# Tag: dew point

## Room Sensible Heat & CFM – Advanced Psychrometrics Part 1

This is the first of a three-part series of articles, which will dive deep into Advanced Psychrometrics. The source material for each of these articles may be found in ACCA Manual P Sections 3, 4, and 5. This article is based on information found in Section 3.

Psychrometrics is the study of the physical and thermodynamic properties of gas-vapor mixtures. In HVAC/R, we are specifically interested in air-moisture mixtures, and how varying properties affect human comfort and equipment performance. The Psychrometric Chart is a tool used to describe all the possible combinations of gas-vapor mixtures, and can be used to calculate the sensible and latent loads associated with HVAC/R equipment.

Using a Psychrometric Chart can be a bit confusing at first, but with practice and familiarity of the formulas, a Psych Chart can be easily used for a wide variety of purposes. Basic Psychrometric education can be found in the Refrigeration and Air Conditioning Technologies Manual (RACT) and in the first two sections of ACCA Manual P. In this article, however, I’m going to show you how you can apply psychrometrics to calculating Design Room CFM and illustrate how psychrometry can be used to help a technician understand supply air properties. All of the information discussed here can be found in Section 3 of ACCA Manual P.

When selecting equipment for a home or building, it is recommended a Room-to-Room Heat Load Calculation be done as opposed to a Block Load Calculation (Wrightsoft is an excellent software for load calculations, just saying). Room-to-Room calculations result in a more accurate representation of the heat gains and losses per zone (room), and can greatly improve the accuracy and performance of system sizing and design. Assuming a Room-to-Room Load Calculation has been done on a building, the next step in utilizing the Psychrometric Chart would be to plot out the Room Sensible Heat Ratio Lines for each zone. Room Sensible Heat Ratio (RSHR) is the ratio of sensible heat to total heat (including latent) for a room (or zone). If, for example, a room had a total heat load of 2,500 BTUh and 1,800 BTUh sensible heat, the RSHR would be 0.72.

RSHR = 1,800 BTUh ÷ 2,500 BTUh

RSHR = 0.72

Now that we know the RSHR, it’s time to plot the RSHR Line on the Psych Chart. To do this, we need to find a “reference dot”.

80℉ db at 50% RH is considered the standard reference dot. Locate and mark the reference dot and then run a line through the reference dot using a straight edge that is lined up with the RSHR (0.72), which can be found on the far right-hand side of the chart.

Now, locate the design conditions for the zone in question. Let’s say the design conditions (on a design day of 90℉) is 75℉ db at 50% RH. Plot that dot on the chart. Now, run a line straight through that dot heading to the left of the chart, making sure it is parallel to the reference line. This line is your RSHR Line. This line may now be used to select a supply air condition that will maintain the design room condition on a design day. However, the supply air condition must fall somewhere between the design room condition and dew point (which in this example is about 51.5℉). Theoretically, the lowest possible supply air condition would involve the evaporator coil in cool mode to be 51.5℉ (dew point), and the supply air leaving the register to be the same. However, this theory is in no way practical when you consider duct gains, air leakage, and bypass factors (let alone the fact no one wants a sweaty supply register). Practically, a supply condition falling somewhere between 80%-95% RH will result in good dehumidification, lower airflow, and low fan power consumption.

Select a supply temperature condition. For this example, let’s choose 55℉ at 90% RH. The next step is to calculate the Design Room CFM. The equation for CFM is as follows:

CFM = Room Sensible Load ÷ (1.08 x ΔT)

Remember, the Sensible Load for this zone is 1,800 BTUh. The difference between the Room Condition and the Supply Air Condition is 20℉.

CFM = 1,800 BTUh ÷ (1.08 x 20℉)

CFM = 83

The required volume of air given an hour of the runtime is 83 CFM for this room to maintain the design room air condition under design load.

But what if my ΔT is lower?

The required volume of air increases. The new supply air condition is 63℉ at 72% RH, giving us a ΔT of 12℉.

CFM = 1,800BTUh ÷ (1.08 x 12℉)

CFM = 139

Both of the different supply air selections will maintain the design room condition on a design day, because they each fall on the RSHR Line. But as the temperature difference between return and supply air decreases, the required CFM increases

What is 1.08 supposed to be?

