Tag: Psychrometrics

This is the second article in a three-part series, where Advanced Psychrometrics are explored. The source material for each of the articles in this series is ACCA Manual P Sections 3, 4, and 5. This article is based on information found in Section 4.

If you followed the previous Advanced Psychrometrics article, you now know how to use a Psych Chart to plot a Room Sensible Heat Ratio (RSHR) Line, and how to calculate Design Room CFM. However, if you followed that exercise, you will note the absence of real-world variables, such as ventilation and bypass factors. Equipment Sensible Heat Ratios are almost never an exact match to the RSHR. This exercise will account for these variables, and walk you through how to plot these properties on a Psychrometric Chart. 

It is worth reminding you that this is an exercise to help illustrate the complexities of psychrometry in the real world. This may not always be a practical method utilized in the design process. 

When outdoor ventilation air is mixed with return air before the equipment coil, the equipment is exposed to latent and sensible loads beyond that of just the conditioned space. This characteristic causes the Coil Sensible Heat Ratio (CSHR) to alter from the RSHR. Remember, the Room Design Conditions will be met only when the supply air properties fall on the RSHR Line. With two different SHRs, we no longer have the luxury of choosing any supply condition we wish. The supply air must be able to cool and dehumidify the space. It also must now compensate for the additional load introduced by the ventilation air. Therefore, the only supply condition that will satisfy the Room Design Condition is the point at which both the RSHR Line and CSHR Line meet on the Psych Chart.

To plot the RSHR Line should be a breeze at this point. For a review on that process, and the first part of this article series, CLICK HERE.

The construction of the CSHR Line, however, is a bit more involved. There is a little trial and error in the construction of the CSHR Line. It’s not impossible, of course, and with practice, you get pretty good at nailing it on the first try. Here’s why a trial and error process is required in order to plot the CSHR Line:

  • The location of the CSHR Line is determined by the Mixed Air Condition (MAT) and CSHR
  • The CSHR and the MAT can’t be plotted without knowing the percentage of Outdoor Air (OA)
  • The percentage of OA can be calculated only when the supply CFM is known.
  • The supply CFM can be calculated only when the ΔT between the room return and supply is known, which is determined by the intersection of the CSHR and RSHR Lines
  • The CSHR Line is the line we are solving for; therefore, it is unavailable.

This is why a trial and error process is required. Simply put, we’re going to use an estimated guess as to what we think our supply air condition will be, then follow the process until we can determine if our selection actually results in the intersection of the CSHR and RSHR Lines. To help aid in the accuracy of your guess, keep in mind that, on average, a Direct Exchange Fan Coil can provide supply air temperatures which may fall between 14-25 degrees below the space temperature at typical relative humidities between 80% and 95%.

To begin this exercise, let’s start with some basic information, which will ALWAYS be available to you from a quality load calculation. This information can be plotted on the Psych Chart with complete certainty:

Room Sensible Heat: 21,700 BTUh

Room Latent Heat: 2,300 BTUh

Room Total Heat: 24,000 BTUh

RSHR: 0.90

Room Design Condition: 75℉ db / 50% RH

Outdoor Design Condition: 95℉ db / 75℉ wb

Ventilation Required: 245 CFM

In this scenario, a Room-to-Room load calculation has been done on a home. The RSHRs have all been averaged together for a mean room sensible heat ratio. We can go ahead and plot what we can on the chart:

Let’s select a 57℉ supply temperature at about 90% RH. Now we can determine the Supply CFM. Since the RSHR is the average of the entire home, the Supply CFM will equal the total system CFM.

CFM = Room Sensible Load ÷ (1.08 x ΔT)

21,700 ÷ (1.08 x 18) = 1,116 CFM

Now that we know the Supply CFM, we can calculate the percentage of ventilation air.

Ventilation =  245 CFM ÷ 1,116 CFM 

Ventilation = 22%

We have a good bit of information here now, but the math starts to get a little confusing without explanation. We now know that 22% of Outdoor Air (at 95℉ db / 75℉ wb) will be mixing with the remaining 78% Return Air (at 75℉ db / 50% RH). To calculate the Mixed Air Condition, complete the following equation:

MAT = (0.22 x 95℉) + (0.78 x 75℉)

MAT = 20.9℉ + 58.5℉

MAT = 79.4℉

We can now plot the Mixed Air Condition on the Psych Chart.

At this point, we have everything we need to construct the Coil Sensible Heat Ratio Line. If you notice on the Psych Chart, there is a list of helpful formulas to the left of the page. We need to solve for Total Coil Heat Load (Qt) if we are to determine Coil Sensible Heat Load (Qs) and CSHR. To do that, we need to figure out the change in enthalpy (ΔH). Enthalpy is heat energy in BTUs per pound of dry air.

