls Royce system, but only wants to pay for a Lada.
as stated you really should employ an expert, but i understand your predicament.
So here are the fixes on the cheap.
Re-position (or add) heater in front of the evap (one presumes a large heater) This will produce false load.
Under low loads turn off/slow down condensor fans (increaseing SCT) reducing refrig capacity, ensuring that the comp is held with working eneverlope.
I not saying this is the best method, but does not require any refrigeration work, only control.
You should be able with the pair mods to keep faily good control with out destroying the comp. (Not so energy efficient)

[chrishaji]

Thanks for that - at least I've got another idea or two!

[Gary]

The key is keeping the air temperature as it leaves the evaporator coil from dropping below 22C.

You do NOT want to heat the air after it leaves the coil.

Therefore, heating the air before the coil, as Mad suggests, is a viable means of adding a false load, albeit an enormous waste of energy.

I don't think you should be messing with the condenser fans... that would be a good way to destroy the compressor if not done properly.

There are all sorts of ways to get yourself in trouble here.

[chrishaji]

The heater is used when outside temperature is low and heating is required.
Obviously, the cooling system is then switched off.
The other reason for the heater, is to help in the dehumidification process - cool to 21C (air will be saturated), then heat 1C to get 22 at 90%RH.
At the moment this is the least of my problems!
What about if I placed the evaporator into a water bath, then used a chilled water battery and a pump. Wouldnt this spread the load and make it easier to control - or am I being stupid?

[Gary]

The heater is used when outside temperature is low and heating is required.
Obviously, the cooling system is then switched off.
The other reason for the heater, is to help in the dehumidification process - cool to 21C (air will be saturated), then heat 1C to get 22 at 90%RH.

How does cooling the air to 21C freeze the coil?

Obviously the air is being cooled far below 21C... all the way down to 0C.

If you cool the air down to 0C and then reheat it to 22C, you have VERY dry air... this is not the least of your problems, it is your primary problem.

[Gary]

The heater is used when outside temperature is low and heating is required.

If you need to add heat, then you also need to add humidity.

[chrishaji]

No - I think you misunderstand. If the air has been cooled to 21C, then its heated by 1C you will end up with air at approx 90%RH at 22C
That is how its supposed to work.

I presume the problems occur when there is only a small load (say ambient is only a few degrees above the desired temperature and at 80%RH) and when the compressor comes on, the cooling effect is much too large for the size of room and the temperature of the evaporator plummets before the control system reacts, and ice forms.

[Gary]

No - I think you misunderstand. If the air has been cooled to 21C, then its heated by 1C you will end up with air at approx 90%RH at 22C
That is how its supposed to work.

If it were cooling to 21C, you wouldn't need to worry about that 1K added heat. But it is cooling far below 21C.

I presume the problems occur when there is only a small load (say ambient is only a few degrees above the desired temperature and at 80%RH) and when the compressor comes on, the cooling effect is much too large for the size of room and the temperature of the evaporator plummets before the control system reacts, and ice forms.

I presume the problems occur when the air flow is reduced and therefore the load is reduced. Set the air flow to 100% and leave it there. Reducing fan speed is counter-productive.

[Gary]

What does the control system sense/control?

I'm betting it controls supply air temp by adding reheat.

[mad fridgie]

There is 1000 and 1 ways to design these, you stated that the client did not want to pay, (penny wise, pound foolish) so why are you know looking at a system redesign.
I wish you all the best is becoming an expert in refrigeration.

[chrishaji]

The control system is simple heat chill - 2 relays with about 2C hysteresis. When the temperature goes above 24C, the compressor goes on and delivers 12KW of cooling - by the time the temperature is down a couple of degrees, a couple of minutes later - the system is heading down and the thermal inertia carries it further - then the heater goes on and there's instability.
I know that its possible to buy a better controller etc. etc. but the basic problem is that under certain conditions there is far too much cooling per unit time.

Nobody has commented on my idea -
What about if I placed the evaporator into a water bath, then used a chilled water battery and a pump. Wouldnt this spread the load and make it easier to control - or am I being stupid?

[mad fridgie]

Time for your client to pay for an expert. Does he give away is mushrooms for free or does he sell them?
Should we give away our "mushrooms"
I think not!
We are here to help those who need it!

[Gary]

The control system is simple heat chill - 2 relays with about 2C hysteresis. When the temperature goes above 24C, the compressor goes on and delivers 12KW of cooling...

When the temperature of ...what?... goes above 24C?

