Chiller dehumidifier
Chiller dehumidifier
(OP)
We have a chiller packaged unit operating as a dehumidifier, ran into a problem and wanting some advice here on this one.
when the humidity level becomes satisfied the chiller stops, of course once stopped it shoots back up and then we have to wait for a set time for the motor protection softwear to count down before it permits a start, by this time we have the humidity shooting up.
Questions are ? How can this be avoided....
Can a soft starter be used to limit the motor stress when starting to allow more starts on the system ?
Can a different type of chiller be used that can take rapid starts with minimal rest times between starts ?
Hope that you can advise me on this one.....
Rugged
when the humidity level becomes satisfied the chiller stops, of course once stopped it shoots back up and then we have to wait for a set time for the motor protection softwear to count down before it permits a start, by this time we have the humidity shooting up.
Questions are ? How can this be avoided....
Can a soft starter be used to limit the motor stress when starting to allow more starts on the system ?
Can a different type of chiller be used that can take rapid starts with minimal rest times between starts ?
Hope that you can advise me on this one.....
Rugged





RE: Chiller dehumidifier
Sounds like a rather huge motor. A soft starter will not help you much since the rotor losses over time will be about the same with or without soft starter. (I am talking about a soft starter with reduced stator voltage). If you use a frequency inverter instead, you can avoid the stopping and starting altogether. Just let the motor run with the speed needed to keep RH just right. An RH controller should be added to make things fully automatic. Starting with a frequency inverter also avoids most of the rotor losses during start (nominal slip during ramping up).
RE: Chiller dehumidifier
Is the indoor-air fan run continuously, or cycled by the humidistat, or something else?
RE: Chiller dehumidifier
Rugged
RE: Chiller dehumidifier
However, as designers don't want to take chances with critical designs, air is cooled down to a generous ADP and hot water coils are used to reheat.
When you start and stop a chiller, inspite of the fastness with which you get the temperature, the inertia of the entire system makes your job difficult.
You can have two options.
1. Introduce a chilled fluid storage tank and maintain a constant chilled fluid temperature in it. Now you can start and stop chiller based on chilled fluid temperature.
2. Go for a desiccant dehumidifier for fresh air AHUs. Don't expect much savings with desiccant dehumidifiers. Your chiller load remains same. Moreover, you have to spend extra energy to regenerate the desiccant medium.
Regards,
RE: Chiller dehumidifier
Were talking large AHU's here and its to maintain air in an equipment hall. Im still working on some ideas and the water one looks particularly intersting indeed. Just now its a simple refrigerant circuit into the AHU that provides the dehimid function. if we were to go the route of a water chilled circuit then this might be the answer.....
RE: Chiller dehumidifier
Even my little window AC unit at home will wait a couple minutes before it will allow a restart -- I wouldn't think motor temperature would be an issue there. . . .
Can anyone confirm this?
RE: Chiller dehumidifier
Used to be that single-phase hermetic reciprocating compressors would not start until refrigerant pressures equalized, taking several minutes—stalling with early reattempts. That was never a problem with three-phase compressors.
Nowadays, the mechanical design of single-phase hermetic “scroll” compressors does not need equalizing off time.
RE: Chiller dehumidifier
Any idea why my little window AC unit has the timer? Is it then in fact to limit the frequency of motor starts?
And here's a related question then which arises from all this -- in a place with critical cooling requirements (a data center) where the HVAC is fed from a transfer switch which might permit a power interruption of a few seconds -- are we unavoidably stuck with a couple minute delay in refrigeration despite the fact that the power was only out for a couple seconds? I've asked this one to a few mechanical engineers and chiller vendors before and have never received any very confident-sounding answers either way.
RE: Chiller dehumidifier
This is usually managed through dual stringing all items. Thus you have the cooling capacity managed over multiple units and then have it done in such a way that there is an inherent redundacy. This will cope with a fail of one string, but if you have a mains fail then this kit is supported through generators and you will have some dead time till the timers time out and the kit starts up again. The cooling is managed by having a good amount of chilled water in the system and then you can rely on that smoothing any bumps out. You just need to have clever operating system linked together and staff on site that know how to deal with a mains fail and on to generator power. Takes a little bit of learning ! remember you go through this again when you drop back onto utility - dont have the wait for generators to sync up and go to bus as the mains is there but you do want to have it under your control so that you dont flick back and forth if the utility network has issues. A call to your local friendly network controller can usually find out if you have any problems to worry about out there. then do your throw back over in your own time. Data Centres are a black art believe me.
RE: Chiller dehumidifier
peebee — Strictly electrical on the 1ø vs 3ø issue.