That is the product of the following equation:

Runtime (minutes) x Isobaric Air Density x Isobaric Specific Heat of Air

60 x 0.075 x 0.24 = 1.08

Some caveats must be addressed regarding this formula, and I credit Alex Meaney with Wrightsoft and Genry Garcia with Comfort Dynamics, Inc. for helping me understand these complexities. Both gentlemen are brilliant-minded experts in their fields, and have contributed (and continue to contribute) to HVAC School.

First, the runtime is specified in minutes, because we are solving for cubic feet per minute (CFM), but also using British Thermal Units per hour. Converting the hour of runtime to minutes gives us 60 minutes, and makes sure our units of measurement are compatible.

Second, you may notice the term isobaric. This refers to any property at a constant pressure. At sea level, atmospheric pressure is around 14.7 psia. At this presumed fixed pressure, the density of dry air is 0.075 lb/ft3, and the specific heat of dry air is 0.24 BTU/lb/℉.

In reality, atmospheric pressure is not fixed, and outdoor air is not always dry. While you may be able to correct for actual pressure and humidity, it may not always be practical. On the other hand, with the ability to use MeasureQuick (which corrects for air density and pressure in its calculations), the processes discussed in these articles may become more practical. It is important to note that manufacturers use isobaric air density and specific heat in their capacity ratings and airflow calculations. Therefore, the argument could also be made that even with this caveat, the end result will (on average) still land you nominally close to the actual air condition requirements. (Please note the wording used here) 😉

So how does this all circle back to practical application? It must be understood that a coil can operate in only one sensible heat ratio at a time, and it may not equate to any of the RSHRs calculated for any particular zone. In the case of a home with multiple zones, you may choose one of the following options when selecting a cooling coil to match the load conditions:

1. If humidity control is critical to a specific zone, use the RSHR for that room to select a coil. All other rooms will vary in humidity, but the critical zone will be maintained.
2. Average all the RSHRs together for a mean RSHR that can be used to select a coil. Each room will vary slightly from its individual RSHR, but it will be minimal and likely unnoticeable.

And that, in a nutshell, is how you may use a Psychrometric Chart and data from a Load Calculation to determine Room Design CFM. This exercise, however, merely scratches the surface of the many factors that must be considered in an HVAC system. This exercise works only for a system that does not suffer from duct leakage, bypass factor, and has no ventilation whatsoever for the home/building. This exercise would fall short of providing any real-world insight into psychrometric properties involving an HVAC system. However, the skills learned here translate into the next phase of advanced psychrometrics! In the next two articles, I will detail how these variables can be accounted for (even solved for). In the end, I hope you will understand a little more about Psychrometrics in general, and how to add that knowledge to your ability to efficiently diagnose a system as a whole (including the envelope and people).

I’ll end this article with a quote from Alex Meaney, and I think it is important to keep this idea in mind throughout the rest of this series of articles:

“I’m of the opinion that local humidity is usually a[n] infiltration/ventilation/return problem, not a supply problem.”

–Alex Meaney

— Kaleb

## A Tale of Two Latents

When the quiz or the teacher asks what “latent” heat is there is generally some reference to it being hidden heat, which is what the word latent means. We then learn that it is heat energy transferred that results in a change of state rather than a change in temperature.

Later on, we hear a lot talk about how much more heat it takes to change the state of water than it does to change its temperature with a graph something like this.

So we learn pretty quick that a lot more energy gets moved when we are changing matter from one state to another and in HVACR we are going from vapor to liquid and back to vapor again in the refrigerant circuit.

In the condensing coil, we see latent heat rejected as the refrigerant changes from full vapor to full liquid at the condensing temperature.

In the evaporator coil, we see latent heat absorbed as refrigerant changes from mixed vapor / liquid flash gas to full vapor at the boiling temperature.

But there is another kind of latent heat we deal with in air conditioning that can leave people confused when we talk fast and loose about latent heat and the evaporator. This latent heat is the hidden heat it takes to change water vapor in the air passing over the evaporator to liquid water on the coil surface.

Which is Which?

Inside the evaporator, there is latent heat of vaporization as heat conducts into the coil and boils the refrigerant. That internal temperature is fixed so long as the pressure remains the same across the coil and the refrigerant is a single component or azeotropic (no glide). There are refrigerant blends that do change increase in temperature through the coil through “glide” but set that aside for another article.

