PDA

View Full Version : Rack Systems



mke005
03-09-2006, 12:28 AM
Hi Everybody!
I have been working for a few years on some light commercial equipment and just got hired by a company that does a lot of rack systems, so I am trying to learn as much about this systems as possible, from operating pressures(R-22) to the CPC controlers they use, I would really appreciate any help, thanks

Dan
03-09-2006, 04:35 AM
Hi MKE005. With rack systems, it is best to break their operation down into easily digested subsystems. For example:

1. Temperature control
2. Oil management
3. Defrost schemes
4. Heat reclaim
5. Discharge pressure controls
6. Computer controls

This is an arbitrary list that I use in my head to simplify discussions and retain focus on subsystem operation. Each of these can be broken down into finer pieces, too.

Why don't you ask a question limited to one of the above subsystems that you are curious about and let's see what our experienced members have to say about it?

smilies
08-11-2006, 06:34 AM
Are you still out there mke005. Where in Mexico are you?

mke005
02-12-2006, 02:16 AM
Hi Smilies! Yes, I'm still outhere, I was living in Puebla, but moved up to Tj a few months ago, sorry it took me too long to reply, haven't been able to log on in a while

jrrefrig
10-12-2006, 06:25 PM
the sub systems for rack trouble shooting is a great heads up. also took a new position that involves multiple racks with novair controls. one reoccuring call we have seems to be temp slow to recover comming out of defrost [temp issue].using the sub system and go right to basic ref trouble shoot proscess seems ok , BUT why do racks intimidate

Dan
11-12-2006, 12:33 AM
Why do racks intimidate? Because you have to choose which subsystem or combination of subsystems are causing your problem. You have an example: Temperatures slow to recover after defrost.

You are wise to look at basic refrigeration theory. The first thing to look at is the suction pressure. Does the suction pressure rise above design, and slowly come down along with the temperature of the refrigerator that is causing your problem?

If the rack suction pressure DOES rise and is slow to pull down, then you have a possible lack of capacity. On the other hand, if suction pressure pulls down before you see improvement in the refrigerator performance, you should look at the performance and superheat of the refrigerator.

Racks intimidate because they force you to look at the large picture, sometimes causing you to ignore the basic pictures. If you only have one system misbehaving, you are correct to address it as a simple refrigeration problem, checking superheats, etc. But the rack might be low on refrigerant, and the system you are looking at might be the first one to warm up when low refrigerant levels are experienced.

In this case, you would have made a mistake if you adjusted the expansion valve before considering that you might be low on refrigerant. But this would be the same mistake if you were working on conventional equipment.

The intimidating part is that there may have been many adjustments made to many systems for all the wrong reasons and once you discover the reason, you have many problems throughout the store, instead of just one problem.

But racks are forgiving, too. If you lose a compressor, the sales of goods are not disrupted because more than likely you can hold temperatures.

I could compare a technician working on a rack to the general manager of the store. I could also compare working on conventional equipment to being a department manager.

Having more responsibility should be more intimidating until you feel that you understand it.:)

Novar controls. Any electrinic controller, hypnotizes most technicians. They look at the display and press buttons and trust everything they see. Always put your gauges on the discharge and suction sides of the rack. Compare your gauges' readings with the electronic controller readings.

Dan
11-12-2006, 01:12 AM
Here is why a rack should not be intimidating.

If you have an air cooled condensing unit operating a line up of meat cases, it is actually more complicated than working on the same system with a rack. A conventional system might have a liquid line solenoid, an expansion valve, a defrost clock, and a temperature control of some sort, that will stop all refrigeration to the meat cases.

On a rack, you do not have to worry that a condenser motor or compressor trip on oil failure will stop refrigeration. A rack will accommodate individual failures of this sort and not spoil the product.

There is more to look at, of course. But each system still has only one suction line and one liquid line.

The rack is less intimidating when you see it as a system that is forgiving when small failures occur... A conventional refrigeration unit will stop refrigeration if a safety control stops the compressor, a fan goes out, a contactor becomes faulty, a circuit breaker becomes faulty, or a compressor fails.

If I were a meat manager, I would be easily sold on the concept of parallel rack refrigeration. And as a contractor, I am more comfortable knowing that I have a system that keeps refrigerating even though I have the aforementioned problems.

Mark
11-12-2006, 07:56 PM
Packs Intimidating, maybe.

Some overlook the concept of a Pack system being any different from the principle of a big condensing unit.

Regards

Mark

taz24
12-12-2006, 03:40 PM
line and one liquid line.

The rack is less intimidating when you see it as a system that is forgiving when small failures occur... A conventional refrigeration unit will stop refrigeration if a safety control stops the compressor, a fan goes out, a contactor becomes faulty, a circuit breaker becomes faulty, or a compressor fails.
.


I agree packs (racks) are only glorified condensing units and should be brocken down to their individual parts. When looked t in this contect they are not as scary as at first thought.

Cheers taz.

Dowadudda
12-12-2006, 05:30 PM
so this is where some of you guys hang when not at HVAC-Talk. I think this is a bit better here maybe.

Dowadudda
12-12-2006, 05:32 PM
I can not understand any place with multiple loads, a perfect example would be a resturaunt, why they have not gone to smaller racks is beyond me. All controls in the machine. So all that ther is at the load is a coil.

I cut my teeth in resturaunt work. I see this as such a huge undefined type of way of doing things in a resturaunt.

The Hussman Protocol system is perfect for them.

taz24
12-12-2006, 08:09 PM
I can not understand any place with multiple loads, a perfect example would be a resturaunt, why they have not gone to smaller racks is beyond me. All controls in the machine. So all that ther is at the load is a coil.

I cut my teeth in resturaunt work. I see this as such a huge undefined type of way of doing things in a resturaunt.

The Hussman Protocol system is perfect for them.

Loads vary in shops and supermarkets. The main cause is the display multideck cases. They are so suseptable to ambient temp change that they need a flexible pack systems to cope. The other big thing that effects load is defrost. If you turn a high temp case off, on off cycle defrost your pack sudenly becomes over sized. The need for flexibility is very, very important.
Freezers do vary as well defrost being the killer.

taz.

taz24
12-12-2006, 08:16 PM
I
The Hussman Protocol system is perfect for them.

I think that the Hussmann pack was good (both enviropack and protocol). I was in a very very small way involved in the initial development back in the eighties.
Hussmann still have the patent for the liquid subcooler using the reciever that imitators can't use.

taz

smilies
16-12-2006, 11:03 AM
but moved up to Tj a few months ago

Oh, I'm sorry.:D Tijuana is something else.:(