[Magoo]

Probably time to consider a chilled water/ glycol system for a softer managable control of coil condition, varable airflow is compounding any DX system if coil feed is 100%, or not partitioned intergrated fead coils. You could consider a VSD on compressor to follow load, this would give some sort of evap stability.
magoo

[chrishaji]

Hi Magoo,

So can I place the existing evaporator into a water bath, then used a chilled water battery and a pump. Wouldnt this spread the load and make it easier to control - or am I being stupid?

[desA]

Ahah... the joys of the mushroom principle & the revenge of the fired refrigeration engineer... bet he has a few keys to the vault...

The Chinese company will have enough funds to do the job properly. This squeeze is unfortunately something often seen in Asia, as is the non-payment of final outstanding balances.

There may be more to this story than meets the eye.

[The Viking]

Chris,
Thanks for the comment about the dogs.

Unfortunately it sounds like these systems are not what the mushrooms requires.
Whatever solution is required will never be reached without having someone "in the know" on site, actually looking at the installation.
The more I read this thread, the more I think "the best option is to rip it out and start again".

Your idea of making it a wet system is possible but not easy or straight forward and will probably end up more expensive than a replacement.

The one thing we haven't discussed is if it really is a cooling demand in the rooms themselves or just for the fresh air intake.
Perhaps the only thing they need is a heat-exchanger, where the extracted air will cool the fresh air. Maybe with an additional cooling unit and humidifier...
OR

OR

...

[Gary]

I will just have to learn more about refrigeration systems and become an expert myself!

Frankly, my impression is that you want to be an expert without becoming an expert... knowing without learning... arriving at your destination without travelling the path.

[monkey spanners]

If the system is cycling between heating and cooling too quickly it might pay to look at where the temperature probe is sighted or if it can have its mass increased to slow down its reaction time, possibly the controller parameters could be altered to improve operation, but it sound like this is just 'turd polishing' a bad system....

[chrishaji]

I'll ignore all superfluous comments on my way to becoming more of an expert than some of my respondents.
My thanks to most of you. I think the most logical way out of my predicament is to hire an expert - but you have to know a certain amount to determine whether someone is an expert or not!
(and I'm getting rather worried about this)

The two ideas that I'm going to consider are:

Monitor the temperature at the condenser and adjust or switch the condenser fans to lower the heat taken off when demand from the evaporator drops.

Convert the system to chilled water as a buffer, and use a chilled water battery evaporator and pump etc.

If anyone has any constructive ideas about these, I'll be grateful. Thanks. Chris

[Gary]

I'll ignore all superfluous comments on my way to becoming more of an expert than some of my respondents.
My thanks to most of you. I think the most logical way out of my predicament is to hire an expert - but you have to know a certain amount to determine whether someone is an expert or not!
(and I'm getting rather worried about this)

The two ideas that I'm going to consider are:

Monitor the temperature at the condenser and adjust or switch the condenser fans to lower the heat taken off when demand from the evaporator drops.

Convert the system to chilled water as a buffer, and use a chilled water battery evaporator and pump etc.

If anyone has any constructive ideas about these, I'll be grateful. Thanks. Chris

When you interview your "experts" tell them your two ideas. Hire the guy who is rolling on the floor laughing.

[The Viking]

Chris,
I do agree with what Gary is saying in his last post.

But, if you think about it and considering all the facts, these units and any solutions involving them will not be the most suitable for the installation you are talking about.

As I understand it, specialist technical knowledge is sparse in the region of this installation. Therefore any solution has to follow the KISS principle.
Have a look at my last post, as an example:
A Daikin VAM unit will more than cope with the ventilation demands (No specialist knowledge required to install/run/maintain).
A small split type A/C system with oversized indoor unit to cool if/when needed (Limited specialist knowledge required to install/run/maintain).
A simple humidifier to boost the humidity if/when needed (No specialist knowledge required to install/run/maintain).

The above kit should be both easy and cheap to source in China, after all they manufacture enough of them. They don't need to know much to get it "reasonably right". It is "common" units not bespoke made so help and spares will be easy to get hold of.

My bet is that something like the above will be cheaper than trying to redesign what you got, even at the initial stage and massively so if you consider the running cost over a 5-10 year period.

[The Viking]

Constructive ideas:

Monitor the temperature at the condenser and adjust or switch the condenser fans to lower the heat taken off when demand from the evaporator drops.

For this you need commonly used fanspeed controllers that act on the pressure in the system.
Nothing wrong with fitting those BUT it will not solve the problems experienced here.

Convert the system to chilled water as a buffer, and use a chilled water battery evaporator and pump etc.

Yes, this is a viable option. That is until you look at the cost of things. You will need a lot of new parts and, again, a specialist to design it.
For the cost of the parts alone you would have had the kit described in my last post installed and even get some change to fly you down there to have a look.

