How to Read a Psychrometric Chart

[Music] welcome back to Tec to this topic is gonna be on psychometric chart which is the most exciting thing in all of HVAC I'm just kidding it's not that exciting but it is important to know how this stuff works and you can solve a lot of problems by understanding what to do with these charts so many of you may have a chart like this if you don't have one and you want one let us know will queue up or come to our TDP classes and we use these quite a bit in there this chart has all the properties of air to find on it right so this is like a hundred-year-old chart not this exact one obviously it's one's laminated right but the concept is 100 years old Willis carrier if you didn't know that a lot of laboratory testing and develop this chart and then HVAC engineers forever have been using this to plot out processes and conditions so we'll take a little dive into this chart and explain how this kind of works let's do that now so we're gonna pull all psychometric try the computer here so it'll be a little bit easier for us to see what's going on so this is our typical psychometric chart is actually almost the exact same one I just showed you a minute ago social few things out here to pay attention to and you can see on the others our ton of lines on here there's just overlapping Cris crossings swooshing lines they're all over the place right so it's a lot to go on so we're going to break it apart piece by piece and look at it that way it's a little bit easier to digest that way so on our chart here these are the main things that we're going to deal with there's even more than this and we'll have advanced training for you guys later on but this is the basic stuff so we were looking for wet-bulb dewpoint temperature dry bulb temperature specific humidity and relative humidity those are the five major things that we're gonna work with and maybe we'll dip it the enthalpy a little bit here in a minute so our chart we build it up from scratch so along the bottom axis here we have our dry bulb temperatures dry ball means as the name would imply there's no moisture involved in the discussion all right have a dry bulb thing you know there's gonna be a wet bulb thing a little bit here right so dry bulb is just straight-up temperature if I'm old school mercury thermometer it'd be dry Pollock just a poor Sonja that goes along the bottom access I'm the vertical access off the side I had the specific humidity now specific humidity is very different than relative humidity it's not measured in percent like most of you probably think of you maybe it's measured in grains so the easiest way to kind of conceptually think about that is every little water droplet if you will is a grain of moisture no grains actually smaller than a water droplet but conceptually it's a bunch of little water droplets all in the air so how many grains of moisture do I have i took a box of air and count up all the grains how many would there be that's on this vertical access here so let go crossed here is honored grains there's 80 grains of moisture and so on so I plot one out in order to plot something on this chart need to know two of those five variables and once I know those two I can figure out what the other three are on the chart that's kind of the purpose of the chart so if I take a typical room that's 75 degrees with 60 grams of moisture how to plot a dot right there I can do that with any of these any of these numbers a few more things on this chart up along that side there knows the chart is missing right and it was missing over here too the chart cuts off right there there's nothing there that's because that end curve line there is the saturation line it means we're 100% saturated the air is holding the maximum amount of moisture that it possibly can hold so there's no reason to build the chart further than that we're never going to be over in that range at least in the HVAC was we're never gonna be over there so that's the saturation line here also known as 100% humidity I can start adding additional lines under the chart so I got my same point I measure my seventy-five degrees my 60 grams of moisture if I was to read the chart by going all the way to the left right here it would be 53 degrees dew point I can read that right here on these numbers but they're hard to see 50 55 60 or I could take it all the way down and read III there the point is if I take this line I extend to a lot I'll be the dew point they probably know the dew point from watching The Weather Channel or whatever the dew point means that if I ever get below the dew point in terms of actual temperature it's going to condense it be fully saturated and since this is the saturation curve up here the dew point is measured at the saturation curve not over here in the middle of the chart so it's then the line that a lot that can read the dew point for take an example we'll use an HB HC example set of a weather example so I have this space above the ceiling where it's really warm right above my drop ceiling and my commercial building on this example it's 95 degrees up there with 100 grams of moisture that's the condition of that space or it's a residential house and it's the Attic 95 degrees up there 100 grams of moisture so I plot 95 degrees with all hundred grams of moisture I can extend that line all the way to the left and see that I have a 67 degree dew point that means if I put anything in the Attic that's below 67 degrees it is going to condense and sweat so I hope there was a cold can of beer it's going to convince and sweat right if I have a dog running in the attic and it's 55 degrees on insulated duct it's 55 degrees it's gonna sweat on the outside of the duct it's going to condense it's gonna drip down into the insulation on the floor of the Attic all right so I need to either get this attic to not have as much moisture going to