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How to Calculate Ventilation Air

Education

MEP Academy Instructor

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October 11, 2024

In this article we’ll cover ASHRAE Standard 62.1 which outlines the ventilation requirements for acceptable indoor air quality (IAQ) in commercial and institutional buildings. The standard uses a combination of the Ventilation Rate Procedure (VRP), which calculates the amount of outdoor air needed based on space type, occupancy, and area.

If you prefer to watch the Video of this article, then scroll to the bottom.

ASHRAE 62.1 Minimum Ventilation Air Requirements

The ASHRAE 62.1 ventilation rate formula is based on three key factors. the number of people in the space, the square footage of the area, and the zone air distribution effectiveness (Ez). The number of people determines the amount of fresh air needed for occupants, while the square footage accounts for the ventilation required to offset contaminants from the building materials and activities. The zone air distribution effectiveness adjusts the airflow based on how well the ventilation system distributes air within the space, ensuring optimal air quality.

Let’s go through three examples using an office, retail store and classroom to illustrate how the ASHRAE 62.1 ventilation rate calculation works in different spaces.

Example 1. Office Space

Given Data.

Occupancy Type. Office space
Floor Area. 5,000 square feet
Occupancy Density. 5 people per 1,000 square feet (as per ASHRAE 62.1 Table)
Outdoor Air Rate per Person. 5 CFM per person
Outdoor Air Rate per Area. 0.06 CFM per square feet

Step 1. Calculate the total number of occupants.

Number of occupants equals Floor Area Occupancy Density. This equals 5,000 square feet divided by 1,000 square feet, multiplied by 5 people per 1,000 square feet equals 25 people.

Calculation for number of occupants per ASHRAE 62.1 — Calculation for Number of occupants per ASHRAE 62.1

Step 2. Calculate the ventilation rate required for occupants.

Ventilation Rate (People) equals Number of Occupants times Outdoor Air Rate per Person. The Ventilation Rate equals 25 people times 5 CFM per person equals 125 CFM for the people.

Calculation for Ventilation Air required for people in an Office

Step 3. Calculate the ventilation rate required for the area.

Ventilation Rate (Area) equals Floor Area times Outdoor Air Rate. This equals 5,000 square feet times 0.06 CFM per square feet equals 300 CFM for the area.

Calculation for the ventilation Air required for the Area of an Office

Step 4. Total ventilation rate calculation using ASHRAE’s additive method.

Total Ventilation Rate equals (Ventilation Rate for the People) plus (Ventilation Rate for the Area). The Total Ventilation Rate equals 125 CFM for the people plus 300 CFM for the area, for a total of 425 CFM.

Therefore, for this office space, the required outdoor air ventilation rate is 425 CFM.

What we haven’t covered is how the layout of the supply and return grilles affect the amount of ventilation air required. We’ll cover this at the end of this article.

Example 2. Retail Store

Retail store ventilation air calculation

Given Data.

Occupancy Type. Retail store
Floor Area. 10,000 square feet
Occupancy Density. 15 people per 1,000 square feet (as per ASHRAE 62.1)
Outdoor Air Rate per Person. 7.5 CFM per person
Outdoor Air Rate per Area. 0.12 CFM per square feet

Step 1. Calculate the total number of occupants.

Number of Occupants equals Floor Area Occupancy Density. This equals 10,000 square feet divided by 1,000 square feet, multiplied by 15 people per 1,000 square feet equals 150 people

Step 2. Calculate the ventilation rate required for occupants.

Ventilation Rate (People) equals Number of Occupants times Outdoor Air Rate per Person. The Ventilation Rate equals 150 people times 7.5 CFM per person, for a total of 1,125 CFM for the people.

Step 3. Calculate the ventilation rate required for the area.

Ventilation Rate (Area) equals Floor Area times Outdoor Air Rate. This equals 10,000 square feet times 0.12 CFM per square feet, for a Total of 1,200 CFM for the area.

Step 4. Total ventilation rate calculation.

