Frick Screw Compressors Pt. 1

I'm Joe Callas, engineering fellow with Johnson Controls Industrial Refrigeration business. Today's presentation is about the basics of operation of screw compressors. So let's start. First, our refrigeration business within Johnson Controls is organized globally. And much of that business focuses on food processing, freezing, and cold storage warehousing. There's other marine parts of the business and gas processing parts of the business, but much of it is focused on food. Our brands globally, we have three main brands, the Frick, Industrial Refrigeration name, Sabro, and also York Process Systems. And those three product lines are used in different parts of the world, and for different customer base. We have an extensive product range lots of compression products, both with rotary screws and recips manufactured in various locations. And then we have the packages that go with those compressors. And we also make multistage centrifocal compressors that's shown by this large package at the bottom left. I have extensive controls offerings both for microprocessor based controllers and PLC controllers and various selection tools. Airside products, we offer evaporators, condensers, hygienic air handlers, pressure vessels and heat exchangers, and of course service in parts to support all of those products We are in contracting in parts of the world, but not in North America. We generally think of our rotary screw compressors as being the heart of many refrigeration systems. We start off with the high efficiency compressor, makes it possible to build high efficiency systems, and there's certainly a very important part of any refrigeration system. The Frick company, it's been in the manufacturing business since 1853 in Lanesboro, Pennsylvania, But we haven't been in refrigeration quite that long. We started built or first refrigeration machine in 1853. And the screw compressors came along, of course, much later than that. But we have forty years in screw compressor development and manufacturing. We started with packages a little bit earlier than that, about 1970 using other manufacturers, group and presser And then we developed our first Frick screw machine about 1980. Since that time, we have produced about 45,000 screw compressor packages and over a 150,000 screw compressor blocks. Screw compressors are widely used in refrigeration for over fifty years. And while they're simple in concept, it's very few moving parts It is however a difficult geometry to visualize. And while we're not trying to make any of you into compressor experts, We have found that folks that understand the basics of how they are supposed to operate, it it certainly helps them in applying them and systems. So as it comes to selecting equipment and and designing refrigeration plants, maintaining those plants, it helps to know the things that are important. So we're gonna start off with a little bit about rotors. We have two example, rotor profiles here. 4+6, the four just refers to the number of lobes on the male rotor. And then six is the number of flutes on the female rotor. So 4+ 6nd a profile, the a just really refers to patents that were involved when that particular profile was designed. And we have lots of different profiles that are in production today, depending upon the application that they're designed for. So we'll use five plus sevens and a slightly different arrangement for our small compressors and we now are have released high pressure machines. And in those, we use a 6+8 profile because it's sub more suited to to operating at at very high pressures. The rotors really are about the only high-tech thing. In a screw compressor. We spent a lot of time on the profile design to minimize noise, to minimize leakage. And to assure that the contact stresses between the rotors are are correct and proper and that, you know, that we have adequate bearings to support the kind of forces that are developed in compressing gas in these in these machines. Manufacturing of rotors has come a long ways. Today, all of our rotors are manufactured on grinding machines. Grinding brought a tremendous improvement to manufacturing of rotors. These are indexible carbide we have indexible carbide cutters for roughing, but the grinding wheels are run at high speed with lots of coolant. They remove a lot of material in a big hurry, and they are continuously dressed while the rotors are being cut. So they are being taken back to the theoretical profile shape all the time. So that means that when rotors come off of this process, they are nearly perfect in the profile shape. In the old days, we would cut these machines with milling cutters that would wear. So it was very important that we would take, for example, the first male rotor that would come off of a a new cutter that have been sharpened. Pair it with the last female to come off of the new cutter so that we could try to control the tolerances as much as possible. That's going away with grinding because nearly every rotor is exactly the same as it come off the process. Ground rotors are certainly closer in tolerance, which reduces leakage pads and improves efficiency, but they also have a very good surface finish, which helps to reduce noise. So