How to DESIGN and ANALYSE a refrigeration system

[Applause] hey there guys Paul here from the engineering mindset.com in this video we're going to be looking at how to design and analyze a refrigeration system uh which is also the same as designing or analyzing an AC system or a chiller system as all of these are pretty much the same thing they're just a slightly different scale of size but also remember that these this is going to be from an ideal Vapor compression cycle so the performance of this would be slightly different to the real world scenario so this is the theoretical version of it now if you you're a new viewer to the channel then I'd highly recommend you check out our other videos as we cover everything to do with hbac and buildings some of the ones you'll probably be interested in are the basics of the refrigeration cycle and also the chiller Basics how they work as well as fundamentals of hbac so if you haven't checked those videos out I highly recommend you go and do so anyway straight into the video so here we've got our basic refrigeration cycle you probably know what all these components are by now but uh I'll just label them up so we've got the compressor uh then the condenser the expansion valve and also the evaporator the compressor compresses the refrigerant and pushes that around the system the condenser rejects the heat The Unwanted heat from the system the expansion valve expands the refrigerant and the evaporator absorbs The Unwanted heat that's coming in from the building it also produces the cooling which goes out to the building now to design and analiz a refrigeration system or cycle then we want to know what the thermodynamic properties of the refrigerant is going to be at four key components being point one here so or stage one between the evaporator and the compressor then stage two which is the uh what state is it in when it leaves the compressor state three is going to be be what the state is of the refrigerant when it leaves the condenser before it enters into the expansion valve and state four is going to be the condition of the refrigerant just after the expansion valve before it enters into the evaporator so the four main things we we want to know about the refrigerant is going to be its temperature its entropy its pressure and its enthalpy at all four of these points now if you haven't seen these charts before then this is the saturation Line This Gray Line here so anything to the left of this is where the refrigerant is a liquid any uh any point where it's in between this is where it's going to be uh some kind of vapor liquid mixture and anything to the right hand side of that is where it's going to be a super heated Vapor so if the line is touching on here then that's going to be a saturated liquid and if it's on this line here it's going to be a saturated Vapor so from these graphs we can see that uh point one here is going to be a low temperature it's also going to be a low pressure and it's going to be a saturated Vapor so we'll start to plot these as we go around state two we can see that it's going to be a much higher pressure so it's going to be a high pressure there it's also a high temperature and it's in the superheated region so it's a super heated Vapor now at point three over here we can see that it's still a high pressure it has reduced slightly in temperature so it's a medium temperature and it is also on the saturated saturated liquid line so it's going to be a saturated liquid and we can also see that point 4 just after the expansion valve when that refrigerant has all been expanded uh we can see that that's going to be a a much lower pressure and temperature so it's a low pressure low temperature and it's in between the Dome so it's going to be a liquid Vapor mixture now just to clarify what some of these acronyms are is put them around so T is going to be for temperature p is going to be for pressure H is going to be for enthropy S is going to be for entropy and X will be the quality of the refrigerant so uh you can see that I've put it's a zero if it's a liquid and a one if it's a vapor so that is just to work out how far between being a liquid and a vapor the refrigerant is when it's in the dome there and we'll see why that's important shortly but you can see for example um just here here x = 1 so that lets you know that it is a saturated Vapor it's right on the line uh there now there's lots of ways that you can start to design one of these systems probably uh knowing the cooling load that you want to achieve would be a very good point to start with so how much uh cooling capacity does your evaporator need but in this video we're just going to start from scratch and really all you need to do is uh watch this video learn uh how we calculate things and how we look the properties up and then you can tweak these to your own version to design the system that you need so we're going to start with the compressor so I've got a compressor that's able to provide or push 7 kg a second of refrigerant around the system I've just added the M do over here as the acronym so you know what that is but M do just means the mass flow rate the dot represents the rate now from the manufacturer's data uh this Chiller is a able to produce uh 1,200 kPa of pressure and it also needs a suction pressure of 320 kPa so we just add those values in there and now we'll start to fill in some of the the data that's missing here so we know that the pressure is 320 kPa and that it is a saturated Vapor on the on the saturation line and then we just need to look up what the values of these are going