Total Ventilation Rate equals (Ventilation Rate for the People) plus (Ventilation Rate for the Area). The Total Ventilation Rate equals 1,125 CFM for the people plus 1,200 CFM for the area, for a total of 2,325 CFM

Therefore, for this retail store, the required outdoor air ventilation rate is 2,325 CFM.

Example 3. Classroom

Given Data

Occupancy Type. Classroom
Floor Area. 1,200 square feet
Occupancy Density. 35 people per 1,000 square feet
Outdoor Air Rate per Person. 10 CFM per person
Outdoor Air Rate per Area. 0.12 CFM per square feet

Step 1 Calculate the total number of occupants.

Number of Occupants equals Floor Area Occupancy Density. This equals 1,200 square feet divided by 1,000 square feet, multiplied by 35 people per 1,000 square feet, for a total of 42 people

Step 2 Calculate the ventilation rate required for occupants.

Ventilation Rate (People) equals Number of Occupants times Outdoor Air Rate per Person. the Ventilation Rate equals 42 people times 10 CFM per person, for a total of 420 CFM for the people.

Step 3 Calculate the ventilation rate required for the area.

Ventilation Rate (Area) equals Floor Area times Outdoor Air Rate. This equals 1,200 square feet times 0.12 CFM per square feet, for a total of 144 CFM for the area.

Step 4 Total ventilation rate calculation.

Total Ventilation Rate equals (Ventilation Rate for the People) plus (Ventilation Rate for the Area) Total Ventilation Rate equals 420 CFM for the people, plus 144 CFM for the area, for a total of 564 CFM

For this classroom, the required outdoor air ventilation rate is 564 CFM.

The Zone Air Distribution Effectiveness (Ez)

Zone Air Distribution Effectiveness (Ez) is a factor used in ASHRAE 62.1 to account for how efficiently an HVAC system delivers and mixes outdoor air within a given space or zone. It reflects how well the ventilation air is distributed to the occupants’ breathing zone, impacting the amount of fresh air needed for adequate ventilation. The effectiveness varies based on how the air is supplied and returned within the space, considering factors like supply air temperature and system design.

How Air Distribution Effectiveness Ez Varies

1. Floor Supply, Floor Return, Heating Mode

When warm air is supplied from the floor and mixed into the space, it can be pulled downward and distribute evenly within the breathing zone, leading to a higher effectiveness. ASHRAE indicates a 1.0 E_z for floor supplied and returned warm air. The 1.0 doesn’t add or subtract any CFM from the previous calculations.

2. Floor Supply, Ceiling Return, Heating Mode

When warm air is supplied from the floor and mixed into the space, it tends to rise and may not reach occupants in the lower part of the room effectively and may not distribute evenly within the breathing zone, leading to a lower effectiveness. ASHRAE indicates a 0.7 E_z for floor supplied and ceiling returned warm air. The 0.7 will add CFM to our previous calculations. For example, our office space was calculated to be 425 cfm. Taking into consideration that the air distribution effectiveness is 0.7, the new CFM would be calculated to be 425 divided by 0.7 equals 607 CFM

Floor Supply of Warm Air and Ceiling Return

Key Factors Influencing Air Distribution Effectiveness

Supply air temperature vs. room air temperature. Larger temperature differences can reduce the effectiveness.

Supply and return air locations. Proper placement of supply and return ducts can improve Ez.

Ventilation system type. Systems like displacement ventilation or underfloor air distribution (UFAD) often have higher effectiveness due to better air mixing.

In general, the higher the Ez value, the more effective the ventilation system is in distributing air, which allows for less outdoor air to achieve the same indoor air quality, making the system more efficient.

This was a simplified explanation of the basic ventilation calculation, as there are a few other key considerations that can change the total CFM required.

Key Considerations

Diversity Factor. In some cases, ASHRAE 62.1 allows the use of a diversity factor to account for spaces that aren’t fully occupied all the time.
System Ventilation Efficiency. Depending on the distribution system (100% Outdoor Air, Multi-zone recirculating, or VAV systems), system ventilation efficiency (Ev) must be factored in to adjust the total outdoor airflow.