On the outside of the coil there is latent heat transfer out of the water vapor causing it to condense on the coil fins so long as the coil is below the dewpoint temperature of the air. This is why we call the ability an air conditioner has to remove moisture at certain conditions its latent capacity.

These two latent heat transfers impact one another indirectly but all the heat that the air moving over the coil imparts on the evaporator is done via conduction through the tubing (or microchannel) walls of the evaporator.

Let’s break that down a bit.

Convection is heat transferred through a moving fluid. So heat moving THROUGH the refrigerant is moving via convection. Heat moving THROUGH the air over the coil is also transferred via convection.

But there is no direct fluid connection between the refrigerant in the tubing and the air moving over the coil is there? (Unless you have a big coil leak).

The heat that moves out of the air and into the refrigerant has to move through the solid walls of the coil and the only kind of heat that can make it through a solid with any significance is conduction.

This means that the only way that the latent heat inside the refrigerant and the latent heat in the air connect is via SENSIBLE temperature difference across the metal walls.

When the air moving over the evaporator has more moisture in it and therefore a higher RH and dewpoint the surface temperature of the coil is increased so long as the coil temperature is below the air dewpoint.

When the surface temperature of the coil is held “higher” by more latent heat of condensation on the coil more heat enters the refrigerant inside the evaporator resulting in a higher evaporator pressure and higher boiling temperature inside the coil (especially in a TXV/EEV system).

This may sound like magic but the quantity of heat that can enter the evaporator is as simple as the temperature difference between the inside of the coil where the refrigerant is and the outside of the coil where the air is. Because we have the potential for latent heat transfer on each side this temperature difference has a lot of contributing factors that can make the math a bit confusing.

Just remember… There are two kinds of latent heat at play

Refrigerant boiling inside the evaporator

Water condensing on the outside of the evaporator

Heat interacting between the two moving from higher temperature to lower temperature via conduction.

Simple!

— Bryan

## What Willis Understood

I’ve been reading a book called “Cool, How Air Conditioning Changed Everything” and it got me interested once again in the history of air conditioning and refrigeration. Like many things the people who are credited with “inventing” are the ones dogged enough to make an idea commercially successful, not the idealists forever tucked away in the lab.

I bought a 1921 version of the periodical “Ice and Refrigeration” and mixed in with the ads for absorption ice machines and “mineral wool” insulation was the advertisement shown above. Willis Carrier understood how to connect ideas and make sense of emerging technology, first to keep paper dry in a factory and later to cool the world with “Manufactured Weather”. Look carefully at the ad, you will notice it mentions many things… but not cooling, the ad is in ICE AND REFRIGERATION and the ad doesn’t mention COOLING.

Many of you know that in 1906 Willis Carrier patented what is now referred to as the “First Air Conditioning System” but do you know what it was that he actually invented?

You may be led to believe that Willis Carrier invented compression refrigeration? Nope, the first commercial attempts at compression refrigeration began in the 1830’s and the patent above actually has no compression refrigeration in it whatsoever. Many will say that he was the first to dehumidify the air, this is also false, there had been compression refrigerated cooling coils in use that dehumidified the air before Willis came along they just didn’t do it on purpose.

What Willis Carrier understood better than anyone else in his day was the RELATIONSHIP between humidity, temperature and saturated air or “dew point” and how to manipulate water temperature, water volume and air volume to produce a CONTROLLED humidity environment first and later a controlled temperature, humidity, and ventilation environment.

The Carrier “Air Washer” was nothing more than water pumped through nozzles that produced a mist of water. The air would blow through the water mist and it would clean the air, drop it to dew point (100% RH) and then continue to sensibly cool the air. Willis worked in northern states with cold groundwater at a time before water use restrictions so the cold water would serve to cool AND dehumidify the air. At the time it seemed like black magic that running air over water could REMOVE water from the air, but so long as the water temperature was below the dew point temperature of the air that is exactly what would happen. All Willis had to do to change the dehumidifier to humidifier was to increase the water temperature or change the dehumidifier to a sensible cooling machine was to use cold water and give the air more dwell time or passes through the water to decrease the sensible temperature.

In the process Carrier and his team made many discoveries about air and in 1911 Carrier presented possibly his greatest work which he called the “psychrometric formulae” which is the founding document on which all of current understanding of psychrometrics is built. Carrier took a VERY SIMPLE idea and pursued it and understood better than the others around him and because of that, we remember him today. He thought about cooling, heating, ventilation, humidity and air cleanliness and combined them together into one machine that controlled it all.

