Mastering Cyclometrics: Understanding the Psychometric Chart for HVAC Applications

psychometrics wait no that's not right cyclometrics is the study of the properties and processes of moist air or

air that is a mixture of water vapor and dry air in other words this is the air we

interact with every day you may not realize it but all air has some water in it even if you go to the

driest desert there will always be some moisture hi and welcome

my name is hermann tabor and today we're going to be learning all about this the psychometric chart and how it can be an

extremely useful tool for hvac applications but first let's take a look at the history of how this chart came to

be let's go back like way back a little more there in the year 340 bc greek

philosopher aristotle published a series of books titled meteorologica which were the first writings to focus on

atmospheric phenomena such as rain clouds mist do and even rainbows

this was the start of meteorology and scientists and physicists continued to develop their ideas gather data and

build improved weather instruments throughout the medieval era and beyond then in 1765 james watt perfected the

steam engine taking the world by storm with the design up to 75 percent more efficient than existing machines it was

one of the main factors that kickstarted the industrial revolution if there was ever a time to further our knowledge on

the relationships and processes between air water and heat this was it steam tables quickly became a useful

tool for steam engine design as a quick reference system that allowed for the bypass of complex calculations

meteorologists were quick to catch on and began using tables for the same reasons but they were tedious and

lengthy to say the least then in 1904 when willis carrier was developing some of the first air

conditioning systems in the world he conceived of a chart that would save him time and effort by displaying

psychometric data in a specific format for his needs this is what he came up with the basis

of the psychometric chart as we know it today in order to learn more about the chart i

traveled to lankster pennsylvania to visit bill griffin bill is the president of engineering and

manufacturing at captive air he is without question the head honcho at this r d facility has the dcv

software been updated keep working on stringing these up in the lab bill deals with psychrometrics every day making him

the perfect teacher when it comes to the chart so thinking about the psychometric chart

and and why we need to know things about it if you know two parameters of moist air you can figure out every other

property of moisture using this chart one example is how do we know when condensation is

going to form on this water bottle well using the psychometric chart we can figure that out so let's take a look at

the parameters all right so one of the first things to start to look at on the psychometric

chart is called the dry bulb temperature which is the the scale on the bottom of the chart for example a 90 degree dry

bulb temperature is constant along this line here this is the temperature at which most

people refer to when they say it's 75 degrees outside that's called the dry bulb temperature dry bulb temperature is

a direct measure of the kinetic energy of air with no regard to its vapor content

the first modern thermometer invented by mr fahrenheit used this principle as the kinetic energy of the air around the

thermometer changes so does the kinetic energy of the mercury within the thermometer causing it to expand and

contract add a scale to it and you've got a bulb thermometer the term dry bulb comes from the fact that the measurement

is taken with a dry bulb so in contrast to dry bob there's something called wet bulb temperature of the air and the wet

bulb temperature is located along what we call the saturation line of the psychometric chart and it's the curved

line to the left side of the chart but the constant wet bulb temperatures are at a diagonal line on the

psychometric chart they start at saturation and fall down to the right the wet bob temperature is the air

temperature when it's fully saturated so for example if you were to take a typical thermometer

soak it in water and move air across it air velocity across it what the reading would be would be the wet bulb

temperature so how does this work when the thermometer bulb is wet and air is moved across it the water around the

bulb evaporates causing the bulb to cool when the air is dry the water will evaporate easily causing a large drop in

temperature when the air is moist the water will not evaporate as fast causing a smaller temperature drop this is a

sling cyclometer it's a pretty neat old-school device and it has a drywall thermometer and a webble thermometer

attached to an assembly in a sling so we go ahead and spin it around causing airflow to interact with both of these

thermometers the drywall thermometer doesn't really care about this airflow it doesn't

change its reading but the wet bulb thermometer is dependent on evaporation so it needs

this airflow today is pretty humid out here so the temperature drop on the wet bulb won't be very high

