I cannot overstate how helpful the graph and animations are. There is simply no better to deeply understand how these variables relate to each other. Thank you for making this!
You’re welcome man! Yeah I had fun with this one. I originally thought I was going to ask someone else to do my video editing for me, but I realised that the animations are actually a critical part of explaining, and I know what it is that I want to show, so it’s best if I do it myself. It also means that often it takes a long time, but I’m not looking to pump out mountains of “content”… I want to make sure that the topics I look into are covered properly and that questions are properly answered. Thanks for the support!
This video and your video about dry vs wet temperature have been so valuable to my understanding of psychrometric terms. You addressed exactly the questions I had, especially the WHY. Thanks so much, keep posting!
This chart really puts in perspective how extreme the dreaded "wet bulb events" really are. To have a wet bulb temperature above 35 degrees you have to be in the very top corner of the chart (or maybe more likely a bit further to the left and out the top at extreme humidity). Dryish air even at death valley temperatures are really not all that bad in terms of wet bulb temperature. As long as you have water you'll be fine. Evaporative cooling is really effektive when at high temperatures. You need the high humidity for it to be truly dangerous.
The short answer is, entropy is a measure of the disorder in a system, that enables us to quantify whether a process is A) impossible, B) reversible, or C) irreversible. The 2nd law of thermodynamics requires that the entropy of the universe, never decrease. This means entropy either can remain the same (for a reversible process), or increase (for an irreversible process). But if entropy of the universe were to decrease, the process would be impossible. An application of entropy, is to calculate the upper limit of performance on processes that trade thermal energy for mechanical work, or vice versa. Such as compressors and turbines. Ideally, there is no heat transfer, or change in entropy, when a substance passes thru a turbine or a compressor, or other similar component. This allows us to find the ideal output state, given a known input state, and calculate the corresponding work associated with the device from other state properties and the mass flow rate.
I have a doubt. @7:00 when moisture in cotton is taking heat from surrounding air then how does temp of thermometer decrease? the cotton should primarily take heat from thermometer's bulb. right?
Hello, Pat (if you still support the channel). Could you, please, help me with this question: then you refer to absolute humidity at some temperature, you also implicitly refer to some air density. Then you change temperature of air, density is also change. I.e 13.5 g/m3 at 30c(303K) is 13,5*(303/293)= 13,96g/m3. Is my calculation correct ?
I don't understand the part where you say that the water from wick enters the air around bulb and increases its RH(relative humidity). But in a large enough room, the water evaporated from wick will have negligible effect on the room's air RH. Then by that logic the temperature can keep falling forever.
No because evaporation and convection will be balanced. To lose more heat by evaporation you will need new dry air in contact with the bulb, but that air will also be hot. So if you put a fan next to the bulb you will both heat it more and cool it more so the final temperature will be the same. I guess in principle you could put the moist, cool air through a heat exchanger with some of the warm, dry air. This dry air will then be cooler, but at the same absolute humidity and thus lower wet bulb temperature. Then you could repeat and gradually approach the dew point. But this process still won't get you lower than the dew point.