Later on, Carrier would begin actively “cooling” the air with compression refrigeration and replaced water sprays with refrigerant evaporator coils to leverage the latent capacity of refrigerants, but it all started with a mist of water an understanding of dewpoint, some dogged determination and some clever marketing for his “manufactured weather”.

— Bryan

To find the catalog where I found some of this information I created a link to the national archives at hvacrschool.com/willis

## What Is Temperature Glide?

We’ve all heard about glide, but what is it really and how does it affect our system?

Glide, or temperature glide, is the difference between the bubble point and the dew point of the zeotropic refrigerant mixture.

Well that wasn’t very helpful, was it? All we did was introduce new terms without defining them and further confused the issue.

So, let’s start with zeotrope or zeotropic mixture. A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor-liquid equilibrium state. Still unhelpful? I thought so, too, so let’s look at what it means to us rather than what the books say.

A zeotrope, is a refrigerant mixture or blend that boils across a range of temperatures at any given pressure. So, unlike water that boils at a constant temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil between across a range of temperatures at that same single pressure. Using r407a as an example, at atmospheric pressure, the liquid would begin to boil at -49°F and will continue to boil until the last droplet boils away at -37.5°F. I know that it’s kind of weird to think of the process of boiling like that, but that’s what is happening with a zeotrope. Boiling takes place over a range of temperatures.
That temperature range is called the glide.

Now that we’ve got a basic concept that we can work from, we can start to understand glide and ultimately get to how it affects a refrigeration system. Let’s start with bubble point. Since we should have a solid understanding of states of matter and the transition between liquid and vapor, let’s assume we have r407a refrigerant in a 100% liquid state at 140 psig. If we start at 66°F, we’ll be just slightly subcooled which is a perfect starting point for this example.. If we start to add heat and raise the temperature of our refrigerant while holding our pressure constant, a single bubble will appear in the refrigerant as it begins to boil. That point is called the bubble point. For our purposes, we can define the bubble point of a zeotropic refrigerant blend is the point at which the first bubble appears.
Still making sense? I hope so.

Continuing with our r407a at 140 psig example, we’re going to continue to add heat to the refrigerant with the same constant pressure. The refrigerant continues to boil, but as the mixture of refrigerant changes, the boiling point changes, slowly rising as the liquid boils away. Eventually, we will have added enough heat to reach a point where one last droplet exists, that point is called the dew point. Like we did with bubble point, let’s state an operating definition for bubble point. The dew point is the point at which the last droplet of liquid evaporates. For our example, that temperature is 75.5°F or very near that. Since it’s boiling over a range of temperatures, it is also true that the refrigerant condenses over the same range of temperatures as we remove heat from it. That will happen in reverse of the process I just described.

What does this mean for the service guy?

Obviously, these different values affect our superheat and subcooling readings. Since the dew point is the point where the last droplet of liquid boils off, we need to know that value to measure and calculate superheat. Similarly, with the bubble point, we need that to calculate subcooling. These are the values found on the PT charts and that are programmed into your digital manifold gauges.

In refrigeration work, evaporator coil temperature can be used for a number of things. Most commonly, we will use it to control fixture temperature and to terminate defrost. It used to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number to a PT chart. We can no longer look at our evaporator pressure and know what our corresponding evaporator temperature is quite the same way.

Let’s look at numbers… say the manufacturer says that you need an 18°F coil temperature. With R22, you simply look at your trusty PT chart, find 40.9# and work from there. Easy enough, right?
Now, let’s look at the same coil with r407a. We have 2 points that are 18°F. The dew point (40#) and the bubble point (52.5#), so which one do we choose?
The correct answer winds up being neither one. Between manufacturer’s recommendations and field experience, I’ve found it best to use something closer to the average of dew and bubble point to find the actual, functional temperature of the evaporator.

52.5+40 = 92.5. 92.5/2=46.25

Looking at a PT chart, this shows us 13°bubble point and just over a 23° dew point. If you look, 18° will land right about in the middle. This isn’t always a perfect setting, but it’s as good a place to start as you can find. Set the control valve there and fine-tune it as needed to get the performance that you need. If we need to use a pressure reading to terminate defrost, we will need to reference bubble point because it is the colder of the two temperatures and will ensure a complete defrost. If we used dew point, the inlet of the evaporator would be several degrees colder than the outlet and frost may still very present.