and if it was raining right now both of these thermometers would essentially read the same thing as there'd be no

evaporation happening on the wet bulb so after we spin it around for a while we get these two readings and we can take

them and plot them on the psychometric chart and from there we can get other useful parameters for example relative

humidity relative humidity is the next most commonly used property of moist air and

relative humidity is along curved lines that are approximated along the entire psychometric chart

from the bottom of the chart the lower axes that is zero percent relative humidity and as you progress up

to the left of the chart the relative humidity becomes higher so this is the 10 percent line 20 percent

line 30 percent all the way up to a hundred percent or saturation one way to easily understand relative

humidity is to think about a sponge if you wring a sponge out the moisture content in that sponge starts to drop so

that means that that sponge can absorb much more water at some point in time so you can think of relative humidity the

same way where a dry sponge would have zero percent and if a sponge has absorbed as much water as possible and

can't absorb anymore that would be a hundred percent so the relative humidity is a ratio of

the current moisture content in the air compared to the state of that same air at a hundred percent saturation it's

worth noting here that relative humidity is highly affected by air temperature the size of the sponge or how much

moisture can exist in the air varies with temperature the hotter the air the more water vapor the air can hold but we

have to be careful here because air doesn't actually hold water as it's somewhat commonly said let's look at

what actually happens in this mix at a molecular level we already know that the motion of these molecules changes with

temperature make it hotter and there will be more energy make it colder and there's less going on

as far as the nitrogen and oxygen in the air are concerned this is all fine and good

but for water things are different as it's a polar molecule like a magnet it wants to stick to others of its kind

this is what we call condensation when enough water molecules bind and turn into a liquid

there are two basic factors that determine how likely a water molecule is to bind how fast it's moving

and the space between it and other water molecules this is also called vapor pressure

as we increase the temperature of the water vapor the molecules move fast so they don't bind easily

but even at this temperature if we keep adding water vapor we reach a point where the molecules are so close

together that eventually their magnetic attraction overcomes their motion and they still condense out of the air

this is what we call saturation where if you keep adding water vapor to the mix it will just keep binding and

condensing out of the mix when we lower the temperature this happens much more easily the molecules

move slower so it's easier for their polar attraction to take over and thus saturation happens even when the water

molecules have a little bit more space that's why relative humidity is so dependent on temperature because

temperature dictates factor one how fast the molecules move one important thing with moisture content in the air and

relative humidity is it impacts the way people feel so on a hot day that is also dry people may feel somewhat cool but if

you add moisture to the air on that same hot day it becomes hot and muggy your body is through the sweating

process as an evaporative cooler and when it's hot muggy outside you can't transmit that energy or that moisture

away from your body and that's why it feels more uncomfortable it also works on the flip side

for example if you're in florida and it all of a sudden becomes cold in those conditions

higher humidity can actually make the air feel colder so it impacts the extremes in both conditions

dew point is the temperature at which moisture will begin to condensate out of the air

dew point starts with the same temperature as the wet bulb temperature along the

saturation line and the dew point is an absolute measure of moisture content in the air so for example a 60 degree dew

point a constant 60 degree dew point would be a horizontal line on the psychometric

chart a common example of dew point is due on grass

let's say we have a certain amount of water vapor in the air during the day and it begins to cool as the sun goes

down as the air cools the water molecules loose some of their kinetic energy and

begin to move slower eventually they move slow enough that they begin to bind and condensate out of

the air the temperature at which this happens is the dew point

some of this water will naturally end up on the grass blades but if the blades are cooler than the

air the process can happen directly on the grass plate itself

[Music] let's go back to this water bottle example

we've just taken this water bottle out of the refrigerator and let's say for example it's 38 degrees

we know that if the dew point of the air around it is higher than 38 degrees the bottle is going to condensate

so an example 75 degree air at 50 rh

that dew point is somewhere around 55 degrees the bottle temperature is 38 we're going

to get condensation 100 of the time the same applies with supply ductwork if we know the air temperature and