These examples show how ventilation rates are calculated based on both the number of occupants and the size of the space, ensuring adequate indoor air quality per ASHRAE 62.1.

How to Calculate Ventilation Air in accordance with ASHRAE 62.1

Flow Meters

HVAC Engineering

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October 6, 2024

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In this article, we’ll look at flow meters, exploring the different types used to measure liquids, gases, and steam. Whether you’re working with water, steam, or natural gas, understanding the right flow meter for your application is critical for accuracy and efficiency. We’ll show you how to calculate the flow based on the measured data.

If you prefer to watch the video of this article, then scroll to the bottom.

Inline Magnetic Flow Meter

Magnetic flow meters measure flow based on Faraday’s law of electromagnetic induction. Two opposing magnets create a magnetic field that the fluid passes through. As the fluid passes through the magnetic field, the electrodes detect the voltage, which is proportional to the fluid flow.

This flow meter will require electrical power or an onboard battery. The information gathered from the meter can be sent to a building automation system for various sequence of operation control strategies.

There are no moving parts or obstructions in the flow path. This provides a very low to no pressure drop. This makes them a low maintenance meter while also being easy to calibrate. You will need at least 3 or more diameters of upstream straight pipe and two diameters for downstream. This is to ensure accurate readings. Check the manufacturers literature as they can vary.

Because these are inline meters and have accuracies ranging from ±0.2% to ±1% of the flow rate they are more expensive than other types especially as the pipe sizes become larger. For these reasons an inline magnetic flow meter would not be the best choice for HVAC applications unless high accuracy is required.

Insertion Turbine Flow Meter

Uses one or more turbine wheels that spin as fluid flows through the system. The rotational speed of the turbine is proportional to the flow velocity. As the turbine spins the meter keeps track of the number of complete revolutions the wheels make. The revolutions per minute is then easily converted to velocity and with a known pipe size a flow rate in GPM or liters per second can be determined.

These meters are inserted into the flow path rather than being inline, minimizing disruption to the system, while making them more affordable. These meters are more commonly specified for HVAC projects for there ease of installation and their lower price. With the use of a hot tap some of these flow meters can be installed into a pressurized and actively flowing pipe. The insertion turbine meter will require additional upstream and downstream straight pipe lengths than that required for the inline magnetic flow meter.

There will be a low pressure drop caused by the turbine wheel, and since they have moving parts, the maintenance will be greater in addition to the time between recalibrations.

Inline Vortex Flow Meter

By placing an object in the flow path that creates vortices that can be measured by the meter a frequency is created that is proportional to the flow rate. There are sensors located along the shredded vortex that measures the pressure changes. The frequency at which each vortex is shed is directly proportional to the velocity of the flowing liquid.

There is also the option to provide insertion type vortex flow meters.

The flow meter has a moderate pressure drop due to the bluff body obstructing the flow, but usually acceptable in most applications.

Clamp-on Ultrasonic Flow Meter

Ultrasonic sound waves are used to measure flow. Transducers are clamped onto the exterior of the pipe, and sound waves are sent through the pipe wall and fluid in both directions. The difference in sound wave transit time is proportional to the flow rate. The transducers act both as a transmitter and a receiver, alternating between sending and receiving sound waves.

There is no pressure drop, as there is no direct contact with the flow or intrusion into the pipe.

The flow meter is ideal for non-invasive measurement of flow in water, natural gas, and liquid applications. Common in applications where retrofitting is required, such as existing pipelines, or where contamination must be avoided.

There is a small handheld meter that can be used for smaller pipes.

Calculating Flow

To determine how much water is flowing through the system a calculation is done to derive at gallons per minute or liters per second. The flow meters determine the velocity which needs to be converted to flow based on the pipe size.

