Refrigeration Compressor and System Types

We’ll be talking about refrigeration compressors and system types just by way of introduction.

So there are many different compressor types out there. It’s a big industry. We talk about things like hermetic and open drive, reciprocating, scroll, centrifugal, screw compressors.

Not all of those are applicable to commercial refrigeration, so we’re limiting ourselves. We’re not going to talk too much about centrifugal and screw.Those are the realms of commercial air conditioning and industrial refrigeration.

1. Compressor Types

Compressor Types

We’ll talk about open drive and hermetic, what’s the difference between the two. And we’ll talk about reciprocating and scroll, because they are widely used.

So the hermetic compressor is the one that you’ll see in the back of your fridge. It’s hermetic, as the name implies. Hermetic : compressor and motor are inside the shell and share a common drive shaft, refrigerant leak is avoided

a. Hermetic Compressor

Hermetic Compressor

It’s a compressor that’s completely sealed. So it’s concealed in a metal case. So on the left hand side, the picture

Hermetic Compressor in Condensing Unit

there shows a hermetic compressor with a perspex case just to show you what’s inside there. But generally, the way you’ll recognize it is by that little black dome thing.

Which if you turn your fridge around, you’ll see that black dome. And it also comes in other colors. And it is one of the common refrigerator compressors used for the smaller applications.

The good thing about it being hermetic, of course, is that there is no shaft seal. So the chance of refrigerant leakage through the shaft seal is minimized. Disadvantage is when it breaks, you throw it away.

So a lot of these compressors end up in Bangladesh where people cut them open and unwind the copper. Because you can’t chuck these compressors straight into a smelter, because it pollutes the steel with copper, so they actually have to separate this.

b. Open Drive Compressor

So the open drive compressor is the precursor of the hermetic compressor. It has a separate motor and a compressor, crankshaft extends outward from the compressor casing, connected to an external power source.

Shaft seal can leak and cause refrigerant loss

Open Drive Compressor

like it’s shown in the picture there. Generally, the older systems. It’s still being used on new systems for bus air conditioning, oddly, because there it needs to drive off the pulley of the diesel engine.

But for commercial refrigeration, it’s mainly an historic relic, and it’s mostly the older systems that still have them. Having said that, there are still many of them out there.

When they built these back in the ’50s, ’60s, and ’70s, they were built pretty well, and many of them are still running. It doesn’t mean they have to continue running. These are the units that we’d be replacing today.

c. Semi Hermetic Compressor

The workhorse of commercial refrigeration is a compressor called a semi-hermetic compressor. The semi refers to it’s not completely hermetic.

Semi-Hermetic Compressor

There are bits of it that you can open up. And you can see on this picture, there are screws, we can undo stuff and you can access parts of the compressor.

But it doesn’t have a shaft seal and the motor is actually also enclosed within that. And the motor is cooled by cold refrigerant going over it.

That is a disadvantage from an energy perspective because all the waste heat that is generated by the motor goes into the refrigerant and therefore has to be rejected through the condenser.

So it’s less efficient than an open drive machine, but it is certainly more convenient and it’s more cost effective.

Scroll Compressor

We also have the scroll compressors, which are vertical units like is shown there. And the characteristic of a scroll compressor where you always recognize it right away and the tip I give is this little knob on the top.

So if you see that little knob on the top of the right hand picture there, that shows you that it’s a scroll compressor as opposed to a reciprocating machine.

Scroll compressors don’t have pistons, they have two snail-like casings that intermesh and they have this interesting compression process. Most commonly used for air conditioning. So your home air conditioning system, if it’s a larger system, may have a scroll compressor in there.

But it is increasingly used for refrigeration. And interestingly, it’s also increasingly used for carbon dioxide refrigeration for the low temperature side. So scroll compressors are common and are actually becoming more common. Now that’s the compressors. The compressors are one component. As you know, there are four components in the refrigeration system.

2. Refrigeration System Types

These are arranged into what we call units. So there’s one arrangement is where you have a condensing unit and an evaporator.

And that’s really the refrigeration technician will buy one of each and put the condensing unit outside and the evaporator in your room and connect the two and he’s done.

The larger systems don’t have condensing units, they have remote condensers and then compressor units, which don’t have condensers built in, and the evaporator. So these are the most common arrangements of the walk-in cooler rooms will be covered by one of these two arrangements.

So what does that look like?

So here we have a compressor rack versus a condensing unit.

So on the left hand side we have four compressors mounted on a rack. This is your classic supermarket or larger commercial refrigeration solution. These compressors work together and they push the gas into a remote condenser. Whereas the unit on the right is a complete self-contained system.

It has a compressor, it has a condenser on it, which has the fan to it, and a few other bits and pieces that I’ll talk about later, which indeed you need to make the system run.

Condensing units can come in various shapes and sizes. On the left hand side is a picture of a condensing unit which is water cooled, as opposed to the right hand one which is air cooled. And the water in the air cooled thing is a very important topic from an energy perspective. The right hand compressor unit also has a variable speed drive built on it.