relative humidity in the space we know and can predict when the supply duct is going to condensate

the moisture content of air is very similar to the dew point of air in that it is also a vertical axis

located on the right side of the chart but constant moisture content

follows a horizontal line across the chart so for example a moisture content of 70

would be constant across the psychometric chart and moisture content is a direct

value of how much moisture is in the air unlike relative humidity where it's relative to the temperature and other

conditions moisture content is an absolute value and the typical units are pounds of

water per pounds of dry air and sometimes we use grains so for a reference point there's 7 000

grains in one pound so it's just another way of measuring

weight of a substance it's hard to have a frame of reference as to how much water exists in the air

in buildings and saying there's 55 grains per pound of dry air isn't exactly easy to visualize so i used some

satellite imagery along with the psychometric chart to calculate how much water is in the air at my local big box

store at 55 grains i'll briefly show the calculations here the final answer is 4656 pounds of water

which equates to 558 gallons of water in the air inside the store this gives you an idea of just how much

water hvac systems interact with in buildings this size really the crux of what we're after in

the psychometric chart is how much energy is in the air and the way we express energy in air is called

something called enthalpy enthalpy is defined as the amount of energy in a pound of air in english

units enthalpy is the btus per pound of air and it's located on the left side of the

psychometric chart in numbers that are at a diagonal and constant enthalpy runs diagonally

along the chart the enthalpy is related to all other properties on the chart for example

if you change the dry bulb temperature or the wet bulb temperature or the rh percentage all that goes into

defining what the enthalpy is or the amount of energy in the air so looking at the chart we can see that enthalpy or

energy increases as we move to the right or increase temperature but the biggest increase in enthalpy actually occurs

when we move up the chart which means increased water vapor but why does more water vapor in the air equate to more

energy let's look at the formula for enthalpy of moist air as you can see it consists of the enthalpy of the dry air

plus the total of the enthalpy of the water vapor multiplied by how much of it is actually in the mix

another term for these two halves of the equation are the sensible heat versus the latent heat the sensible heat

enthalpy is easy to calculate we just need to know the temperature and specific heat of air specific heat is

just how much energy it takes to heat or cool a substance one degree in the case of air it's 0.240

btus per pound degree fahrenheit the latent heat also takes into account the specific heat of water but it adds

this huge number which is the heat of evaporation of water because of the strength of water's

hydrogen bonds it takes precisely 1061 btus per pound to turn liquid water into a gas as it exists in the air and all

this energy is still present in water vapor in the form of kinetic energy if it weren't for this heat of evaporation

this side of the formula would be almost negligible but because of it the amount of water vapor in the air is a huge

factor when determining the energy of the mix as a whole even a little bit of water vapor can add

significant enthalpy enthalpy is very useful in calculating energy to process air so for example if

if you're going from one point to another point on the psychometric chart you know the difference in enthalpy

and btus per pound of dry air so if you know that difference in btus

you can convert that to tonnage which is then used to size hvac equipment the last parameter we will

look at right now is specific volume specific volume is defined by these diagonal lines across the chart and the

units are cubic feet per pound of dry air you can think of specific volume as the

inverse of density for example the air inside a hot air balloon is low density which means it has a high volume per

unit mass at least when compared to the air around it it's common to say that when humidity is high the air feels

heavy but using the chart you can see that at the same temperature humid air is actually lighter than dry air as it

takes up more volume so the saying is a bit wrong but why do engineers care about specific

volume i'll give you one practical example here when you're heating air

since blowers are constant cfm or constant volume machines when the air is very cold entering the

blower you can put more heat into that airstream because the air has more mass

versus when you try to heat warm air you can't put as much heat into that air very easily

because there's less mass of air now that we've gone through all the parameters it's clear that some of these

relationships are complex giving the chart all kinds of curves and angles with a little practice though it's a lot

easier to navigate the chart than it is to calculate these long formulas so let's give it a try