ASHRAE provides recommended velocities for various pipe sizes and whether the system is constant or variable flow. Chapter 22 of ASHRAE’s fundamentals handbook recommends that for an 8-inch pipe in a variable flow system with greater than 2,000 operating hours a velocity of 8.98 feet per second or 2.74 meters per second be used. We’ll use 9 feet per second.

To calculate flow, we’ll need to know the area of the pipe because flow requires us to know velocity and area. For an 8 inch or 200-millimeter pipe we need to determine the area. For an 8-inch pipe the area equals 0.349 ft2. The GPM equals velocity times the area times the conversion factor. The conversion factor equals 448.8312 which converts cubic feet per second to gallons per minute as follows. 1 cubic foot of water equals 7.48052 gallons.

GPM equals feet per second (Velocity) times square feet (Area) times 448.8312 which is the conversion factor.

GPM equals 9 feet per second times 0.349 square feet times 448.8312 equals 1,410 GPM

Each of these flow meters has specific strengths and limitations, so the best choice depends on factors like the fluid being measured, temperature requirements, and system pressure drop constraints.

Flow Meters Explained – Magnetic Inline, Insertion Turbine, Vortex Inline and Clamp-on Ultrasonic

Understanding Dry Bulb, Wet Bulb, and Wet Bulb Depression

HVAC Engineering

MEP Academy Instructor

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September 25, 2024

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Understanding Dry Bulb, Wet Bulb, and Wet Bulb Depression

Dry Bulb Temperature and Wet Bulb Temperature are both essential in understanding air properties, especially in HVAC applications. We’ll explain these two temperatures and how they relate to evaporative cooling and relative humidity using a psychrometric chart.

If you prefer to watch the Video of this presentation, then scroll to the bottom of this page.

Dry Bulb Temperature

Dry Bulb is the temperature of the air measured by a regular thermometer, without considering moisture. It’s measured using a standard thermometer exposed to the air but shielded from direct solar radiation. Dry bulb temperature is what people commonly refer to as “air temperature.” It indicates the heat level of the air and is crucial for thermal comfort and HVAC system design.

When trying to understand the current air conditions, one must also look at the wet bulb temperature, not just the dry bulb temperature, as it accounts for humidity and provides a more complete picture of heat stress and cooling potential.

Wet Bulb Temperature

Wet Bulb is the temperature a parcel of air would have if cooled to saturation (100% relative humidity) by evaporation. The wet bulb temperature will always be lower than or equal to the dry bulb temperature because evaporation absorbs heat. It’s measured by wrapping a wet wick around a thermometer bulb and allowing evaporation to cool the bulb, with the resulting temperature reflecting the cooling effect of moisture in the air.

As water evaporates, the temperature drops, and this lower reading is the wet bulb temperature. Wet bulb temperature helps assess the amount of moisture in the air. It is used in processes like evaporative cooling and determines the cooling efficiency in such systems.

Wet Bulb Depression

The Wet Bulb Depression is an indicator of how much the air can cool down through the process of evaporation. The larger the temperature depression, the drier the air, which means it has more capacity to absorb moisture.

We can see on this psychrometric chart that a dry bulb temperature of 85 Fahrenheit minus the wet bulb temperature of 55 Fahrenheit equals a wet bulb depression of 30 Fahrenheit or 16 Celsius

Wet Bulb Depression = Dry Bulb Temperature – Wet Bulb Temperature

A high Wet Bulb depression means that there is significant potential for evaporative cooling. For example, in hot, dry environments, where the Dry Bulb Temperature is much higher than the Wet Bulb Temperature, evaporative cooling (like using a swamp cooler) is very effective.

A low Wet Bulb depression (where Dry Bulb Temperature is close to Wet Bulb Temperature) indicates the air is near saturation with moisture, so there is less cooling potential through evaporation.

Relative Humidity (RH)

Relative Humidity is the percentage of moisture the air holds compared to the maximum it can hold at that temperature. When the Dry Bulb Temperature and Wet bulb temperature are close together, the relative humidity is high because less evaporation is occurring. When Dry Bulb and Wet bulb temperatures are far apart, the air is dry, and relative humidity is low, as there is more capacity for moisture to evaporate into the air.