–Jeremy Smith

## How to Reduce Indoor Humidity

Sometimes I beat around the bush too much in these tech tips, so let’s get right to the nitty-gritty! (as Nacho Libre would say)

Humidity inside a home should be maintained between 30% and 60% relative humidity.

I like to shoot for 50% in humid climates when possible (and by possible I mean financially feasible for the customer because anything is possible).

Causes of High Indoor Relative Humidity

• Low Heat Load / Short Equipment Run Time / System Oversizing
• High External Humidity Drivers / Humidity Entering the Home
• High Internal Humidity Drivers / Humidity Being Generated Inside the Home
• Poor or No Spot Ventilation in Kitchen’s and Bathrooms (or it Isn’t Being Run)
• High Evaporator Coil Temperature / High System SHR / High Evaporator Dew Point Temperature
• Insufficient Total Dehumidification Capacity
• Low Space Temperature
• Relying on the A/C alone to Dehumidify

This is the list of everything that causes high relative humidity in a home or building. Total humidity drops when you pull out more water than you put in and it increases when more moisture enters the space than you pull out.

Before we cover what to look for and how to fix it let’s first address some common fallacies that often crop up.

Truth = Lower Temperature Alone Means Higher Relative Humidity

The evaporator coil running below dew-point and water leaving the pan and going out the drain is what dehumidifies the space. This is called latent heat removal and it’s our friend when we are looking to drop the RH% in a space.

Sensible cooling is decreasing the space temperature and while this is a necessary part of comfort in most seasons, it is the enemy when it comes to dropping indoor RH%.

When air is cooled without being dehumidified the relative humidity in the space actually INCREASES because the lower the temperature the less water vapor the air can contain before turning into liquid water.

When we dehumidify with cooling equipment it is the water leaving the drain that matters (latent heat removal) not dropping the temperature of the space (sensible).

For dehumidification getting water out (latent heat removal) = good

dropping room temperature (sensible heat removal) = bad

Truth = Adding Insulation Will Decrease The Heat Load and Generally Increase the Relative Humidity

In order for an air conditioner to pull out humidity and drip it down the drain, it needs to run. In order for it to run it needs to be warm enough in the space for it to run.

When you add typical insulation in the ceiling, floors and walls you decrease the heat load without changing the humidity load. This will result in the RH% going up.

There are some insulation materials such as closed cell foam that will also act as an air & vapor barrier helping to block moisture from making it in. This can help reduce humidity but it is the air/vapor barrier portions that do it not the insulation.

Truth = Many Humidity Issues are Caused by Abnormally High Moisture Not the A/C

The air conditioner needs to be properly sized and selected with sensible and latent capacity that matches the building design. There are many cases where homes aren’t built or lived in exactly to design and cases where the weather doesn’t act like the models predict.

In Florida we have a lot of Hurricanes and tropical systems, In these cases we get tons of moisture, high winds that create big pressure differential across our homes and forces it in, low sensible temperatures so the A/C doesn’t run much and power outages that keep it from running for days in some cases.

For months afterward owners will complain of condensation, biological growth, high relative humidity etc… and everyone tries to “solve” the issue by messing with the air conditioning. These tropical weather events increase the amount of moisture in the home while at the same time impacting the ability of the equipment to remove the moisture.

My own house is another example of an extreme internal moisture condition. I have great insulation, good vapor and moisture barriers and excellent HVAC equipment (if I do say so myself).

However… I have 9 kids and we homeschool so they are home most of the day, we live in the country so we do tons of laundry (lot’s of dirt and mud) and we cook 3 meals a day at home …

Needless to say, our home has internal moisture loads that no model will be able to account for. This is why we added a whole-home dehumidifier to keep that humidity in check.

Final case study…

Many years ago I had a customer who always had high humidity in the main living area and the vents in the ceiling would sweat. I kept going back and messing with the equipment over and over and nothing I did seemed to help. I finally asked another tech and he laughed and said; “they have a pool don’t they?” I thought about it and sure enough, they did have a pool. “How did you know that?” I asked. He smiled and said “They are leaving the slider open when the kids play in the pool to keep an eye on things or they are in and out all time, that’s why the issue is always in that room”, I’ll be darned, he was right. You may be able to use a data logging humidity sensor to find these sorts of client caused intermittent issues.