we're going to talk about the basics of screw compressors and the first thing we have to talk about is how do they work. So suction gas is coming in the generally the top of the machines, flows back into the rotor position where the male and female are riding side by side in the bore. Gas is compressed from one end to the other, from the top to the bottom. And then it's pushed out the discharge port on the bottom. We'll also have a couple valves, which we're gonna talk a lot about the slide valve and slide stop. And we'll have bearings that are supporting the rotors as to keep them from touching the housing. Tolerances are very tight, rotor clearance to the housing is a few thousands of an inch. Clearance to the discharge case is a few thousands of an inch. So generally very close tolerance machines. And here you can see a little bit about how the gas flows through the rotors. So again coming in the top these cross hatched regions are the port area. So this would be the suction port that's located on the suction in the in plane. And the gas would be drawn through the machine, pushed out toward the discharge where it again has two ports kind of a radial and an axial port. That the compressed gas is pushed out. I like to describe the process of how this group compressed or works by referring to a reciprocating compressor. I think in general most people understand how reship work and it's a nice it's a nice comparison. So we'll look at the reset first. That the beginning of the suction stroke on a reciprocating oppressor is when the piston leaves top dead center and starts its way down the cylinder bore. As as the piston moves down, there's a vacuum or a low pressure area that's created above the piston. And when that pressure is lower than the pressure that's in the suction, cavity. The suction valve will open and allow gas to flow into the machine. Screw compressor works very much the same way. The male lobe is meshing in the female flute on the on the suction end of the machine. And as it rolls out, it creates a void or an empty spot, a low pressure area, and suction charge is is drawn into that low pressure area as the rotors rotate. So this one is rotating this way. See that females rotating this way. And that suction or a low pressure area will get larger and larger, then as the rotors continue to unmesh. The end of the suction stroke on a reciprocating compressor is when the piston gets to bottom dead center. So it's a drawing in all the suction, pressure gas, it can get into the machine. We have a special name for that. I use the VS just for volume at suction. You'll see later that we use that term some. It has another name in a reciprocating compressor. It's called displacement. The displacement of a machine is how much volume the piston displaces as it goes from one extreme to the other. Screw compressor has the same volume at suction, but it's shaped a little different. So these these two threads that are wrapped around the rotor are the maximum volume that can be drawn into the screen machine, while still at suction pressure. Before the gas is trapped, fully trapped and before it's the pressure starts to raise on it. The next step in the process, the piston leaves bottom dead center, starts its way up the bore so that volume at suction or displacement becomes smaller as the piston begins to compress the gas. On the screw compressor, we have to flip the rotors up and look at the bottom nail. So the male lobe rolls back into the female flute as the as the compressor's routine this way. Forms the back end of a trap pocket on a V-shaped wedge of gas where I've got these circles in the in the pocket. Now there's a close tolerance housing all around the rotor tips, so the gas can't leak out once it's been trapped in that area. But continued rotation will reduce the size of that trap pocket and raise the pressure on the gas. So here we are a little further along compression. You can see the piston now is more than halfway. The pressure is continuing to be raised above the piston. You'll notice in this slide both valves are still closed, so nothing has left the cylinder yet. It's just in the compression. The same way in the screw. We have moved the gas down on the bottom of the machine along the high pressure cusp. Of the of the rotor set. But the gas is still trapped. The volume has been reduced. The pressure has been raised. But nothing has left the compression area again. As the piston goes higher, in the reset, we'll see there's a change up here now. Our volume has gotten smaller, but now the discharge valve is open. So in order for that to happen, it means the pressure in the cylinder below the valve had to exceed the pressure in the discharge manifold above the valve, and that's what opens the valve and starts to push the compressed gas out. Screw compressor, this is where they differ a little bit because there are no valves in a screw compressor. So as we compress the gas up to our desired volume and discharge, The only way to stop the compression is to time the opening of the discharge port so that once this trap pocket or the leading edge of the trap pocket passes over a discharge port, then the compressed gas can be pushed out of the machine into the discharge housing. Here