We can see that the relative humidity is at 10%.

If the wet bulb temperature increased to 60 F or 15 C, then the wet bulb depression decreases to 25For 14C. We can see that the relative humidity has increased to 20%.

If the wet bulb keeps climbing and the dry bulb stays the same, the wet bulb depression keeps shrinking, while the relative humidity increases. This informs us that as the wet bulb gets closer to the dry Bulb Temperature the relative humidity increases. For efficient use of a swamp cooler or evaporative cooler, a wet bulb depression of at least 15°F to 20°F (8°C to11°C) or more is generally required. This means the difference between the dry bulb temperature (ambient air temperature) and the wet bulb temperature should be at least 15°F (8°C), indicating low enough humidity for effective evaporation and cooling. Evaporative cooling is best suited for hot, dry climates with low humidity.

Dry Bulb, Wet Bulb and Wet Bulb Depression

Here the dry bulb and wet bulb temperatures are the same at which point we have 100% relative humidity, and we have also reached the dew point line where condensate occurs. The wet bulb depression is zero because the dry bulb and wet bulb temperatures are the same.

100% Relative Humidity equals a Zero Wet Bulb Depression

Dew Point Temperature

Dew point is the temperature at which air becomes fully saturated (100% Relative Humidity) and moisture condenses into liquid (dew). If Dry Bulb Temperature falls to the dew point temperature, condensation occurs, leading to dew or fog. When the Wet bulb temperature is close to the DBT, the air is near saturation, and the dew point is close to the current temperature, meaning high humidity levels.

Understanding Dry Bulb, Wet Bulb and Wet Bulb Depression

A Guide to Refrigerant R454B and R32

MEP Academy Instructor

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September 10, 2024

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Why are we changing refrigerants again? As the battle against high Global Warming Potential refrigerants rages on, air conditioning manufacturers are left feeling like they’re in a never-ending game of limbo—constantly asking, how low can you go with each new refrigerant mandate. Refrigerant R454B and R32 are becoming the new darlings of the industry for smaller commercial and residential systems.

If you prefer to watch the video of this presentation, then scroll to the bottom.

R454B has a lower Global Warming Potential than R410A. R410A is being phased out like R11, R12 and R22 were. R454B has a Global Warming Potential of 466, while R410A has a value of 2,088, which is above the new threshold of 700. The higher the value the worst the refrigerant is for the environment. R32 comes in at 675, just under the mandate.

The questions we’ll answer are, how much does R32 and R454B cost compared to other refrigerants? Will I need new tools and equipment to work with R32 and R454B? Can I use R32 or R454B as a drop-in replacement for an existing R410A, or R22 system?

Effects of Global Warming Potential on Refrigerant Cost

The cost per pound of refrigerant is influenced by its Global Warming Potential (GWP) and whether it is being phased out or has restricted production:

Refrigerants with higher GWP values are more environmentally damaging and are increasingly subject to regulatory restrictions. As regulations tighten, such as those under the Kigali Amendment to the Montreal Protocol, the demand for low-GWP refrigerants rises. This demand shift can lead to a decrease in the availability of high-GWP refrigerants, driving up their cost.

The effects of GWP on Refrigerant Cost per Pound

When a refrigerant is phased out or its production is restricted, as seen with R11, R12 and R22, the supply diminishes while existing systems still require the refrigerant for maintenance. This limited supply, combined with ongoing demand, results in a significant increase in cost per pound. R410A is currently available at a reasonable cost per pound but that will start to change as production decreases and other refrigerants with lower global warming potential values are produced and installed.

Refrigerants with high GWP values and those subject to phaseouts typically become more expensive over time due to increased regulatory pressure and reduced availability.

Daikin is currently a manufacturer of R32 and residential units that use R32. New R454B AC systems will become more available in 2025. The manufacturing or importing of R-410A residential and light commercial air conditioning products is prohibited starting January 1, 2025.

Your price per pound will vary based on how much refrigerant you buy from your supplier.