Condensing Unit

So you can buy better units today that are fully speed driven, which allows you better control of your energy use and of the performance of the system. So the condensing unit is a compressor and a condenser and lots of other stuff built onto one unit.

So yeah, like I say, simple to install. Just add an evaporator and you have a refrigeration system. Compressor units are, as I say, you need those for bigger systems. Supermarkets in the past used condensing units. Today, most of them will use compressor racks.

Same applies with carbon dioxide. You will continue using that concept. So you get larger capacities, you get better control, but you need a remote condenser, which is then typically located on the roof.

Now lets explain some terminology quickly.

Here I have a picture of a classic condensing unit. And just for further reference, you might want to refer back to this, some terminology relating to the compressor unit. So what we see here is it’s a nice compressor unit with four fan condenser. So the items that you’ll see are the actual compressor, which is a semi-hermetic in this case. Behind that is the condenser with its four fans.

Then we have a suction accumulator, that black thing on the right, which is just a device that protects the compressor. We have an oil separator, the blue thing on the left, which maintains the oil to the compressor. Then we have devices like the HP and the LP switch.

3. Condenser Types

Condensor Types

We’ve spoken about compressors, lets talk about the condensers. There are different types of condensers. Air-cooled condenser is the most common. And the reason why it’s common is because it’s cheap and you don’t need anything else.

So air-cooled condensers use dry air. But then you have various other systems that use that are water-assisted, as we call them. So you have the pad-cooled adiabatic units, which have basically a weted pad, which it draws the air through the weted pad and it causes it to cool.

We have spray-cooled adiabatic-cooled condensers, where a water mist is used to cool the air before it goes on the condenser. And then we have the water-cooled ones, where you don’t have air at all. You just have water that’s being passed throughand the water is then separately cooled in a cooling tower.

Now a few little rules that I find are quite helpful.

The lower the suction pressure or suction temperature of a refrigeration system or the higher its condensing temperature or pressure, the more energy the system uses or how much more.

And this rule is quite handy and it’s more or less accurate. So if you can increase the condensing temperature by one degree or if you reduce the evaporation temperature with the room temperature perhaps by one degree, you’re incur about a 2% to 3% energy penalty.

Or vise versa, so if you manage to reduce it or raise it, depending on what are you doing, you’ll gain energy efficiency. So this is a powerful tool.

What does that mean in a practical context?

An air-cooled refrigeration system will use 20% to 30% more power on a 40 degrees ambient day than on a 30 degrees ambient day, all things being equal. And that is the reason why in summer we have things called brownouts.It’s because every air conditioning system, every refrigeration system out there, they’re all air-cooled.

And their temperature goes up 10% and everything goes up, the energy use of all of those go up by 20% or 30%, and bang, we have a brownout. So these systems are very sensitive to that. On the other side of the refrigeration system, a freezer running at negative 20 compared to a cool room running at zero will be a 40% to 60% difference in power.

So clearly, running a freezer room costs you a lot more than running a cool room. Kind of obvious. I mean, we know that. But this gives you a number. So we talk about dry bulb and wet bulb, these are the technical terms. Dry bulb is the air temperature that we measure with the thermometer. There’s a thing called wet bulb.

And the adiabatic-cooled systems, they run to the wet bulb temperature. So I’ve got two rags here which have been sitting here on the table for a while. And these rags are now at the room temperature. I can measure that with my trusty infrared thermometer. So if I measure them now, they’re at about 20 degrees.

So that is, in fact, the dry bulb temperature that we have in this room right now. So if I take one of these rags and I put them into this glass of water, which has also been standing out for a while, so it’s not cold water, it’s just normal tap water, and I wet the rag. Some of you might remember the old weather man who goes round with his swings thermometer.

This is what he does, he just has a wet rag and he swings that. And the rule is you have to swing it 20 times, which I’m not going to do, but then it cools down to the wet bulb temperature. So if you’ve swung this enough times, then it equilibrates. And now if I take my trusty gun and I measure the temperature of that, it’s now 16 degrees.

So if you were to cool, if you were to use dry air you can’t cool below 20 degrees. But if you use wet air, you can cool down to 16 degrees here. If it’s a dry day, this could be 10 degrees or less. So you get a lot more energy efficiency. This is only four degrees difference, but remember, 2% to 3%, so you’ll easily get 10% better power consumption if you use wet cooling.

And as example on a hot summer’s day, you could have 40 degrees dry bulb temperature and less than 20 degrees wet bulb. So imagine the energy benefits you had if you could make a system run wet. Now, that’s not so easy, of course. It’s called the temperature depression.

But using the temperature depression is a really handy way to reduce the energy use of your refrigeration system. I’m clicking the wrong thing. So air cooled or water assisted, what’s the pros and cons?

If we use dry air, well, the cooling medium for air cooled is dry air. The cooling medium for water cooled or water assisted is either wetted air or water. And summer operation, the ecosystem is clearly hotter.

The water assisted system will be colder. Therefore, the energy use of the air cooled will be higher. Energy use of the wetted system will be lower.

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