Whichever way you decide to go, good luck.

[Gary]

Chris,
I do agree with what Gary is saying in his last post.

But, if you think about it and considering all the facts, these units and any solutions involving them will not be the most suitable for the installation you are talking about.

As I understand it, specialist technical knowledge is sparse in the region of this installation. Therefore any solution has to follow the KISS principle.
Have a look at my last post, as an example:
A Daikin VAM unit will more than cope with the ventilation demands (No specialist knowledge required to install/run/maintain).
A small split type A/C system with oversized indoor unit to cool if/when needed (Limited specialist knowledge required to install/run/maintain).
A simple humidifier to boost the humidity if/when needed (No specialist knowledge required to install/run/maintain).

The above kit should be both easy and cheap to source in China, after all they manufacture enough of them. They don't need to know much to get it "reasonably right". It is "common" units not bespoke made so help and spares will be easy to get hold of.

My bet is that something like the above will be cheaper than trying to redesign what you got, even at the initial stage and massively so if you consider the running cost over a 5-10 year period.

- OR -

Install an EPR in the suction line.

Move the electric heater to the other (inlet) side of the evap coil.

The EPR maintains minimum evap pressure.

The heater (controlled by low pressure switch) provides minimum false load to keep the suction pressure positive.

[mad fridgie]

This again has gone from helping some one get out of the s***, to design. There is know no need to guess, you have to start at the PROCESS, "growing mushrooms". (from experience each grower has his/hers own secrets)
What is the maximum and minimum ambient conditions
Max and min air flows
Max and min process requirments
How many processes (rooms)
How important is energy efficiency
Whats the liklyhood of expansion
Local or remote controls and monitoring.
Access to spares and expertise.
Cost of loss verses breakdown.

[mad fridgie]

I have seen standard AC equipment used on mushroom farms (as per Viking) I do not like this option, as the fin spacing is to small, you tend to get fouling problems (blocked air flow), and pipe wall thickness is thin and tend to corrode away quickly.

[Emmett]

Chris,
I have a couple of suggestions for you, but I need to understand better this lowering of the fan speed. How is this accomplished? for how long? etc any information regarding this subject may be helpful.

[The Viking]

I have seen standard AC equipment used on mushroom farms (as per Viking) I do not like this option, as the fin spacing is to small, you tend to get fouling problems (blocked air flow), and pipe wall thickness is thin and tend to corrode away quickly.

Fully agree with you here.

The only reason I said A/C was price/availability and technical demands.

[chrishaji]

I can see what you mean about the cost of converting the unit into a chilled water system, but surely (and I've already said that I'm not a refrigeration expert) this means that if you immersed the complete evaporator into a large tank of water you could then run the compressor until the water was down to 5-6C and use the existing simple control system to keep it around that temperature. Meanwhile, you could have a simple pump to pump the water around to a chilled water battery - this would be controlled by another simple control system. Why is this so expensive? Why do you need a 'specialist' to design it?
In answer to Emmets question - at various stages of the cropping cycle you need fine control of airspeed around the mushroom beds (and you need fine control of RH) so as to prevent scaling of the mushroom surface. Go to your local supermarket and you can infer the methods of control used by the mushroom producer concerned.
I also dont understand why control over condenser fanspeed in proportion to condenser temperature wont be a solution. Surely its just the laws of physics?
Chris

[The Viking]

A. It's not just dropping the existing evaporator in a bath tub, you will need a new evaporator designed for liquid to refrigerant heat transfer.
You would also require a (probably bespoke made) cooling coil for your duct, a pumset and the pipework to connect it all. All sized and matched to compressor and controls. (Not to mention, requirement)

B. Condenser fan speed to control Condenser pressure/temp is normal but that's not what you need to achieve here, you need to control the evaporator temp and duty.

None of the solutions to your existing systems mentioned in previous post would be sorted out even by an apprentice in his (hers?) final year.
You do require some serious help from somewhere.


Also, look at the people that given you 'serious' replies. You have tapped in to a couple of centuries worth of refrigeration experience, why bother to do that if you don't like the answers you get?

As I said before, Good luck.

[Magoo]

Chrishaji.
my best advise would be to employ a competent refrigeration design/ consulting company. This will save you a lot of greif and heart acke and the client may possibly be half happy with spending more money to resolve situation. You become the shinny a##ed supplier. ( check your public liability insurance policy first though and manage the risk ).
I can now understand why the first bloke was dismissed from project.