to be on the firm thermodynamic properties of refrigerant R134a or whichever refrigerant you're using fact I'll just add that in there so we know which refrigerant we're using so if you check out our website the engineering mindset.com and specifically if you go to this page here thermodynamic properties of refrigerant R134a and we then scroll down and we'll see we want to find this is the saturated refrigerant tables so we want to find 300 20 kPa which is this line here and we want to know what the temperature eny and entropy are at this point so on this line here change that a little bit uh we can scroll to the top and see that this column is the temperature this is the enthalpy and this is the entropy and obviously we're on the saturated Vapor line so uh we got the saturated Vapor of that one and saturated Vapor line for the eny as well so we're going to want this column this column and this column so we want this value here this value here and this value here so let's just zoom out and copy that over so we'll take this cell here and we'll just paste that into Excel you don't have to do this you could write it by hand as well I'm just using Excel it's a bit bit easier and quicker then we'll take the enp at the saturation saturated level and just paste that in as well then we just take the temperature and we paste that one in as well and I'll just update that front sheet there and now we'll have a look to find what the properties of the refrigerant are at state two now to look at the uh properties of the refrigerant up on the tables we need to know two points uh to reference that so at the moment we just know what the pressure is and that we know it's a super heated Vapor but because we're doing this as an ideal cycle that means it's I the compressor is isentropic and that means that the entropy at State 2 equals that at State one so I'll just drop that figure in there that's come from here and now we can look in the super heated Vapor tables uh to find the uh enthalpy and the temperature given the pressure and the entropy so if we come back to the website and we scroll down through the properties of the refrigerant R134a and you'll see we'll come to the superheated refrigerant so we're looking for 1,200 kPa so these are too low come down a bit more bit more we got one for all of them over here there we go 1,200 kPa now the entropy we're looking for is 0.931 K per kgr per Kelvin but looking down the this list you can see it's not quite there it's actually in between these two here but that's okay we'll just have to use some linear interpolation to uh find that value so if we highlight and then copy and paste uh these values here and just drop them in Excel and uh we'll just drop the values in there now we can use this formula here to calculate what that's going to be so we know these three A's and we also know the uh B1 and B3 but we don't know B2 and what I mean by that is we know uh at 50° what the entropy is going to be and at 60° what the entropy is going to be but we want to know what the enthalpy and entropy is going to be at 0.931 so therefore these will be our A's and these will be our B's now I've already pre-made the C calcor for this um so I'll just show you the formula there it's just this formula here but in the Excel version uh so we'll just drop the numbers in there so we'll take the 50° celsus drop that in there uh 60° onto here and then we'll take these three U numbers there and we'll drop them in and that will give us the temperature there which is now 50.9 de C so I'll add that to the table and then we can find out what the enp is by doing exactly the same just moving uh this enthalpy over and this enthalpy over using that same formula and that will give us what the enthalpy is at the correct entropy that we need so we'll just copy and paste that over into our table obviously we know that the entropy was the same as this cell here and we'll just update the overall table there now as this is an ideal system there is no uh resistance in there so there'll be no pressure drop in the real world obviously there will be some pressure drop but for this ideal scenario then we're just going to take the pressure from here and we'll assume that is the pressure at state three so now we've got the pressure and we know that it is a saturated liquid and that means we can use the saturated liquid tables to find out the temperature eny and entropy so coming back to the site and the properties tables uh we'll just scroll down we've got the saturated refrigerant tables there and we're looking for 1,200 kPa there's the pressure on this line here and we know that it's a saturated liquid so we want the saturated liquid column so the entropy we want this one here the enth will be we want this column here and obviously we've got the temperature over there so we just scroll down and uh so we want this value here and this value here as well as this temperature here so we'll just copy and paste those into the Excel sheet so there you go we've just dropped these in there and we also know that this pressure is going to be the same as P2 and I'll also just update the front sheet there and now we need to know what the properties of the refrigerant are going to be at State 4 and this is slightly more tricky because obviously it's in between the vapor Dome so it's part liquid part vapor and we're not sure exactly yet how much vapor or liquid it is but that's okay CU we can just work it out so we know here that the temperature is going to be the same as 0.1 and that the enthalpy will remain constant through the expansion valve so that means