let's say we know two parameters a dry bulb of 70 degrees and a relative humidity of 50 percent what else can we

get on the chart let's find the 70 degree dry bulb line here

then the 50 relative humidity line here where those two lines cross we can plot a point

from that point we can gather all kinds of useful information from the chart starting with the wet bulb we see that

this point is below the 60 degree wet bolt line so here's 59 and 58 it's right about in the middle so we can

interpolate a wet bulb of about 58 and a half degrees fahrenheit then for dew point we can draw a line

straight to the left and see where it intersects the saturation line it looks like it's at about 50 degrees

looking at enthalpy now these diagonal lines our point is slightly above this line which is 25

so let's call it 25 and a half btus per pound of dry air we can also get the humidity ratio

aligned straight to the right and it looks like it's right around 55 grains per pound of dry air

and finally the specific volume this point is just to the right of this diagonal so maybe we can interpolate to

13.55 cubic feet per pound of dry air so you can see this is one very

practical example of quickly getting all kinds of data from just knowing two points now let's say we have a second

point and just arbitrarily place it here the parameters for this point are all different so what happens when we try to

change air from one point to another for this we go to part two once you establish your starting point

on the chart different directions you move from that point describe different processes let's break it down to left

and right at first and we we'd find that as a sensible process so for example going from left to right on the chart is

called a sensible heating process so there is zero exchange of moisture in that in that process it's just sensible

heat going from right to left is called sensible cooling and there is no change

in absolute moisture in the air in that process conversely moving up and down on the

chart there is a gain or loss of moisture and no

gain or loss of heat moving vertically upward on the chart is a humidification process and moving vertically downward

is a dehumidification process so that then you can combine those different directions so for

example if you're moving up on the chart and to the right that is a heating and humidification

process so common applications in hvac a traditional rtu

would be a sensible heating or cooling process where we're just moving to the left on the chart for cooling

or to the right on the chart for heating when you get to more advanced products

like dedicated outdoor air systems they introduce a latent component and are heavily biased towards the latent

removal of moisture so for example a doze may have a 70 percent latent component to it

versus a 30 percent sensible so it's more heavily removing moisture from the air

the primary example for latent moisture removal would be when you have a cooling coil and you actually see moisture

falling off of the cooling coil and out of a drain pan that's a good indication that you're latently cooling the air

because moisture is actually being rung out of the air when you're cooling and you don't see

any moisture on the cooling coil you know that's a sensible cooling process one thing to really put this in

perspective is when you're along a sensible process you're more parallel with constant

enthalpy and when you're operating in a latent process you're moving more perpendicular

to the enthalpy line so it removes a lot more energy that way that's why it takes more energy to

perform a latent process than it does a sensible process one unique process is

moving up on the chart and to the left and that's called evaporative cooling the interesting thing about moving

diagonally on the chart especially to the upper left is it's an adiabatic process so there's no energy exchange

because you're moving along a constant enthalpy line so you can move diagonally parallel with enthalpy on the chart and

exchange zero energy evaporative cooling is a technology used as early as 4000 bc by

the egyptians who build wind towers to catch and funnel hot and dry desert air and push it through tunnels lined with

wet blankets or reeds effectively providing cooling for buildings this energy neutral process

has been adapted with modern equipment for cooling and dry environments like the southwest

so the reason why you'd want to move a certain direction on the chart is to try to bring conditions into a more

comfortable zone and ashrae has many guidelines on how to define that comfort zone based on

the amount of moisture the amount of air movement how much clothes you're wearing so if you know where you're at

and you know where you need to go that will help define the process that you need to undertake to get there

so let's go through a little bit of a of an example here showing

the process of of cooling on the chart say i want to maintain 70 degrees in a space

if i delivered air directly to the space at 70 degrees it would not be able to maintain 70 degrees in the space so you

have to actually over cool and overshoot with your cooling process to bring the building back to a comfortable condition

so say our target is 70 degrees dry bulb and 50 rh we'll draw a dot there that's our target