Can I convert an R410A System to Refrigerant R454B

R410A systems are not compatible with R454B due to differences in refrigerant characteristics, including pressure, temperature glide, and flammability. R410A is a class A1 refrigerant, while R454B is a class A2L refrigerant which is slightly more flammable. As a result, retrofitting R410A systems to use R454B is not advisable. New systems specifically designed for R454B will be required.

R410A operates at higher pressures than R454B, making the compressor and condenser less compatible with a lower pressure refrigerant. Additionally, the components in systems designed for R410A are not suitable for use with lower flammability AL2 refrigerants like R454B. Before charging the system with R454B, you must replace these components with ones that are designed to safely handle a slightly more flammable refrigerant.

Refrigerant Comparison Chart. Can you drop-In R454B or R32 into and Existing R410A or R22 system

R-454B is not a drop-in replacement for R-410A or R22. While R-454B shares many characteristics with R-410A, its use is restricted by codes and regulations to systems specifically designed for it.

The same is true for R32. R32 is not a drop-in replacement for R410A or R22.

Can I use my existing tools and equipment on a R32 or R454B system?

A refrigeration technician might be able to use their existing R410A or R22 manifold gauges, leak detectors, vacuum pumps, refrigerant recovery machines, and other tools directly with the new R32 or R454B refrigerant systems. You will need to confirm with the manufacturer to see if it’s approved for multiple refrigerants. This is because R32 and R454B are classified as an A2L refrigerant. These refrigerants are mildly flammable, and may require tools that are specifically rated for use with A2L refrigerants.

We have found new gauge manifolds that are rated for all four refrigerants discussed here. It’s just a matter of buying the right equipment and tools. Never use a tool or piece of equipment that is not specifically approved for the refrigerant in question. You may have older tools that weren’t built to handle the new refrigerants, in which case you’ll need to buy new ones.

To work with R32 and R454B, technicians will need to use tools and equipment that are compatible with A2L refrigerants. This includes gauge manifolds, recovery machines, and vacuum pumps that are designed to safely handle the flammability of refrigerants that are classified as A2L refrigerants. Using the correct equipment is crucial to ensure safety and compliance with regulations when working with R32 or R454B systems.

Can I use the existing Refrigerant Piping

When changing from an R22 or R410A system to an R32 or R454B system, the refrigerant piping generally does not need to be replaced. This is provided that the existing piping is in good condition and appropriately sized for the new refrigerant. R32 often requires smaller pipes than R22.

However, it’s crucial to ensure that the existing piping is thoroughly cleaned and free of any residual oil or contaminants from the previous refrigerant. R454B, like other A2L refrigerants, requires the use of specific lubricants (such as POE oil) that are compatible with the new refrigerant. The system may need to be flushed to remove any incompatible oil or residue before charging with R454B.

Additionally, the piping should be carefully inspected for leaks and pressure-tested to ensure it can handle the operating conditions of the new refrigerant. If the existing piping is in poor condition or not properly sized, replacement may be necessary.

See how the latest Refrigerants compare to the older R410A, R22, R12 and R11

Example 1. Office Space

Example 2. Retail Store

Example 3. Classroom

The Zone Air Distribution Effectiveness (Ez)

How Air Distribution Effectiveness Ez Varies

1. Floor Supply, Floor Return, Heating Mode

2. Floor Supply, Ceiling Return, Heating Mode

Key Factors Influencing Air Distribution Effectiveness

Key Considerations

Inline Magnetic Flow Meter

Insertion Turbine Flow Meter

Inline Vortex Flow Meter

Clamp-on Ultrasonic Flow Meter

Calculating Flow

Dry Bulb Temperature

Wet Bulb Temperature

Wet Bulb Depression

Relative Humidity (RH)

Dew Point Temperature

Effects of Global Warming Potential on Refrigerant Cost

Can I convert an R410A System to Refrigerant R454B

Can I use my existing tools and equipment on a R32 or R454B system?

Can I use the existing Refrigerant Piping

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