Hey Maksim! You are absolutely right, which is why a true WBT measurement is one in which the air around the wick has reached saturation, and if it's saturated the velocity would not affect the rate of evaporation. I have copied and pasted my reponse to a similar kind of question from my main WBT video, which might be interesting for you. Let me know if it helps: "When measuring WBT we are losing energy due to water evaporation; by energy balance, the material of construction of my thermometer has a specific heat, and the higher this specific heat, the less the temperature of that thermometer will drop, affecting the reading, right? But here's the deal - we can then go and replace the air we have just saturated with a bunch of fresh, unsaturated air, and cool further. So the question becomes more about time taken to get to the reading more than the material itself. You will eventually cool it down to the WBT reading. But it gets more interesting and more complicated. What stops me just repeating this process indefinitely? I can keep replacing the saturated air with fresh air, keep evaporating water, and keep cooling. Does that mean we have infinte cooling to as low a temperature as we want? The answer is no. There are actually three different tempratures here at the start: the temperature of the air (dry bulb), the temperature of the thermometer, and the temperature of the water. These three can be, and probably are, different to begin with. As you start evaporating we start getting heat transfer between these three entities until you get to some sort of equilibrium temperature profile (note PROFILE - I won't realistically get all three copmponents to same temperature). Eventually you cool down the air and thermometer, and you cool down the water. Since the water temprature drops, so does its vapour pressure AND HENCE ITS TENDENCY TO EVAPORATE AND COOL. The air is saturated when the partial pressure of water in the air equals the vapours pressure of the liquid. Learn this - this is a definition. So you can't cool infinitely because you cool down the liquid and diminish its vapour pressure. This touches on something called the psychrometric ratio, which ackowledges that there are two temperatures here: 1. There is the thermodynamic wet bulb temperature - the "true" or theoretical limit of the WBT that I achieve when any gas becomes saturated with any liquid (not just air and water). This is the WBT I describe in my video "The difference between WBT and Dew point". To get this I would need a perfectly insulated box in which I saturated & cooled the gas by evaporating liquid inside the box inside this box and and acheived a unifrom temperature inside. Note that I don't care what sort of thermometer I am using to measure this temperature in my perfect box. 2. Then we have a realistically measured WBT, you could probably refer to this as simply an equilibrium temperature which is what we usually measure. This temperature reading stabilises at some value based on the mass transfer rate of water into air as it evaporates (mass transfer coefficient) and the convective cooling rate (a heat transfer coefficient). Depending on those two transfer coefficients there may be a large difference between the thermodynamic WBT and the equilibrium temperature measured during a WBT experiment in your kitchen. For water in air there is no significant difference purely by coincidence."
@@ProcesswithPat But when the air around the wick has reached saturation, sure air velocity won't affect the WBT because it'll be equal DBT and there won't be any evaporation at saturation. I still don't get it.
It’s the delta from whatever the initial humidity is TO saturation… So the cooling effect from initial to 100% humidity creates the delta between dry and wet bulb. So I think you get it, but maybe I’m struggling to give you the “aha!” moment. Yes, the air velocity will affect it, but in theory imagine you locked their around the bulb at initial humidity, and allowed the water to evaporate with zero heat transfer elsewhere. Then the “velocity” (I know it’s a little dumb cos we’re in closed box, but pretend we can control the velocity without adding energy) would determine how QUICKLY you arrive at the wet bulb reading, but not the actual final reading itself. Make sense?
Hey! To be honest I just Google things as I need them. Occasionally I use an old university text book if needed, but nothing specific really… Also, just call me Pat!
You tried SO hard to make this understandable, and I'm still completely lost. It's not your fault, my brain just cannot comprehend it. Air doesn't have pockets to carry stuff in. It makes zero sense to me.
I cannot overstate how helpful the graph and animations are. There is simply no better to deeply understand how these variables relate to each other. Thank you for making this!
Hands down the best video explaining the Psychrometric chart. Great job!
Great explanation once again Pat! They really do sound like the same thing, but this helps clarify the difference very well 👍
The effort u hav put into making this video is amazing.. great explanation and perfect use of animation..good job Pat
You’re welcome man! Yeah I had fun with this one. I originally thought I was going to ask someone else to do my video editing for me, but I realised that the animations are actually a critical part of explaining, and I know what it is that I want to show, so it’s best if I do it myself. It also means that often it takes a long time, but I’m not looking to pump out mountains of “content”… I want to make sure that the topics I look into are covered properly and that questions are properly answered. Thanks for the support!
This video and your video about dry vs wet temperature have been so valuable to my understanding of psychrometric terms. You addressed exactly the questions I had, especially the WHY. Thanks so much, keep posting!
what a nice explanation. this is the first time i understood this chart. Thank you
You’re welcome!
You using the psychometric chart really helped in the understanding of wbt, dbt and dew point. Thanks!
I don't usually comment on TH-cam videos, but I have to say this was impressive.
Very cleary explained....shows good knowledge
I took a three days course for Water Damage Restoration Tech, didn't get it but you explained within 10 minutes 🙏🙏 , Thank you so much!!