Data-center admins seem to be actual/self-appointed Nervous Nellies—sometimes with managers that have wide-open signature authority. On refrigeration-startup delay, conscientious HVAC manufacturers hawk a conservative stance on equipment reliability {or their perceptions of their clients’ perceptions of reliability needs.} A little randomized staggering of larger-motor starts is an easy solution to this. If it isn’t available already, some salesman will be proud to rollout their newest “fully addressable neural-control” mechnical equipment {soon with autonomous N+1 RS485/ethernet/fiber ports} as the latest advance in restart-control finetuning. Men’s room toilet-paper monitoring with automated refill can’t be far off.
For data centers, it may be that part of the “Black Art” is sales-droid vultures taking advantage of considerable middle-management paranoia.
Window air-conditioner restart timing could be to limit light-flicker complaints.
RE: Chiller dehumidifier
During offload conditions your AC unit will be stopped and stagnant air may not add the required heat to evaporate the liquid refrigerant in your evaporator. If you immediately start your compressor, there are fair chances for the liquid to go inside.
In other words, Busbar is right to the point. When the pressures are equal, you can ensure that it is gas everywhere.
With industrial chillers, the time delay is in between the start of chilled water pump and the compressor.
Regards,
RE: Chiller dehumidifier
A data centre for a financial institution is a pretty important piece of kit, so it follows that you do what you can to eliminate any risk of any failure from occurring.
Not an easy task and one that means that the shirts and ties throw lots of money at to try and control. If you can remove the risk then you can limit the exposure to risk. Not an easy task.....
You should see the method statements and risk assesments that are required and the 'you do not do anything unless it is clearly written down' way of working that prevails. Get it wrong and well you lose a lot and that hurts.
Some people say that hospitals dont work to the same controls but you know that is one person on the table in a hospital. In a bank it is thousands of lives at stake peoples savings and such, so yes its fraught.
Rugged
RE: Chiller dehumidifier
Interesting perspective. After spending most of my career in the data center area (and previous to that in wafer fab and pharmaceutical manufacturing), I'm now starting to do some hospital work.
Not only do hospitals have lousy power installations compared to banks. The surgical areas are positively disgusting compared to a real pharmaceutical or fab clean room.
I appreciate your point about it being one person vs. thousands. But at the same time, jeez, I sure hope I never have to go under the knife.
RE: Chiller dehumidifier
But hey are we not better and more resiliant than mere machines ?
I served my time in a hospital and I know where you are coming from, can be scary seeing behind the scenes ! But you know hospitals get no where the same money as what a data centre gets - private versus public. This can be seen here in the UK when you look at private hospitals, very well done and look the part till you scrape the surface and look behind the scenes. With a data centre its reversed - especially dark sites - nothing to look at and all done to set spec, but the equipment wow nothing spared - best of the best and no risk taken with any kit what so ever - back ups for the back ups....
Clean rooms are worth seeing understanding principles and how they work, getting to grips with environmental control. Its look like we will be going back to water cooling, going full circle. This was abandoned when chips got smaller and more power effective but now chip densities are getting to the point that they need extra cooling. Put in a localised chiller in the unit and you have to dump that heat in the room adding to the heat load, get the point of dumping out of the room and you reduce the heat load presented so its looking like were going back to chilled water cooling at point.
It is an interesting game to be in and participate in.
Rugged
RE: Chiller dehumidifier
The larger the motor, the more restrictive, and the fewer allowed starts per hour.
A typical AC motor draws 3 to 6 times its rated full load current during startup, when the motor and its load accelerate from stop to rated speed. This extra current (and increased torque) during startup places tremendous thermal and mechanical stresses on the motor. To deal with the stress, most motors require minutes of cooling or equalizing time after each start; by running uninterrupted at rated speed, coasting or remaining stopped. Again, the larger the motor the worse the starting effect and the longer the wait between starts. Also, heavy loads placed on a motor during start up - due to unequalized refrigerant back pressure, for example - makes the situation even worse.
RE: Chiller dehumidifier
How about with VFD's? It would seem that with the soft starting afforded by VFD's that we could start the motor repeatedly. But I'm under the impression that the time delay exists even with VFD controls.
Someone mentioned that the time delay is there to let the pumps spin up. But if power has only been dropped for a few seconds, we can't have drained THAT much water from the system. Maybe a flow switch or pressure switch would permit us to get the chiller on-line faster?
And another question, just slightly off topic -- any thoughts in general on the value of putting a Multilin-type relay on a VFD? Is there any value to this or is it just a waste of money?
RE: Chiller dehumidifier
Another possibility is to use an evaporator bypass valve that allows the condenser to cool the compressor while it is idling.
Is your dehumidifier the kind that uses some of the condenser heat to reheat the air like household dehumidifiers do? If so, does it have 2 condensers, 1 for reheat and the other for disposal of excess heat?
Mike Cole, mc5w@earthlink.net