and our outdoor conditions are 95 degrees and 78 wet bulb so those are our starting conditions

the theoretical cooling process would start at the 9578 point and sensibly cool to the left until we reach

saturation and then we would ride the saturation line down

to a point where the equipment was capable of hitting and then from that point

the air being delivered to the building would be very cold and relatively saturated and the natural process would

be for the air to reheat itself back up to the space condition and that follows a sensible heat ratio line

in in actuality what happens is you start at the 9578 and you ride a general curve down

to about 95 relative humidity and then the building naturally heats itself up so there's a difference in the

theoretical cooling curve and the actual cooling curve and that has a lot to do with coil design and velocities through

the coil the reason that the theoretical and practical cooling curves look so different is because it's almost

impossible to just completely sensibly cool the air as the air goes through the cooling coil not every molecule

interacts with the surface of the coil in the same way some particles may never touch the coil as they travel through

while others touch or get very close for a long period of time this means that while the average temperature of the air

hasn't reached saturation some water vapor molecules will still get cold enough to condense out of the air and as

a result we get this more gradual curve representing the loss of moisture when you know your target you have to know

your internal loads or your internal heat gains to know where to end the cooling process so if your internal heat

gains are going to naturally increase the air from the from the unit that's called your sensible heat ratio

line you know where to end your cooling process some very typical internal heat gains

are lighting which is improving with led lighting by the way

computers people in the space any appliances that generate heat also

there's heat gains through windows and walls and roof that must be accounted for so anything that adds heat to the

space it goes into what's called the sensible heat ratio and that's how the building naturally heats itself back up

in order to know where to end the cooling process we have to know what ratio of this heat is sensible versus

latent and it's a simple formula sensible heat divided by the total heat so for example a building that

experiences mostly sensible loads would have a high sensible heat ratio close to one

a building with predominantly latent loads like a room full of people or a kitchen would have a low sensible heat

ratio if we know the shr the psychometric chart has one more trick up its sleeve

some charts at least have a pre-existing dot somewhere near the center and a scale to the right

if we draw a line from this dot to the shr value in this case i'll use 0.70 the slope of this line will correspond

with how the building will naturally heat back up this is very useful as we now know that the cooling process needs

to end somewhere along this line in order for the building to achieve our final target point

with modulating equipment and with advanced algorithms we can actually control the exact point

dynamically where we need to end the cooling process to allow the building to naturally reheat itself to the exact

point we need there's a lot of emphasis put on exactly where we need to end the cooling process

through modulating components to produce perfect conditions in the space

so now that we've reviewed the cooling process let's take a look at the heating process and how that differs on the

chart so say for example we want to maintain our 70 degrees dry bulb and 50

rh condition in the space and our air temperature coming into our hvac unit is 55 degrees and nearly

saturated the heating process just moves left to right on the chart so we're going to go

from the left point directly to the right

towards the space conditions just like cooling has loads heating has loads as well so for example

assume you're in northern minnesota and it's 70 degrees inside and minus 20

outside well heat will be moving out of the building in that case so you actually have to bring in warmer air to

maintain a space condition so for example if it's 70 degrees in the space that's what you're trying to shoot for

you may have to deliver 90 or 100 degrees out of the unit to maintain that 70 degree temperature in the space

before making this video i was under the impression that the heating process has a drying effect and this is actually a

common misconception using the chart we can visualize that while relative humidity does decrease as we move to the

right the humidity ratio stays constant meaning the actual quantity of moisture in the air does not change

is it comfortable in here [Music] well we must be inside of this box then

so we really have to know where we're where we need to be

to know how to size equipment so there's been a lot of industry study on the exact comfort range inside of a

psychometric chart and ashrae has established a comfort zone box on the chart and it's basically

approximately 70 to 75 degrees and 40 to 60 percent rh and that chart will sometimes shift

depending on how much activity you have and if you're wearing heavy clothing or

light clothing but that's the general premise around the chart so it's good to know where where we want to