Glad it helped! That’s what it’s there for! Best of luck!
I think this video is one of the best video so far I have. Thank you for making video
One of the best examples I’ve found so far thanks.
Good explanation and easy to understand..And wet bulb temp is used in cooling tower and dew point for compressed air system..please do video on this.
I've been studying for the FE and this was a great explanation
Awesome, best of luck!
Extremely well explained video. Thank you so much.
Wonderfully explained, thank you!
Great explanation. I am quite new in this and just trying to learn especially for HVAC system. Thank a lot.
you are a great teacher!
very truly helpful, thanks
One of the best videos in this topic!
Excellent explanation, thanks for making such video
You are a great teacher, sir
I am an I&C Engineer. Thanks for making this video. Thumps up from Pakistan .
You are most welcome!!
And Pakistan is one of the places in the world where the concept of wet bulb temperature risks becoming very relevant with climate change.
Thanks for your great effort!
I am looking this type of videos. 😊
This chart really puts in perspective how extreme the dreaded "wet bulb events" really are. To have a wet bulb temperature above 35 degrees you have to be in the very top corner of the chart (or maybe more likely a bit further to the left and out the top at extreme humidity). Dryish air even at death valley temperatures are really not all that bad in terms of wet bulb temperature. As long as you have water you'll be fine. Evaporative cooling is really effektive when at high temperatures. You need the high humidity for it to be truly dangerous.
Amazing explanation!
Very well explained. Could you make video on entropy and its uses in real life
The short answer is, entropy is a measure of the disorder in a system, that enables us to quantify whether a process is A) impossible, B) reversible, or C) irreversible. The 2nd law of thermodynamics requires that the entropy of the universe, never decrease. This means entropy either can remain the same (for a reversible process), or increase (for an irreversible process). But if entropy of the universe were to decrease, the process would be impossible.
An application of entropy, is to calculate the upper limit of performance on processes that trade thermal energy for mechanical work, or vice versa. Such as compressors and turbines. Ideally, there is no heat transfer, or change in entropy, when a substance passes thru a turbine or a compressor, or other similar component. This allows us to find the ideal output state, given a known input state, and calculate the corresponding work associated with the device from other state properties and the mass flow rate.
fantastic Video
great explanation boss!
Top videos champ! The live graph helps immensely, you speak well and are thorough in explanation and make it easy for our brains to process.
Great explanation.
Thanks, Project Pat! 😆
(Project Pat is an old school rapper.)
Great Video!
Good presentation 👍
What's the duty cycle of a dehumidifier? Usually it's not perfectly efficient, so the temperature goes up a bit while the absolute humidity goes down
Thank you
I have a doubt. @7:00 when moisture in cotton is taking heat from surrounding air then how does temp of thermometer decrease? the cotton should primarily take heat from thermometer's bulb. right?
amazing work
Thanks a lot!
So is a swamp cooler essentially moving left along a line of constant enthalpy?
Hello, Pat (if you still support the channel). Could you, please, help me with this question: then you refer to absolute humidity at some temperature, you also implicitly refer to some air density. Then you change temperature of air, density is also change. I.e 13.5 g/m3 at 30c(303K) is 13,5*(303/293)= 13,96g/m3. Is my calculation correct ?
I found an obvious error in my logic: y-axis is g of water per kg of air, not a volumetric value.
I don't understand the part where you say that the water from wick enters the air around bulb and increases its RH(relative humidity). But in a large enough room, the water evaporated from wick will have negligible effect on the room's air RH. Then by that logic the temperature can keep falling forever.
No because evaporation and convection will be balanced. To lose more heat by evaporation you will need new dry air in contact with the bulb, but that air will also be hot. So if you put a fan next to the bulb you will both heat it more and cool it more so the final temperature will be the same.
I guess in principle you could put the moist, cool air through a heat exchanger with some of the warm, dry air. This dry air will then be cooler, but at the same absolute humidity and thus lower wet bulb temperature. Then you could repeat and gradually approach the dew point. But this process still won't get you lower than the dew point.
air velocity affects the rate of evaporation from the wick. why does it not affect the wet bulb temperature?