you can kind of visualize that there is this V-shaped compression pocket coming along the bottom of the the rotor bore along the top of the slide valve, And when it gets to this radial discharge port, the leading edge of the trap pocket, there's always a leading edge and a trailing edge. That's that's what's trapped between the rotor tips. But when the leading edge gets to the discharge port, that's the end of compression. Then you're just pushing gas out at whatever pressure there is in the discharge housing. We use two different discharge ports in a screen machine will have a radial discharge port that's on the high pressure end of the sliding valve. And we also have an axial port that's located on discharge end wall between the two rotor bores. So this all this green area, then it's where the high pressure gas would be pushed out. Then we'll go back to see the end of the compression process in the reset when the piston gets to top dead center. It can't compress the gas anymore. If there's always a little bit of space left at the top, that's really just because you can't put the piston into the valves because it would do mechanical damage But it does leave a little bit of clearance volume. There's some compressed gas left at the top of the stroke. On the next suction stroke that gas will re expand and it will actually take space that could have been used by more incoming suction charge but you cannot get gas into the piston on the next suction stroke and tell the pressure in the cylinder is lower than the pressure in the suction housing. So that clearance volume actually produces a bit of a loss in a reset. Screw compressor does not have that issue exactly. We flip the rotors around the other way now. We're looking at the other end. So this is the high pressure in wall of the router set and the bottom of the machine would be here. So we've pushed the gas out of this last trap pocket into the discharge line. And the last little bit of trapped gas and actually oil in these machines and most of them are oil flooded So there is a fair amount of oil in the compression. That oil and gas has to be pushed out this little area. But when it passes through the final parts of compression, There is nothing left. There is no gas that's carried back to suction to produce the same concept as a clearance volume. Because group compressors will have a very flat volumetric efficiency curve against compression ratio. They there is no gas to re expand. So they all pump almost the same thing at twenty to one that they do at at two to one compression ratio, unlike a reset. The concept of volume ratio is not talked about with reset. And really it's because of the discharge valves. The discharge valve decides when the compression process is over. The screw compressor has no valves, so this concept of volume ratio then is really well, we know we trapped a certain volume at suction. We compressed it up to the point that it left the machine going out the discharge ports, and there is a reduction in the volume in a particular ratio. That's what the volume ratio is. So I can design a machine for a two to one volume reduction. Actually, BI stands for volume index but it's we usually call it volume ratio. I could design a machine for ten to one volume reduction. So it would be a much smaller volume left on the discharge side if the compressor were designed for a much higher volume ratio. To make those changes in a machine actually to design for low volume ratio I like to use this model So it's schematically you could picture this screw thread is open to suction. So it's filling. This one is filling. This one is filling, but then once the trailing edge of the trap pocket, this trap pocket passes the open suction port, this volume is trapped. So there is my volume at suction. And as the rotor continues to rotate, the volume will get smaller and smaller, until the leading edge of the trap pocket opens to the discharge board. So to build a low volume ratio machine, we're basically putting in a short slide valve and put this discharge board fairly early in the compression process. Now, to build the same machine as a higher volume ratio machine, It's actually pretty simple. We just put in a longer slide valve. So that moves that radial discharge port later in the compression process and we keep the gas trapped longer. We raise it to a higher pressure before it's allowed to escape from the screw threads. So, again, we got a longer slide valve. The radio port's located further from suction. PS is trapped longer, and that's how we build a high volume ratio machine. Now the the issue and the reason we talk about this is because refrigeration systems are particular in in one way that the compressor does not decide what the suction pressure will be nor the discharge pressure. Those are determined by the temperatures that you're operating at. That was a concept that was hard for me to understand when first came to the repatriation industry, but those of you that have been in this industry for a while would well understand if you want to freeze peas at minus twenty eight, then your evaporator pressure has gotta be zero pounds gauge. And if you want to chill water at plus forty, then your evaporating pressure is gotta be