raise this temperature of the inside the duct or need to wrap and insulate adopt that's how I could tell them something's gonna happen like that relative humidity which is the humidity you verbally talk about when you tell your friends oh yeah it's gonna be eighty percent you made out tomorrow or whatever right that humidity is kind of special I it doesn't mean the same thing all the time the relative part means the humidity is relative to the temperature you're at so 80% humidity on one day is very different than 80% humidity on another day and the way that works is if I take my same point in this case 75 degrees with 60 grains of moisture the relative humidity is these curved lines so there's the 50% curve there's the 40% curve on there in this case I'm half of this means I want to call it the 45% curve anything on that red curve is 45% humidity but you can see I 60 grams of moisture on here if I was to come over here and say it's 90 degrees and I'm on the red curve 45 percent now it's actually 95 grains of moisture different amount of moisture these numbers here are real moisture these curved lines are the moisture as relative to the temperature around it with the idea of being the hotter the air is the more moisture can hold the cooler it is the less it can hold and the most it can almost 100% which is the saturation pair of hair so I want to figure out the relative humidity we've got my 75 and 60 grains if I draw a line straight up and say well it's 75 degrees how much moisture could I hold before there was a condensation problem at 75 degrees so though all the way up to 100% humidity and I read it across I could hold 132 grains of moisture but I only have 60 so I have 60 out of a maximum of 132 that's 45% relative humidity that's how I'd be reading that do a quick example a common example is a residential window in the wintertime all right sometimes they sweat and they drip and the windowsill it gets all wet and all that stuff so if it's really cold outside let's just say it's zero degrees outside that means it might be 35 degrees on the inside of a double pane glass window so the glass surface inside the home is 35 degrees if my house is 75 degrees inside with 23% humidity I'm good at this point right here if I stay at the 23% maybe that's the most I can hold but if I was to raise the humidity up to 30% I have a condensation problem if I lower my humidifier down to 20% I would not have a condensation problem I can set my humidifier up and down to avoid the windows from condensing or I can keep the windows would that be kind of hard to eat 30 windows in someone's house sling psychrometer I'm sure all of you guys out there at home had one of these I'm sure you carry it on your truck you use it all the time I'm just kidding if I don't have one of these but you may know dude if the officer is old enough to have one I don't even have one anymore and I'm becoming the old dude around here lately but I use a digital one for the past I don't know seven eight years I've used this digital she's probably ten years now it's kind of nice that shows me temperature in this case on air and relative humidity so it's 72 degrees with 20 percent relative humidity and unfortunately as I hold it and as I breathe on it it keeps going up so it's kind of weirding the video here but I can also rotate it between showing me the dew point 33 degrees the wet-bulb 53 degrees back to the temperature so I got into a little mini psychometric chart built into here which is kind of nice so I can see what the wet-bulb and dew point are without having to measure the dry bulb and go into this and convert it over I can see all those variables at the same time in the old days of user sunshine comet err which is nothing more than a thermometer we put a little wet sock on it and then fling around the movie air acrostic which was the swing part of the discussion and that wet sock would evaporate into the air and the temperature would be lowered because that so I've read the dry bulb on there and then I'd read the wet bulb on there with a wet sock if you will and I'd use that to come on here and plot it then I'd figure out everything from there relative humidity two-point etc now modern tools you can do it all in one [Music] the wet bulb is right on the chart in a little bit of a different way so that same point we talked about before 75 degrees with 60 grams of moisture if I read across this way I get the dew point at 53 but if I read out this 45-degree angle line I get to a wedding bulb of 61 so what bob was on is very right on the heart on the what balls are in the diagonal line dew point is registrate across temperature is read straight down so look at all of at the same time right so wet ball is measured on the diagonal dew point straight to the left dry Bob straight down relative humidity to the pair of line specific humidity actual humidity is horizontally across to the right so I can read any one of those five variables I know two of them I could plot the point and get the other three from there because getting a specific humidity is really hard to measure the field so I have to read probably what Bob and drive up in the field and calculate Geller's from the chart and then we'll have additional training modules down the road to do more stuff with these we can plot out application processes the beautification processes things like that any direction on this chart you move you can add each remove heat add moisture remove moisture so account some combination of those things we'll talk about that in future training values so with that ever have to part ensure about to the second mushrooms thank you [Music] you

Channel: Tec Tube

Views: 189,508

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Keywords: hvac, cooling, heating, psych chart, psychometric

Id: s7J6R9wECh8

Length: 11min 20sec (680 seconds)

Published: Wed Mar 21 2018