we can use the enthalpy from state three so if we drop those figures in there and also drop them into our Excel sheet that's State four now we also know what the pressure is because that is going to be equal to State one and that means we only need to find out what the entropy is and the way we do that first is by finding the quality of the refrigerant so how much or what part uh liquid or vapor do we have and for that we come back to the saturated refrigerant tables and we scroll down until we find the 320 kPa line and then we want to copy and paste all these values or just the entropy enthalpy uh temperature and pressure and we want to take those for both the saturated uh liquid and Vapors and now to find the quality of the refrigerant we're going to use this formula here so X being the quality and H representing the enthalpy so we already have the value of H4 because that was equal to the enthalpy at point or state three and we also know what HF and HG are because we've got them here on the table so HF is the saturated liquid and HG is the saturated Vapor value and we took these from the uh charts on the website so now uh we'll just use this formula here but I've put them into this cell here and you can see the formula up here um is exactly the same as this one here and when you drop these numbers out you'll see that this comes out as a decimal because it's a ratio and so it's 31 almost 32% so now we use that uh quality that we've just calculated there to work out what the entropy is going to be at State four and we do that using this formula here so we've just worked out what the quality is at State 4 um we know what uh SF and SG are going to be because we've taken them also from the tables and you can see SF is the saturated liquid and SG is the saturated vapor uh for entropy sorry so if we uh use this formula here just this one here exactly the same there just uh in the Excel version and we'll see that S4 drops out at 0.443 6 uh K per kilogram per Kelvin so that's all the uh properties of the refrigerant worked out but we still got some more calculations to do so let's work out the amount of work done by the compressor so we can do that using this formula here um so we've got the enthalpy of State 2 and state one and the mass flow rate of the refrigerant so how much refrigerant is the compressor pushing around and uh we can use this formula up here which is using these cells to uh work out it's the compressor is doing 82.2 n Kow of work on the system and we can also work out how much cooling or the cooling load on the evaporator by using this form here and it's almost exactly the same formula we're just using the enp from 0.1 and the enp from 3 and we use this formula here to work out that it is providing 402 kilow of cooling and we also want to know or calculate what is the heat rejection by the condenser and we can work that out using this formula here again very simple and similar formula uh and just using this formula here and that we can see it's 480 485 KW now you notice that that is higher than the cooling provided and that's because we've got to get rid of the heat uh produced by the compressor as well so you can add these together to uh to check if your figures are correct so if you added the EVAP uh the cooling load plus the work done by the compressor you should equal the heat rejected by the condenser if they don't equal then you've done something wrong and you need to go back and just have a look through some of your figures and then we can calculate the efficiency of the system or the coefficient of performance using this formula here which is just the cooling load divided by the work done by the compressor and we can use this formula here very simple just a division the ratio uh and that will work out that this system we've just looked at here is producing or has a coefficient of performance of 4.89 so for every kilowatt of electricity you put in you'll get 4 .89 KW of cooling and that's a very efficient system now if you want to know what the temperature of the air coming off the coil so the air on the in this case the evaporator and the condenser are both fan based so if you want to know what the temperature of the air coming off of this is going to be then you can have a look at this video here which is the hbac cooling Coe plus calculations video and in there you will see that we've already done the formula for that for cooling coil Outlet air temperature and that's the temperature of the air coming coming out uh there's a couple of bits you need to know such as the temperature of the air going in etc etc have a look at that video I think it will really help you and I'll also add a link at the top here so you can see that too but anyway that's it for this video thank you very much for watching I hope this has helped you please don't forget to like subscribe and share and if you have any questions or comments please leave them in the comments section below also don't forget to check out our website the engineering mindset.com once again thanks for watching

Channel: The Engineering Mindset

Views: 193,238

Keywords: Design, ac, chiller, hvac, refrigeration, system, condenser, mechanical, thermodynamics, evaporator, central plant, chiller plant, ahu, chilled water, condenser water, chilled water system, air handling unit, compressor, refrigerant, expansion valve, building services, engineering, how to, calculate, hvac hacks, hvac training videos, hvac basics, fan coil, Online HVAC Training, HVAC Training, Online HVAC Class

Id: TPabv9iDENc

Length: 18min 26sec (1106 seconds)

Published: Sun Aug 06 2017