thirty five. Pounds gauge. So the temperature of the process decides what suction pressure is needed. Same on the discharge side. The discharge pressure is decided behind by how hot it is outside or the temperature at which you're condensing. The condenser decides what the discharge pressure is. Now the compressor is pushing the gas to the condenser, but the compressor can't decide what the pressure is. So the system volume ratio, you could say that based on these temperatures of operation, that decides how much the gas needs to be compressed to satisfy these two pressures. Now it changes over time with a a drop in the ambient, your condensing temperature can can drop or will drop. If there's anything that would cause an evaporating temperature to change, for example, if you're processing different types of foodstuffs and one is at lower temp than another, then that would require a different evaporating temperature level. Compressor that's chosen for one design condition may not be optimum, off design. These are PV diagrams. So we got pressure on the y axis and volume, on the x axis, It's kind of an attempt to describe what's going on inside of the machine. So you can picture we start at the suction pressure level and we begin to draw gas into the screw threads and we draw it in until we trap our volume in suction and we know what that means now. And then we begin the compression process and the only thing a screw compressor knows how to do is reduce the volume of gas that was drawn into it in its design volume ratio. So it will keep it trapped until it reaches this volume at discharge point. At that point, the discharge port would open to the system pressure. And in this case, I've shown over compression, what if our system pressure is lower? So my condensing pressure is low today. It's colder outside. I've already compressed up to the pressure that was determined by my volume ratio. But then I'll have an expansion as the screw thread opens and it will drop down to whatever pressure is in the the discharge port, the discharge housing, that's controlled by the condensing temperature. This results in lost work on PV diagrams this area under the curve is the work that's required to perform the compression. So this is not ideal to have lost work. The way that we would have saved that extra energy would have been to stop the compression process a little bit sooner. So that we would not have the over compression work. And if we could have stopped at this point, then we would have no re expansion into the discharge line. Our total power would be reduced by the the area of this green triangle. Now the opposite case also waste energy. So in this case, I will start off here again. My suction pressure and zero volume, I trap everything that the compressor is able to trap until I have my volume at suction. I then reduce it in whatever volume ratio it as designed for and I'll let the charge go. In this case though, my condensing pressure today is higher. So like missing pressure is higher than what I designed for. So as soon as that trap pocket opens, the discharge pressure gas will flow back into the screw threads raise the pressure almost immediately to help to the full system condensing pressure, now the threads have to work against that higher pressure for the remainder of the thread meshing period rather than work against a gradual buildup in pressure if the volume ratio had been correct. So how would we have corrected this case? The volume of discharge would have had to be a little bit smaller. The guess would have had to be trapped a little bit longer. The ideal case is pretty obvious. You would like to have your internal volume ratio match whatever the system needs. In order to avoid over and under compression losses. Now in practice as we build these machines, what this would be sort of looking at down through the rotor. So the red lines are the center lines of the rotors. We're looking down on the top of the slide valve. And you can see here there's my low BI port opening. So it's fairly early in the compression. The bottom one, shows a longer slide valve, so that's again a higher higher BI machine. We know now what those mean. Crick was one of the first companies to ever develop the concept of variable volume ratio where we put two movable slides in the bottom of the compressor bore, a slide valve, conventional slide valve, and a slide stop. So these two can move back and forth and reposition the discharge board and this this gap at an infinite number of positions. And in that way, we're able to adjust volume ratio while the machine is is running. In the past, you would build a machine with a particular slide valve If you wanted to change it, you would have to take the compressor apart physically change parts. So variable BI has made it much simpler to just adjust the AI while the machine is running and maximize efficiency in that way. Here's a three-dimensional view. Of the slide valve, which is the blue part and the slide stop behind it and see the parting line between the two. Anytime that they are touching, machine would be running at full load. We'll talk more about that. So here you can see back to our side view of the machine anytime my slide valve and my slide stop are