Advantages of the Compressor Rack System

Advantages of the Compressor Rack System

By

Chris Savage

 

Here at Carlson & Stewart Refrigeration we have the opportunity to work with a lot of different types of refrigeration systems.  We typically break these systems into two major categories, Industrial Refrigeration & Commercial Refrigeration.  For all intents and purposes Industrial Refrigeration usually refers to large Ammonia systems.  This is morphing and changing…..but that is a discussion for another time.  The second major category for us here at Carlson & Stewart is Commercial Refrigeration.  This is usually the category where Rack Systems reside.

Within the realm of commercial refrigeration there are 2 types of systems that dominate the scene.  First, and probably the most common, is individual condensing units combined with remote evaporators.  Condensing units are factory assembled units which include the compressor and condenser, with all its appropriate accessories, in one nice little package.  These packages are designed to be either indoors or outdoors.   These condensing units are matched and piped to a remote evaporator, or a couple of evaporators.

The second type of commercial system is often referred to as a “Rack” System or a “Parallel Compressor” system.  This is what I want to put a little focus on in this article.

First of all, I will get it out of the way right away, Rack Systems will be more costly up front than an equivalent Single Condensing Unit system.  I will also say that this extra cost is often worth it and here are the reasons why.

Efficiency

Assuming the same refrigerant and type of compressor a properly operated and maintained rack system will always be more efficient than individual condensing units.  I am not going to get into any of the numbers in this article but in general here is why a rack system is more efficient.

Imagine a fairly large system that includes multiple single condensing units with many evaporators, or in the case of a supermarket many cases.  Every one of the evaporators on this system see different heat loads and heat load profiles.  So each one of the condensing units has to react to its induvial load.  So each system has slightly fluctuating suction pressures, a compressor that is starting and stopping to control suction pressure and usually a fixed condenser with no modulation capability.  From a power consumption standpoint, every time you start one of these many compressors there is a spike in power usage.  Fluctuating suction and discharge pressures also results in spikes in power usage.  These spikes are what drive power usage costs up.

In a Compressor Rack system all of these evaporators or cases are connected to a single header system.  Each evaporator/case reacts to variable loads by modulating the expansion valve and opening and closing a simple solenoid valve to turn on/off.  As each evaporator/case does it’s thing, the rack system sits there and holds a very stable suction pressure, keeping evaporator efficiency optimized. 

Instead of having many different compressors in many different condensing units starting and stopping, the rack system has just a few compressors, many of which will have some sort of modulation capability, less frequently starting and stopping.  All of it being controlled by a single control system, programmed to hold suction pressure stable by efficiently modulating compressor capacities and starting/stopping as necessary.

The end result is a much smoother energy profile, resulting in significant energy savings.

Capacity Control

In many ways, this advantage goes hand in hand with the efficiency advantage.  Think about how a single condensing unit has to react in a low load situation.  This is very likely to happen because we design refrigeration systems for worst case scenarios which is usually the hottest time of the year.  What happens when it gets cold outside?  For a condensing unit this usually means the compressor is starting and stopping very regularly.  This is hard on the compressor and comes with a large energy penalty.

With a rack system, it simply means you will need to run fewer of the compressors on the rack.  Keep in mind that most rack systems that I have seen have somewhere between 2 and 8 or so compressors.  So in the very hottest part of the year you may need to run 7 of 8 compressors and as the weather cools you may be able to drop to 6 or 5 or fewer compressors, all the while holding a very stable suction pressure for the evaporators/cases.

Redundancy

There are a couple of different ways to look at redundancy when thinking about a Rack System.

First of all, think about that remote freezer case or small, highly critical storage room.  If the refrigeration for this case or room is operated by a single condensing unit and anything on that condensing unit fails…..you are down until it can be fixed.  You are going to need to scramble to move product to save it.

Having that same case or room on a central Rack System eliminates this concern.  Even when you lose a compressor on the rack, there are still other compressors there, doing the same work. 

The other way to look at redundancy is to look at the rack itself.  Most racks are designed with redundancy in the form of a totally spare compressor all piped in and programed to run whenever necessary.  The failure of a single compressor isn’t going to shut you down.

Hot Gas Defrost

The last advantage I’ll mention is not one that is always used with a rack system but when it is used properly it can be a significant advantage.  That advantage is the ability to use the excess heat within the system to defrost evaporators.

As you know, refrigeration systems use mechanical energy and a change of state of the refrigerant to move heat energy from where it is unwanted to someplace it can be rejected, usually the atmosphere.  In a central system like a rack system we can take advantage of this heat and instead of simply rejecting it all to atmosphere we can use a small part of it to defrost evaporators.  Using this available heat negates the need to use other means, such as electric heaters, to defrost.

This type of defrosting does not work in single condensing units because the compressors need to be running, generating the heat, during the defrost cycle.  With a single condensing unit the compressor automatically shuts off when defrost is initiated, so there is no heat available for defrosting.

In a rack system, one evaporator can be defrosting while all of the others are still refrigerating.  Leaving compressors operating and generating heat that can be used to defrost the evaporator that is in defrost mode.

Conclusion

All in all, there are some very significant advantages to a rack system over single condensing units.  The larger the system gets, the more of a benefit these advantages become and the lower the cost difference between the 2 gets.  The lines separating the cost difference and the advantages is not linear nor is it always clear.  Carlson & Stewart Refrigeration has the experience and expertise with both systems and can help you weigh the differences.  Just know that you have options.