ΔH = 30.6 – 23.4

ΔH = 7.2

Now let’s plug our ΔH into the Total Coil Heat Load calculation. (4.5 here is Air Density x Run Time in minutes. 0.075 x 60 = 4.5)

Qt = 4.5 x CFM x ΔH

Qt = 4.5 x 1,116 x 7.2

Qt = 36,158 BTUh

Solve for Coil Sensible Heat Load. To do this, make sure you are using the entering air condition the equipment will actually see: MAT.

Qs = 1.08 x CFM x ΔT

Qs = 1.08 x 1,116 x 22.5

Qs = 27,119 BTUh

We can finally solve for Coil Sensible Heat Ratio at this point:

CSHR = Coil Sensible Load ÷ Total Coil Load

CSHR = 27,119 BTUh ÷ 36,158 BTUh

CSHR = 0.75

We can now plot the CSHR Line on the Psych Chart.

If you look closely, you may be thinking, “Wait a second, the CSHR Line does not intersect with the RSHR Line.” You would be absolutely correct. This is why the trial and error solution is necessary. However, if you notice, the CSHR Line is extremely close to our selected supply temperature. The CSHR Line is just slightly above the RSHR Line.

What does this mean?

We can still use the design CFM and supply condition, and the equipment will satisfy the sensible load, but will maintain a slightly higher humidity level in the space than what was designed. Take a look at the actual grains of moisture for the Mixed Air Condition in comparison to the Supply Air off the coil at 57℉.

The equipment will be able to dehumidify from 73 grains of moisture/lb of dry air down to 63 grains of moisture, rather than the ideal 62 grains. We’re talking about a difference of 1 grain of moisture. This can be acceptable, and the difference likely unnoticeable. In cases where a coil selection will not match the latent load requirements of a space, a viable option would be to add supplemental dehumidification to deal with the remaining latent load. Ultra-Aire Ventilating Dehumidifiers are an excellent option, and will also help lessen the additional latent load from the ventilation air.

Lastly, let’s talk about Bypass Factor. Remember, the ideal supply temperature would be the apparatus (equipment) dew point. However, there is a small percentage of air that will bypass the coil and not transfer its heat to the coil. This can be calculated using the known apparatus dew point. The Bypass Factor formula is as follows:

 Bypass Factor = (Supply Air Temperature – Apparatus Dew Point) ÷ (Mixed Air Temperature – Apparatus Dew Point)

Bypass Factor = (57 – 53.5) ÷  (79.4 – 53.5)

Bypass Factor = 3.5 ÷ 25.9

Bypass Factor = 0.14

At this point, you would need to look up a manufacturer’s extended performance data for their equipment to ensure that the coil you select will meet a sensible capacity of 27,119 BTUh and a total capacity of 36,158 BTUh at 1,116 CFM, with an entering condition of 79.4℉ db / 65.6℉ wb and an outdoor condition of 95℉ db / 75℉ wb. Let me translate that to something you might actually see on a Performance Table:

Entering Air Condition: 80℉ db / 67℉ wb

Outdoor Air Conditions: 95℉ / 75℉ wb

Total Capacity: 36,000 BTUh

Sensible Capacity: 27,000 BTUh

Airflow: 1,100 CFM

If you can select a coil that will match these criteria, you will be able to maintain an indoor air condition that is nominally close to your design.

To see how this chart would look in another scenario (without going through the step-by-step process), here is a psych chart based on my house and ASHRAE Design Conditions:

Room Sensible Heat: 16,800 BTUh

Room Latent Heat: 7,200 BTUh

Room Total Heat: 24,000 BTUh

RSHR: 0.70

Room Design Conditions: 75℉ db / 50% RH

Outdoor Design Conditions: 90℉ db / 80℉ wb

Ventilation Requirement: 46 CFM

In this case, my selected supply air condition happened to fall perfectly at the intersection of the CSHR and RSHR Lines. The tricky part, however, is finding a coil that will meet the sensible and latent heat requirements under the design conditions. I would need to look for a coil with 18,000 BTUh sensible capacity and 29,000 BTUh total capacity. I’d have to settle for a 2.5 ton (30,000 BTUh) coil with a close CSHR under design conditions, and potentially add supplemental dehumidification. (A Carrier FB4C–030 would actually fit the bill quite nicely.) Remember, the equipment selection performance table will have actual capacities that differ from the nominal rating; thus, care must be taken when using manufacturer performance tables to select equipment.

If you’ve made it to the end of this exercise, congratulations: you are as nerdy as they come! I hope this helps illustrate the complexities of psychrometrics. If nothing else, the take away should be a new-found respect for psychrometrics, and its integration into a technician’s daily diagnostic toolbag.

Stay tuned for Part 3, where we will dive into ACCA Manual P, Section 5. There we will learn how to account for duct gains, and how reheat dehumidification looks on a Psych Chart.

 

–Kaleb Saleeby

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 = Room Sensible Load ÷  Room Total Load

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

For access to the Testo Psych Chart I used for this article, click here.

 

— Kaleb

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

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