touching each other then I'm in a position of running full load. If I am back in the position, it's gonna be a lower BI position. So this is really running fully loaded at a two point two winery show. Now these should point out perhaps the slide stop is controlled by this hydraulic piston that's attached to it. We'll have hydraulic control mechanisms to feed oil to one side or the other. And the slide valve was controlled by this the second piston that's in a separate sort of a compound cylinder. It's blocked here in the middle. And putting wall pressure on one or the other side of this piston will change the capacity or the slide valve position in the machine. Now here, all we've done in this slide is adjust to five volume ratio. So both slides have moved forward. We've kept the the two slides together. All we've done is move the discharge port later in the compression process so we keep the gas trap longer. You carry it to a higher pressure, presumably, in this case, because condensing pressure has increased. So benefits, a variable volume ratio. Really, it's all about energy consumption. We're trying to maximize the compression efficiency of the machine to reduce the overall energy consumption. It's a simple system. The operator doesn't really have to understand how this works. It's all controlled by the microprocessor controller. And it's measuring the system compression ratio. The system suction pressure, the system discharge pressure calculates the ideal place to put these two slides and automatically adjust it. It also gives application flexibility that you can design systems that can run at various compression ratios and and handle greater swings in condensing pressures without a loss in efficiency. So really, it's about matching load and pressure changes to reduce total energy consumption. That's why that's what variable BI is for. Now we're gonna move on to capacity control. There's two main methods used to control the capacity of a screw compressor, slide valve control, and we're we'll talk about that along with variable BI because they interact with each other, and then variable speed are the two main methods that can be used today. So we've seen this slide before that in the last time we used it, we were talking about what's going on with the discharge port And when we talk about slide valves, there's one of the main purposes of the slide valve is to open this recirculation slot. Part way down the compression. So you can see, again, back to our schematic, this thread is still drawing in suction gas, this thread is trapped. But as soon as we would start to reduce the volume in that thread, if the slide valve has moved out, then there's there's an opening, and this recirculation slot goes right back to suction pressure. So a slide valve is essentially a low pressure bypass It delays the beginning of compression by taking some of this suction pressure gas and pushing it right back to suction and it doesn't retrap the volume that will be delivered to discharge until the back end of the particular thread process over the back end of the slide valve and and then that amount of gas would be carried onto discharge. The slide valve can be positioned at an infinite number of places. So in this position, we're pumping a hundred percent of the volume the machine is designed for. And generally we can adjust any position down to about ten percent on most machines. This has been one of the things that that has made screw compressors so popular is the ability to adjust capacity exactly to what's needed in the system. In system control generally, that means very precise suction temperature control. So if something changes in your process, you move in another skid of something, would tend to cause the suction pressure to rise. The slide valve will sense the rise in suction pressure and it will load up a little bit. Until it can exactly match the capacity that's needed. So it's a And it'd be an inch of a slug now. So, again, here's the showing the gap at the back of the slide valve. So when we're unloading, we have to open this gap between the two valves. And then we don't reestablish compression until the v shaped wedge passes over the back end of the slide So only what's trapped from that point on would be delivered to discharge. And here you could imagine some portion of this crap gas, just recirculating back to suction, and only the rest of it then carried on to discharge. So here we can see a slide valve fully unloaded and I think in this case our volume ratio is in the minimum position, wouldn't have to be, but it's shown that way in this slide. As we begin to load the slide valve up, you can see it's moving toward dissection into the machine, you're worried about forty percent load and then about seventy percent load. And then as they touch, then we're at a hundred percent load. Now we talked about slide valves and as a means of unloading, slide valve is a good method to unload. And this is power full load horsepower against percent of full load capacity. Let me say it again, percent of full load, break horsepower against percent of full load capacity. Perfect would be this straight line here. So let's say you get forty percent power for forty percent capacity, that would be