Hey Maksim! You are absolutely right, which is why a true WBT measurement is one in which the air around the wick has reached saturation, and if it's saturated the velocity would not affect the rate of evaporation. I have copied and pasted my reponse to a similar kind of question from my main WBT video, which might be interesting for you. Let me know if it helps:
"When measuring WBT we are losing energy due to water evaporation; by energy balance, the material of construction of my thermometer has a specific heat, and the higher this specific heat, the less the temperature of that thermometer will drop, affecting the reading, right? But here's the deal - we can then go and replace the air we have just saturated with a bunch of fresh, unsaturated air, and cool further. So the question becomes more about time taken to get to the reading more than the material itself. You will eventually cool it down to the WBT reading.
But it gets more interesting and more complicated. What stops me just repeating this process indefinitely? I can keep replacing the saturated air with fresh air, keep evaporating water, and keep cooling. Does that mean we have infinte cooling to as low a temperature as we want? The answer is no.
There are actually three different tempratures here at the start: the temperature of the air (dry bulb), the temperature of the thermometer, and the temperature of the water. These three can be, and probably are, different to begin with. As you start evaporating we start getting heat transfer between these three entities until you get to some sort of equilibrium temperature profile (note PROFILE - I won't realistically get all three copmponents to same temperature). Eventually you cool down the air and thermometer, and you cool down the water. Since the water temprature drops, so does its vapour pressure AND HENCE ITS TENDENCY TO EVAPORATE AND COOL. The air is saturated when the partial pressure of water in the air equals the vapours pressure of the liquid. Learn this - this is a definition. So you can't cool infinitely because you cool down the liquid and diminish its vapour pressure.
This touches on something called the psychrometric ratio, which ackowledges that there are two temperatures here:
1. There is the thermodynamic wet bulb temperature - the "true" or theoretical limit of the WBT that I achieve when any gas becomes saturated with any liquid (not just air and water). This is the WBT I describe in my video "The difference between WBT and Dew point". To get this I would need a perfectly insulated box in which I saturated & cooled the gas by evaporating liquid inside the box inside this box and and acheived a unifrom temperature inside. Note that I don't care what sort of thermometer I am using to measure this temperature in my perfect box.
2. Then we have a realistically measured WBT, you could probably refer to this as simply an equilibrium temperature which is what we usually measure. This temperature reading stabilises at some value based on the mass transfer rate of water into air as it evaporates (mass transfer coefficient) and the convective cooling rate (a heat transfer coefficient).
Depending on those two transfer coefficients there may be a large difference between the thermodynamic WBT and the equilibrium temperature measured during a WBT experiment in your kitchen. For water in air there is no significant difference purely by coincidence."
@@ProcesswithPat But when the air around the wick has reached saturation, sure air velocity won't affect the WBT because it'll be equal DBT and there won't be any evaporation at saturation. I still don't get it.
It’s the delta from whatever the initial humidity is TO saturation… So the cooling effect from initial to 100% humidity creates the delta between dry and wet bulb.
So I think you get it, but maybe I’m struggling to give you the “aha!” moment.
Yes, the air velocity will affect it, but in theory imagine you locked their around the bulb at initial humidity, and allowed the water to evaporate with zero heat transfer elsewhere. Then the “velocity” (I know it’s a little dumb cos we’re in closed box, but pretend we can control the velocity without adding energy) would determine how QUICKLY you arrive at the wet bulb reading, but not the actual final reading itself.
Make sense?
❤
Woo woo woo what's your book source sir😁
Hey! To be honest I just Google things as I need them. Occasionally I use an old university text book if needed, but nothing specific really…
Also, just call me Pat!
You tried SO hard to make this understandable, and I'm still completely lost. It's not your fault, my brain just cannot comprehend it. Air doesn't have pockets to carry stuff in. It makes zero sense to me.
Still trying to understand it?
Air does have pockets to hold stuff in. It’s is mostly empty space.