be because we know where we're at and if we know where we're at we know

where we want to be we can calculate the energy required to get to that point and that leads to

how big of a hvac unit do we need to do this process all right so let's use a real world

example here to calculate the amount of cooling capacity or cooling tonnage we need in an hvac unit

so to do that let's start with where we want to end up and we'll use our example here of 70 degrees

dry bob and 50 rh and we keep using that condition because it's a relatively comfortable condition at about 52 degree

dew point and we know our btus there are approximately 25.5 btus per pound of dry

air now let's take another point a starting point an outdoor air condition

a 95 degree dry bulb and 79 wet bulb and we know our b2 there is approximately 42 and a half btus per

pound so subtracting those two enthalpies gives you a difference of 17 btus per pound of dry air and that's

from where you're starting to where you're going now that we have delta enthalpy where do we plug this in well

we have four different cooling formulas all useful for different scenarios let's use the second one to calculate total

btus per hour so the formula for total cooling capacity is

btus equals cfm's which is the amount of air you're moving cubic feet per minute times a constant 4.5

times your change in enthalpy so let's assume for round numbers a thousand cfms

and if you do a thousand cfms times 4.5 times your change in enthalpy you get a total of 76

500 btus per hour there's a there's a nice little conversion in the hvac industry of

12 000 btus per hour is one ton of energy so if you divide that 76 000 divided by 12 000 you get

6.375 tons so you would need a at least a 6 ton unit

to cool a thousand cfms from the starting point to the ending point that would be just a nominal way to size a

unit so when we're sizing hvac equipment we

don't just guess at conditions we don't say it's really hot in phoenix so we need a lot of capacity

ashrae has published specific data on every major location around the country and we

use these this data to accurately size equipment we'll take miami for example and it's

important when you're looking at cooling to look at both dry bulb conditions maximum dry bulb conditions and maximum

dehumidification conditions because it can make a difference on how you size units especially when we treat outdoor

air so we'll take the example of miami like we mentioned before and ashrae publishes something called point four

and one percent data and point four percent data basically says the conditions are going to be

at these extremes for 99.6 of the time you'll be at these extremes or less so it's pretty a pretty good reference to

size equipment too so we'll look at dry bulb cooling first from miami

and assuming we're going to end up at 70 degrees and 50 rh in the space so we we know

just by looking at the psychometric chart that 70 degrees and 50 percent rh produces an enthalpy of of 25 and a

half btus per pound of dry air

looking at the dry bob cooling conditions at miami at 0.4 percent we get a condition of 91.8 dry bob

at a 77.6 wet bob condition so if if you look that up on the psychometric chart you get an enthalpy

of 41.04 btus per pound of dry air that's good for dry bulb conditions but you also have to look at the

dehumidification conditions so for miami for example where it's very humid looking at the dehumidification

conditions at 0.4 percent conditions we're at 78.4 degrees dew point

at 83.5 degrees dry bob and if you look that point up on the psychometric chart you'll notice that the enthalpy is 43.2

btus per pound of dry air so just doing the math here we know our starting conditions and we know where we

want to end up at 70 degrees and 50 rh just looking at the dry bulb conditions the difference in enthalpy is 15.5

and looking at the dehumidification conditions the difference in enthalpy is 17.7

so you can see there's a different amount of energy required when you're dehumidifying versus looking at dry bulb

conditions and if you do the math using the formula total btus per hour equals cfms times