perfect. Slide valves aren't perfect, but they're pretty close. And the lower the compression ratio, the better they are. So 2.5:1, that's kind of water chiller duty. And you can see at fifty percent capacity I'm taking well maybe 57% power pretty good. It's a pretty good reduction in power as you unload the machine. That is what has made slide valves popular. Now as we go to higher and higher compression ratios, so here I'm showing nineteen to one. So that would be a single stage blast freezer, not that that's done very often, but At that condition, you've got fifty percent capacity is now taking about 76% power. Okay. That's not very good. There are better ways to unload on high compression ratio systems, or you build two stage systems for those kind of high ratio. Machines. So then you're more back down at about the 6:1 if it's a stage system. So they're unloading maybe about 65% power for 50% capacity. But regardless, the slide valve does give a significant improvement in efficiency rather than just say, a Hotcast bypass, which is typically what's done on centrifugal machines. The other way to provide reduced capacity on screen machines is with variable speed drives. So variable speed drive or variable frequency drive drops the rotation speed of the motor, and as the compressor spins slower, it pumps less. So that's the other common way to reduce capacity. And so we'll talk about that a bit. Here are the same curves that I showed a few minutes ago. Varying compression ratio. So the blue line is our water chiller again and and line on the top is our single stage blast freezer. The very bottom line is a variable speed drive. We we call Viper, but any VFD that's reducing the speed in order to control capacity does improve the efficiency at at any compression ratio. Now, there's a few things to consider with this. We'll walk through. It is significant savings at high compression ratio. Not quite as significant at low compression ratio. And really the amount of energy that's saved helps to decide how much money you should spend to put in a VFD or if the expense is worth it. I will look at this example a little bit more detail. So here is a six compression ratio application. So that's about minus, that's about ten degrees Fahrenheit evaporator with 95 condensing. So a slide valve machine in the in the blue and then a variable speed drive machine in the brown. And there's a couple things to look at there. First, There is always an inefficiency in any VFD. The electrical loss in the drive is on the order of two percent. On a good modern VFD. Now there can also be losses in filters. There are electronic filters that are in a lot of these drives that can actually add a little bit more to the loss. So if you've got an application where the compressor is gonna run fully loaded all the time, a VFD does no good because if it's actually gonna lose lose energy because of the the losses, the electrical losses. But then we start to look at what we can save as the speed is reduced. If we compare the energy used with the slide valve at fifty percent capacity compared to the energy used with the VFD at fifty percent capacity. We can save eighteen percent of the power. Now that is eight. That's taking this curve and dividing it by the value of that curve. So energy savings at that point is significant. Most motors can go down to about 50% speed without too many concerns. So there's a lot of applications that can be run this way. Now, we can also allow turn down further if everything is designed for it. So Frick machines are using anti friction bearings, which are very good at low speed. Sleep bearing machines typically can't go allow about 50% speed. So on our machines, we can run down to twenty percent in most cases 20% of full RPM. In a lot of cases, then it means the motor has to be chosen. The motor becomes of restriction. And we will quite often use external fans on the motor in order to allow it to cool properly at the low speeds. So we can buy motors that will let us go down to this twenty percent speed range and and our compressor is designed for it. But then there's significant savings at those low speeds. So if you have an application that varies widely in the capacity requirements, then there can also be significant savings for allowing them to turn down further. We refer to that as 5:1 turn down. So from a hundred down to twenty percent gives us 5:1. Case saves a lot of energy at the lower capacities and it does a few other good things. It takes the slight valve out of frequent motion. If you're gonna use speed control almost exclusively, then there's a lot less wear on the the slide valve and the mechanical parts. It also gives the ability to load and unload very rapidly. That is a big plus in applications where the load comes and goes. But for example, bottling plants. The the break of bottle on a bottling plant line, the load disappears, and then it also will come back quickly once they fix the break in the line. So we often put VFDs on bottling plants. Lower speeds also reduce noise, And the condomizers which we haven't talked about yet, but we will in our next session can be used at all percent look. Quite often, we turn them off as the slide valve unloads. But with the VFD, you can can