[Emmett]

Chris,
The question I asked was "HOW" is the fan speed control accomplished not why! Is the speed manually adjusted based on some observation of the grower? The reason I ask is this,based on your explaination of the problem it appears all agree that the expansion device cannot control adequately at low load conditions!(oversized maybe) appearantly brought on by the evaporator fan speed being reduced. Another way one might approach the problem is this, instead of reducing the fan speed, use full out door air and divert the excess air to somewhere other than the growing space,(new ductwork and a damper) assuming the ambient conditions are greater than the conditioned space a false load would be applied to the evaporator. this is obviously an in-efficient use of the system, however it may be a very inexpensive alternative to modifying the refrigeration system. If the out door ambient conditions are not capable of providing the false load consistently then you would have to add an electric heater to the inlet of the evaporator, you would not want to move the existing heater as you properly stated earlier it is necessary to cool the air below set point, a saturated condition and then reheat the air to lower the %RH

[Gary]

you would not want to move the existing heater as you properly stated earlier it is necessary to cool the air below set point, a saturated condition and then reheat the air to lower the %RH

Reheat is not needed. The bypass factor alone will add 1K... and then there is the heat from the fan... and the duct gain. Then as the air enters the room it will warm much more than 1K.

The very last thing this system needs is reheat.

[Gary]

The bottom line, most important condition is the temperature/humidity in the growing room... but instead of telling us the growing room design conditions, chrishaji has told us the design conditions in the supply air duct.

He tells us that the cooling comes on at 24C, but he does not tell us where that 24C is measured. Is this the temperature of the growing room?... the supply air?... the return air?... the mixing box?... the outdoor air?... other?

I understand that he is not a refrigeration engineer, but I would expect a software engineer to be more specific in his descriptions... or to at least answer the specific questions he is asked.

[Gary]

We can't see the system from here. We can only see the description of it.

We have been provided with a picture of the condensing unit, but that is only part of the system. Where is the picture of the evaporating unit?... the mixing box?... the evaporator?... the heater?... the fan?... the controls?

We have been told that the control system is simple. Simple is not a description. Simple is not a picture. Simple is not a wiring diagram. Simple is just an opinion. Simple tells us nothing.

[Gary]

And here's something else you need to think about:

As soon as the outdoor dewpoint drops below 21C, any introduction of outdoor air will give you a reduction in humidity. If there is a direct water spray on the product then perhaps this is not a problem. Else you will need to add humidity to the air.

[chrishaji]

Sorry Emmet, the fan is controlled by a variable frequency controller (3ph 0.75KW) and is manually operated by the grower.

I can design circuitry and software to automatically control the fan, but the problem is the need to reduce airflow on the beds sometimes, and this leads to ice-up.

I have a couple more ideas -
1. Put in an extra
duct pointing to the ceiling - this should reduce airflow across the beds, whilst still enabling max flow across the evaporator.
2. buy 2 compressors each half capacity and make it a 2 stage system. Would this be cheaper than putting a variable speed drive on the existing compressor?

I dont understand why its not possible to place the existing evaporator in a tank of water to obtain a chilled water buffer system - surely the heat transfer beween evaporator and water is far more efficient than air transfer. Does this require 'sizing'?

I agree with you all that I need an expert, but for financial and geographical reasons that is not an option. I need to learn how to do this myself.
Chris

[Gary]

Time for your client to pay for an expert. Does he give away is mushrooms for free or does he sell them?
Should we give away our "mushrooms"
I think not!
We are here to help those who need it!

Well said... and herein lies the problem with helping people online:

We won't get paid for it. We won't get credit for our ideas and suggestions. We will be fortunate to even get a polite thank you. Likely as not they will not even come back and tell us the end results. Where is the incentive? We must be insane.

Sorry about the rant. I am just not in the mood for all the crap today.

[brunstar]

I would leave the current system in place to take care of the peak demand and have a smaller additional system in place which will match your partial load demand.

[Gary]

I agree with you all that I need an expert, but for financial and geographical reasons that is not an option. I need to learn how to do this myself.
Chris

Start here:

http://www.refrigeration-engineer.co...ad.php?t=19701

[Gary]

1. Put in an extra
duct pointing to the ceiling - this should reduce airflow across the beds, whilst still enabling max flow across the evaporator.

Finally an idea that makes sense.

[Gary]

2. buy 2 compressors each half capacity and make it a 2 stage system. Would this be cheaper than putting a variable speed drive on the existing compressor?

Let's see if I can make this a little clearer:

An EPR valve reduces evaporator capacity and therefore system capacity. It does this automatically and only to the extent currently needed, no more and no less. EPR's are used extensively in applications where precise (high) humidity control is needed.

BTW, '2 stage system' has very specific meaning, referring to very low temperature freezer systems. I'm thinking you are referring to 2 parallel compressors (very complicated) or perhaps 2 separate systems whose operation is staged (less complicated).