What to Do About High Humidity

There are many approaches you can take on this depending on the types of tools you have at your disposal, as well as the severity and the budget and patience of your clients. I’m not going to give every possibility and test but here is what I would suggest for the average HVAC tech even if it makes my more hardcore building science friends cringe a bit.

1. Make sure you have a few good quality psychrometers/hygrometers. I use the Testo 605i as my go-to. Never trust a cheap tool with humidity measurement.
2. Ask the customer about how often they cook and note if they have a range hood that vents outdoors.
3. Ask the customer if they use bath vent fans when bathing and showering.
4. Look for roof leaks, proper grading around the home, ponding water etc…
5. Test the space humidity, temperature, and dewpoint at various locations around the home. Often you can find the source of an issue this way. keep in mind that the closer you get to the ceiling the dewpoint tends to increase due to that fact that water vapor is less dense than air.
6. Check the HVAC equipment in detail. When humidity is a challenge setting up the equipment for 350 CFM per ton is generally a good practice. Make sure it all wired properly if it is multi-stage or has dehumidification features. Confirm the system airflow, for newer equipment using the total system static and fan chart method is usually the easiest for a tech. I use the Testo 510 and 440dp for this.
7. Inspect the ductwork and seal any leaks. Leaking ducts cause pressure imbalance in the home and can either drive air in or out of the home.
8. Make sure there are no dryer vents, bath fans or kitchen ventilation leaking or discharging into attics or crawlspaces. Make sure the dryer vent is well-connected to the dryer.
9. Check and measure any incoming fresh through fresh air intakes, ERV’s or HRV’s. If it is too much it may be reduced but proper calculations and likely blower door testing will need to be done before reducing outdoor air.
10. Look for can lights, gaps around boots into the space, holes in walls between the attic and crawl space and the living space etc.. Sealing these can greatly reduce the moisture drivers.
11. Check seals, sweeps & weather stripping around doors and windows
12. Make an assessment if the equipment may be significantly oversized. If so then do a Manual J calculation to determine.
13. Discuss supplemental whole-home dehumidification with the customer, especially when the issue is a big priority for them.

The goals in inspecting the home and equipment is to make some of the following recommendations that can reduce indoor humidity when they are appropriate

• Run or Install Point Ventilation in the Kitchen and Baths to Remove Excess Moisture at the Source When in Use
• Alter Habits (like leaving doors open) That Lead to Moisture Issues
• Install New Weatherstripping and Door Sweeps
• Seal or Install Sealed Can Lights, Seal Around Boots and Seal Other Gaps Between Attic/Crawlspace and the Home or Walls
• Make HVAC System Settings Changes to Run Longer with a Colder Evaporator Coil (Reheat is an extreme example of this)
• Advise Properly Sizing Equipment or Installing Whole-Home Dehumidification Where Appropriate

Quick caution. It is possible to seal a building so tight that it can become unhealthy. Whenever sealing is in order it is best to do a before and after blower-door test on the space and decide if mechanical outdoor air needs to be brought in.

When this is the case I generally suggest a ventilating dehumidifier (and an ERC in some cases) in humid climates, otherwise, you can just make the situation worse.

Also keep in mind that when you run a colder coil the equipment, ducts and vents will be more likely to condensate as they will also be colder. While a colder coil will decrease the space humidity it may not be an option if it results in excessive equipment, duct and vent sweating. This is situation dependent and often dictated by where the equipment and ducts are installed… attics are the WORST for this.

When condensation occurs you can either drop the dew point (humidity) of the air around it or increase the temperature of the surface that is sweating. Sometimes decreasing the dewpoint of the air is very hard (like ducts in an attic) so we are left with increasing the temperature of the duct with either more insulation or warmer air going through it.

Another thing worth mentioning is that varible speed blowers and multi-stage compression paired with humidity controls can help a lot with the coil temperature and run time side of the equation. Even then, they aren’t a silver bullet to fix all issues and if you over promise you may end up with a dissatisfied customer.

Once more… For lower humidity in a home, you want..