leave them on and gain additional system efficiency. This is a little bit of a complicated slide, but I like it. This is trying to show the variable volume ratio, ability to adjust against capacity from all stream. So our two point two BI is the lowest we generally go. Five BI is the highest we generally go. But as we start to reduce capacity, in an application. The BI, particularly for unloading from five BI, the BI will reduce with the capacity It's just the geometry of the slide valve and the slide stop. So by the time we're eighty percent capacity, we've dropped our volume ratio from five down to about four. So that can introduce some under compression losses in that application as we start to unload. When we look at only unloading 20% and already losing 20% of our BI, then that's something we'd like to avoid. And a VFD allows us to do that. So you can picture that if we can leave the slide valve and the slide stop at whatever position is optimum port location, district port location, and just change the capacity by changing the speed, we can get perfect BI control over the entire capacity range from a hundred percent down to however low our motor speed can go. So I've shown 20% here. So this is significant. You can see the huge difference in volume ratio that we can achieve at the low percent capacity. This is really one of the things that makes the FDs quite desirable and also more so at high compression ratio because you can see the low compression ratio curve is here as we're unloading from this point, there isn't such a big drop in BI, just as a slide that one learns, but we can still maintain this BI with the VFD as as well. Some of you might know that Frick also makes a range of machines that are adjustable at a lower volume range. So from one point seven to three point o, what we call chiller ports or chiller compressors, it's like water chillers. They are typically operating in a lower compression ratio range, a lower BI range than normal refrigeration applications, So but again with VFPs, we can perfectly optimize the vi on those machines as well. All way down the capacity range. So we'll look at how you would put systems together with the FDs. In most applications like cold storage or processes where the load is fairly fixed, we would generally put one VFD per suction temperature level. And you'll trim with the VFD machine And when you need more capacity, you'll turn on a slide valve machine, load it up all the way, but all the while still just trimming with the VFD machine. And as I mentioned in applications with rapidly fluctuating process loads, there we might put VFPs on all the machines. Okay. One of the last topics in this section is bearing. So I wanted to touch on bearing type So there's two basic types of bearings used in Scrum sheets, sleeve or journal bearings, and quite often the tilting pad thrust will go with those and then anti friction bearings. So we're using cylindrical rollers in place of the sleep bearing. We use a ball bearing, thrust bearing in place of the tilting pad. Now, when compressors are designed, there's two basic force directions on both the male and the female rotor. The radial force is up and down. Axial force is in and out. The thrust bearings are used to hold the rotor in the axial position so that we we don't strike the discharge end wall. As the forces change on the machine, and the roller bearings are reacting to this radio load. So they're holding the the rotor in position up and down and side to side. We like anti friction bearings, all of the Frick compressors, most of the Frick compressors are designed with anti friction bearings. And for a couple reasons, the first one, they're called anti friction bearings because they have less friction. Sleep bearings will consume more power primarily because they constantly share the oil film in the sliding surfaces, to support load, anti friction bearings are more of a rolling motion. So there's much less parasitic power loss in turning an anti friction bearing over and supporting the load. They have less internal clearance. If you look at the radial clearance in a cylindrical roller bearing, versus a comparable journal bearing at the same load, there's less than half of the clearance required in the bearing. That means we can design with tighter, rotor to rotor positioning, which helps to reduce gas leakage. That can be very significant on high compression ratios. Enter friction bearings also are tolerant to a loss in oil supply because they're sitting in cavities that contain static oil, they can start up without having the requirement for an oil pump in most cases. So we have a cross section again showing these red areas, which are oil reservoirs, You could say that drains out of those cavities are kinda high. So all of the oil will never leave the bearing cavities. So in the first revolution of the shaft, the rollers will dip through these oil reservoirs and the balls, and they can run on that static oil supply for quite some time while the flowing oil is beginning to come through the machine and put more oil into the cabinets. Now, One of the things we have to consider in refrigeration system is a behavior of oil in screw compressors. Again, a difference between a reset and a