4.5 times your delta enthalpy for dry bulb design you end up with 69 930 btus per hour assuming a thousand cfms

for dehumidification you end up at 79 650 btus per hour assuming that same

thousand cfms so what that means is if you're sizing a unit for dry bob conditions in miami you would need an

exact unit of 5.8 tons versus dehumidification at 6.6 tons so that's why we look at dehumidification loads

along with dry bulb loads and we typically size units to handle the larger of the two

these examples were a bit simplified to focus on tonnage calculations and the differences between peak sensible and

latent cooling remember we don't actually cool air in a straight line from outdoor conditions to

space conditions we tend to over cool to account for building heat gains there are also other processes like reheat

which we'll talk about more later or limitations of the equipment that must all be taken into account to determine

real leaving air conditions for more accurate calculations the starting conditions are also not

always as straightforward as our examples for simplicity we assume that the air entering the unit was a hundred

percent outdoor air matching the peak outdoor conditions but when recirculation is used in a

building the entering air conditions or our starting point will be different a quick way to obtain this new starting

point using just the chart would be to draw a straight line from the space conditions to the outdoor air conditions

if for example the equipment is set to 50 outdoor air our starting point would be fifty percent of the way along this

line if it was thirty percent outdoor air the starting point would be thirty percent

of the distance these factors are just the tip of the iceberg and we will cover them in more

depth in a future video the examples bill just worked through used the second formula here for total

btus per hour we can also calculate the specific btus for sensible and latent heat we just need to know the change in

drive of temperature for sensible btus and the change in grains of moisture for latent btus

the chart can also be used as a quick way of isolating specific sensible and latent enthalpy changes if you draw a

line from the starting condition to the target condition and continue drawing a right triangle

the vertical and horizontal vectors represent the change in latent and sensible enthalpy respectively

if you went to an hvac manufacturer or supply house and asked for a 5.8 ton unit

someone would probably laugh at you because there is no such thing as a 5.8 ton unit

they're rated a nominal capacity so most residential caps out at five tons and then above that you go five

seven and a half ten ton units in increments like that the nice thing about modulating

equipment is you can select the seven and a half ton unit and it will modulate to that exact 5.8 ton capacity when

needed [Music] imagine these two scenarios you're

driving a car in a desolate straight road and as the law abiding citizen that you are you decide to follow the speed

limit of 70 miles an hour in scenario 1 you set your cruise control to 70 and the car maintains 70

miles an hour the ride is smooth quiet and efficient in scenario two

a couple things are different your cruise control is broken and the throttle pedal will for some

reason only work in two positions full throttle or no throttle at all

you still try to maintain 70 miles an hour but you constantly go over and under

the ride is uncomfortable and you notice you're using a lot more gas the average speed for both scenarios

might be the same but wouldn't you rather be in this car this illustrates the basic differences

between modulating and stage hvac equipment the former maintaining perfect target conditions for comfort and

efficiency even in changing conditions [Music]

the thing that we have to work with is software compared to traditional rtus in the past that were just on off or stage

so we have the luxury of working with some higher end equipment that is fully modulating and controlled with software

and what that allows us to do is to balance perfect conditions with energy savings that's why you see

high ieeer ratings with modulating equipment for example we know we have variable conditions coming into the unit

and once we know what we want to deliver and how we want to deliver it we can write algorithms around

controlling our equipment and modulating our equipment to exactly hit that point

the general differences between truly modulating equipment and staged equipment is with modulating

equipment we can pinpoint an exact point on the psychometric curve where staged equipment

pinpoints a circle or an acceptable range that's the biggest difference so it enables us to narrow that target down

as closely as possible with stage equipment the target becomes much larger because

stage equipment cannot hit such an accurate point one area of optimization is reheat

a lot of applications require a reheat process from a comfort standpoint so we'll give you two examples here one

with using reheat and one without using reheat so we'll start at our standard conditions here 70 degrees and