• Longer run times
• Colder evaporator coil
• Less moisture coming in from outside
• Less moisture being generated from outside
• Higher indoor temperatures
• Extra moisture removal with dehumidification when required
• Spot ventilate when cooking or bathing

— Bryan

## What is the “Mid Point” of a refrigerant blend

As we have mentioned in several previous articles, many blended refrigerants have glide, which simply means they boil and condense over a range of temperatures instead of just one temperature.

As an example consider refrigerant R407c, it is a zeotropic blend which means it has enough glide that it makes a big difference if you fail to take it into account.

For example, on an evaporator coil running R407c the refrigerant leaving the TXV will begin boiling at the bubble point, let’s say that the pressure in the evaporator is 80 PSIG that bubble temperature will be 40°.

Now as the refrigerant continues boiling the temperature will begin increasing towards the Dewpoint which is 50.8°. Any temperature gained ABOVE 50.8° on a R407c system at 80 PSIG is superheated, meaning the refrigerant is completely vapor.

So we calculate superheat as temperature above the dew point and subcool as temperature below the bubble point and the condensing temperatures and evaporator temperature aren’t fixed but they GLIDE between the bubble and dew and back again when the refrigerant is changing state.

But what does this mean for evaporator and condensing temperatures when calculating target head pressure (condensing pressure) and suction pressure (evaporator pressure) also known as evaporator TD and condensing temperature over ambient?

The simplest way is to use the midpoint between the dew and bubble points to calculate CTOA and DTD.

In the case above you would simply calculate 50.8° + 40° = 90.8 | 90.8 ÷ 2 =  45.5° average evaporator temperature or midpoint

Emerson points out that evaporators would be better calculated using 40% of bubble and 60% of dew but the extra complexity generally doesn’t make enough difference to mention.

I made this video to demonstrate further

— Bryan

## Understand Dew Point and Absolute Moisture, The Right Side of the Psych Chart

Let’s first state the obvious. Most techs are intimidated by Psychrometric charts and Mollier diagrams, we JUST ARE. While there are some pretty complicated formulas that back up these diagrams, using them isn’t a big of a deal once you understand the different elements and then focus on one at a time.

BUT WHY DO YOU CARE?

Dew point is one example of a very useful measurement to understand, design for and test for in an HVAC/R system. Take an evaporator coil, do you know how to calculate the exact temperature at which that evaporator coil will start to condense moisture? can you tell the exact temperature at which a surface inside of a space will start to condensate and possibly grow mold? These are both cases where a basic understanding of a psychrometric diagram can help a technician.

While some of the elements on the chart are represented by curved or slanted lines, dew point temperature and humidity ratio / absolute moisture content are just straight lines horizontally across the chart.

So if we focus on a 65°F(18.33°C) dew point on the right side of the chart you will notice it crosses  over 92 grains (there are 7000 grains of moisture per lb) of moisture line and then goes all the way across until it intersects with the curved 100% humidity line on the left side. This shows us that at a 65°F(18.33°C) dew point the air always contains 92 grains of moisture per lb.. ALWAYS.

This also shows us that when the air is at 100% relative humidity the dew point, wet bulb and dry bulb temperatures are ALL THE SAME.

If we have a dew point of 55°F(12.77°C) the air contains 64 grains of moisture per lb. If the dew point is 30°F(-1.11°C) the air contains 24 grains… you get the point.

So now if you find the dry bulb temperature and the relative humidity you can easily calculate the dew point at which that same air will reach saturation and begin to form condensation.

Let’s say we have 75°F(23.88°C) dry bulb air at 50% relative humidity. We would simply draw a line up from the bottom at 75°F23.88°C) until we hit the curved 50% line. Then go right (or left) until you bump into the the grains of moisture and then the dew point scale. Now you know at what temperature that same air mass will start to condense water.

So we can see that this if this 75°F(23.88°C) dry bulb 50% relative humidity mass of air comes in contact with a surface that is 55.5°F(13.05°C) or less, it will begin to condense water. We also know that this air stream contains 65 grains of moisture per lb of air.

Forgive me for saying so, but I think this is pretty cool.

— Bryan

P.S. – If you want a good quality Psychrometric chart you can use THIS ONE

## Psychrometrics Basics w/ Jamie Kitchen Podcast

In this episode of the podcast Jamie Kitchen from Danfoss comes on to talk about Wet bulb, Dry bulb, Relative Humidity, Dew point, enthalpy and latent heat. As well as what it all means and why you care.

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE

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