screw. A reciprocating compressor has its oil stump on the low pressure side of the of the housing. This crew compressor has its oil stump in the oil separator, which is on the high pressure side the housing. So the oil is sitting under relatively high refrigerant gas pressure. It is a characteristic of of any liquid sitting under high pressure gas that a certain amount will be absorbed into the oil. And whenever that oil then goes through a pressure drop, like I've shown a valve here, it doesn't look like oil when it comes out. It looks like shaving cream or foam. If you've ever drained oil out of a screw compressor system for oil analysis, you know that you have to wait a while until the shaving cream turns back into something that looks like oil. So that That's the nature of what we are lubricating with. We have a very foamy oil after any pressure drop. So because of that, we have to know that our oil is behaving that way. So we're absorbing refrigerant into the oil. It'll come through an oil cooler. Through filters, all of those have pressure drop, it is not desirable to put a foamy oil into a journal bearing. Because the journal bearing it supports load by collapsing all those bubbles and producing a hydrodynamic film. As bubbles collapse, you can get metal to metal contact anal journal bearing. So for that reason, you really don't see journal bearing machines designed with this type of system. The difference is an oil pump. So if you got a journal bearing machine, you'll typically have a full time high pressure oil pump that will keep the oil pressurized into the journal bearings so that all those bubbles are kept in solution and you have a solid oil to support load. In the in the journal bearings. Now when we look at anti friction bearings, they're a little different. Anti friction bearings don't have as much a requirement to avoid foaming. So we have these cavities that we're injecting oil into them. We know that we have the potential to have bubbles coming in with the oil and actually we intentionally make it worse. We vent these cavities to low pressure. That causes more foam, and it will cause the oil to de gas. And there's a couple reasons for that. One, it takes less power to run these bearings in we'll call it shaving cream than it does in a solid oil, so we can reduce power by the the foaming and the bearings. And also the rotation of the bearings will beat the bubbles, the gas out of the oil and give us, say, and oil supply that's sitting in these in these cavities, that's very refrigerant free. And the viscosity of this oil is much higher than the oil got all of the refrigerant in it, we can typically see three to one 5:1 one increase in viscosity. Between what's in the oil separator and what's actually lubricating our bearings. So what that really means is in most cases we don't need oil pumps on the anti friction bearing machine in most applications. We can save the cost of the pump, the energy cost of the pump and design with the lagoon. And the cost of an oil pump can be significant. We look at a three horsepower oil pump on the kilowatt usage. It's about say thirty, two hundred dollars a year at ten cents a kilowatt hour. And you know, as the pumps get a little bigger, of course it's more, but they don't run for one year. I mean, these systems go in and many of them operate twenty years. So over the life of a machine, a five horsepower oil pump can cost sixty five thousand dollars. So we certainly don't want to put oil pumps on if we don't need them, and anti friction bearings allow us to do that. We have one other trick we do on our packages. We use something called a cold start valve. The cold start valve has a high pressure spring in it and it's on the discharge line. And it basically restricts the pressure and doesn't allow the discharge pressure to drop quickly but it allows because it's the build rapidly and discharge pressure as soon as the machine starts. So we get our oil supply pressurized and get it flowing to the machine quickly. As soon as the compressor starts rotating. And then once we're up to pressure, then this this valve will power open and it will allow and no pressure no pressure drop essentially through that through that discharge, cold start valve. Now, we've heard some of our competitors. Story lines along these, they'll say, oh, we Frick never supplies oil pumps. So the compressor could start for oil and there's no pre lube or post lube of seals and bearings could cause failures. The fact is we do sell demand pumps when we need them. So if it's a booster machine or a low differential machine, we will put pumps on those. And the cold start valve is really what gets oil flowing fast without up pumps. So we can get all to a machine very quickly using this cold start valve. Anti friction bearings aren't sensitive, like sleep bearings, so they don't need that instant oil supply. And they're all of our bearings and seals are sitting in oil reservoirs. They stay wetted on shutdown. So really, why maintain and feed horsepower or kilowatts to an unnecessary oil pump? Well, unless you have sleep rings. Okay. So we're gonna stop this session at this point and we will we will continue on with our next session. Thank you for your attention very much.