50 rh that's where we want to end up and we'll start with outdoor conditions of 95

and 78 our typical starting point here and we'll say we have a relatively flat sensible heat ratio line there's not a

lot of latent load so we'll draw the sensible heat ratio slope

on the psychometric curve so we've got that established so there's two ways of doing this we

could start at the outdoor air conditions sensibly cool and then latently cool ride the

saturation line down all the way to the sensible heat ratio line no reheat

the building itself would naturally raise the conditions of the of the leaving air

at that point we'd have to cool the air down to a 45 degree dew point so we'd have to deliver air at 45

degrees into the space which can lead to very cold drafts

and some condensation of ductwork so what's the alternative well the alternative is to reheat the air and a

reheat is a sensible heating process where we take hot gas right off the compressor

and cycle it through a coil a reheat coil which is downstream of the cooling coil

and then that hot gas then goes back into the condensing coil to be condensed we'll start at the same point

we will sensibly cool the air and then

latently cool it say down to 50 degree dew point and then reheat the air back up

to approximately 65 then ride the sensible heat ratio lineup so there's two different ways to get to the same

point one with reheat and one without reheat and there's pros and cons of either way a lot of people

think it's just free energy because we're already over cooling that air and we have refrigeration you know

to work with but one thing that they don't consider is that there's air energy required to push

air through a second coil and there's refrigeration energy required to push

refrigerant through a second coil and a valve so reheat comes at a cost but

comfort is king right now and if you're not comfortable you're wasting your time and money

so if you're going to go through the investment of buying high-end air conditioning equipment

you shouldn't take the risk of being uncomfortable an economizer in theory is a

device that allows for better efficiency of a unit the theory behind an economizer is if

you have a cooling load inside of a space and your outdoor air conditions

are cool you would lock out your mechanical cooling of an hvac unit and just bring

in that outdoor air to cool the space so that's why some people call economizers free cooling

but it really isn't free for a variety of reasons let's take an example of a just a simple dry bulb economizer and

let's go back to our original example we're trying to maintain 70 degrees dry bob and 50 degrees into space

let's say our dry bob economizer has a set point of 60 degrees to open up

so anytime there's a call for cooling and the outdoor temperature is 60 degrees or less

the economizer will open up and the unit will lock out cooling and bring in outdoor air

looking at 60 degrees there could be a whole range of relative humidities associated with 60 degrees

let's use an example of 60 degrees and 85 percent relative humidity if that's the case if those are the

outdoor conditions you'll actually be bringing moisture into the space

because your dew point at 60 degrees and 85 rh is higher than your space conditions at 70 degrees and 50 rh so

you will be actually adding moisture to the building in that case that's why

dry bob only economizers don't make a lot of sense so

now looking at another option for economizer is called an enthalpy economizer and we'll use the

same point 70 degrees and 50 and we'll draw our enthalpy line there

and that will be you know our 25 and a half b2s per pound of dry air so say our enthalpy setting there on our

enthalpy economizer is 23 btus per pound of dry air so anything less than 23 btus per pound

of dry air on the outside will lock out mechanical cooling and bring that outdoor air in well the same

situation exists where we could be at a lower enthalpy outside

say 55 degree dry bob but a 90

rh so we could be bringing in more moist air or wetter air from the outside

into the space so you have to look at all types of conditions to make sure economizers make

sense and on a large percent of jobs we've seen them disabled by users because they actually end up making the

space more uncomfortable the intention of economizers are to save energy but they sacrifice

comfort and quality in those some of those situations looking into the future of equipment we

know that there's energy considerations to pay attention to so there's going to be a lot of resources placed into

optimizing every process with hvac equipment including cooling reheat

how we deliver air into the space so we have to look at all those factors and how it relates to the psychometric chart

and balance energy at the same time today we've really gone through a lot of material and i hope you've enjoyed it

for me the biggest realization while learning and researching psychometrics is that the chart is truly a culmination

of efforts by many people from various times and places and that the way we use it today continues to evolve as

engineers develop new products and techniques to handle air and lastly i just want to say that while

we live in a world where the trend is to use an app to calculate just about anything

learning to use a chart gives you a much more complete and comprehensive picture of what's actually going on with moist

air and it really has no competition i hope this summary was was good for

everyone and i encourage everyone listening to this to become familiar with the psychometric chart and learn

everything about it because it can make your job as an engineer much easier and you can talk more intelligently

about how hvac equipment operates and the energy associated with it so yep plus you need to know it to pass

your pe exam [Music]