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Flow Meters

HVAC Engineering

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October 6, 2024

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

VAV Laboratory Fume Hood Control

MEP Academy Instructor

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September 2, 2024

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In this article, we’ll explore one effective method for managing VAV systems in laboratory settings, focusing on the sequence of operations that ensures a safe and energy-efficient environment.

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

A common strategy for addressing exhaust and supply requirements while minimizing energy consumption is to implement a variable air volume (VAV) lab control system. This system can be as straightforward as a single exhaust fan connected to two variable-volume fume hoods, or it can scale up to include multiple fans that activate based on laboratory demand.

No matter the size of the lab, each VAV fume hood is equipped with an exhaust airflow control valve to regulate airflow through the hood. Typically, hoods used for standard research are connected to a manifolded exhaust duct that leads to roof-mounted fans. Additionally, the laboratory space is supplied with fresh air through a duct with a modulating damper to control the air’s volumetric flow.

A VAV lab system is managed to maintain a safe environment by use of the following.

Space Pressurization

Laboratories typically operate under negative pressure to contain fumes and odors. To achieve this, the control system is configured to exhaust more air than is supplied. The amount of exhaust air required is directly related to the Air Changes required, Fume Hood airflow capacity, and or the amount of conditioned supply air needed based on heat gain driven labs.

Chemical Fume Hood Flow

The VAV fume hood is equipped with a fume hood monitor, sash position indicator, controller and an airflow control valve that adjusts the airflow based on the position of the sash. While exhaust velocities vary by application, the use of 100 feet per minute is often used. If you have a 6-foot fume hood with an open sash height of 24 inches, then the volume of air would be 6-feet times 2-feet equals 12 square feet times 100 feet per minute, equals 1,200 CFM. An 18-inch open sash would equal 900 CFM. In a lab that measures 64 feet in length by 32 feet wide with a 10-foot ceiling at least 2,048 CFM would be required to achieve the 6 air changes required for this lab. If you want to learn more on how to calculate air changes per hour see our other video.

Laboratory Fume hoods with Venturi type airflow Valves

Lab Airflow Valves

A VAV laboratory controller maintains negative pressure by dynamically adjusting the fume hood exhaust valves, general airflow exhaust valves, and supply airflow valves in response to real-time conditions.

Fume Hood Exhaust Valves

The controller modulates the fume hood exhaust valves based on the sash position. When a sash is opened, the valve opens further to increase the exhaust airflow, ensuring that fumes are effectively captured and removed.

Laboratory Airflow Valve - Phoenix Style Lab Airflow Valve — Laboratory Airflow Valve

General Airflow Exhaust Valves

These valves control the overall exhaust from the lab space, beyond just the fume hoods. The controller adjusts these valves to maintain the desired negative pressure in the lab, ensuring that air flows from adjacent areas into the lab, preventing contaminants from escaping. If the fume hoods are not being used, then the General Exhaust valve will increase its airflow to maintain a negative pressure based on the position of the supply valve.

Supply Airflow Valves

To maintain balance, the controller also modulates the supply airflow valves. As exhaust airflow increases or decreases, the supply airflow is adjusted to ensure that the lab remains at a slightly negative pressure. This ensures that the lab draws in clean air from adjacent spaces, rather than allowing potentially contaminated air to escape the lab.

By coordinating the adjustments of these three types of valves, the VAV laboratory controller effectively maintains a consistent negative pressure, ensuring a safe and controlled lab environment.

Sash Position

As the sash opens, the sash sensor detects the change and signal the fume hood controller to adjust the exhaust airflow control valve to open, ensuring the desired airflow and maintaining a safe hood velocity. When the sash closes, the valve adjusts to a specific level to sustain the required face velocity. As the sash is moved to a new position the controls will respond within seconds to reposition the exhaust airflow control valve to maintain the required face velocity. This requires that the exhaust airflow control valve be able to adjust flow (CFM) from full load to the minimum required. The valves are pressure independent and will maintain the required flow when the overall system pressure fluctuates.

Fume Hood Monitor

The fume hood monitor will indicate the current velocity. There may. be a green light indicating normal operation, and a yellow light when the velocity drops below normal. Once the velocity drops too low or there is a system failure the fume hood monitor will show a red light and produce an audible alarm to alert the room occupants of a potential unsafe condition and to close the sash.

If the user spills a toxic or hazardous substances within the hood, the user can push the purge button on the hood monitor. This will close the sash to contain the contaminants.

Duct Pressure and Bypass Air

Monitoring duct pressure between chemical fume hoods or other exhaust points ensures the system maintains the appropriate negative pressure. This is to keep the lab space negative, even when VAV hood sashes open suddenly. This pressure monitoring also prevents excessive negative pressure, which could damage the exhaust valves or ductwork.

A crucial aspect of controlling duct pressure is adjusting the capacity of the exhaust fan(s) that draw air from the system. While there are various methods to regulate fan capacity, including variable frequency drives. The most common approach is to use a bypass air plenum with a modulating damper.

The bypass air plenum is located either beneath an inline fan or next to a scroll-style centrifugal fume exhaust fan. It includes a modulating isolation damper that disconnects the fan from the duct system when not in use. When sashes are closed and duct negative pressure increases, the bypass damper allows outside air to flow into the fan. This reduces the amount of air pulled from the exhaust system when hood demand is low. The benefits of this setup include:

Laboratory Exhaust Fan with Bypass Damper control

Lab Exhaust Fan

Stable operation of the exhaust fan, ensuring consistent nozzle velocity and plume rise. Exhaust fan stack velocity is crucial in laboratory fume hood exhaust systems because it ensures that hazardous fumes and contaminants are effectively dispersed into the atmosphere, away from building occupants and nearby areas. A sufficient stack velocity creates a high-velocity plume that carries pollutants to a safe height.This minimizes the risk of re-entrainment into the building’s air supply and reducing the exposure of nearby personnel to harmful chemicals. This helps maintain a safe and compliant laboratory environment.

Quick response to changes in duct pressure.

Energy savings, as reduced exhaust from the lab lowers the demand for tempered supply air

The opposite occurs when the sashes begin to open. As the VAV hood valve opens, the duct pressure becomes less negative, and the bypass dampers begin to close. This adjustment draws more air from the lab exhaust system, while the make-up air increases to maintain proper system pressure.

Summary

Lab exhaust systems are essential for ensuring a safe environment in and around laboratory facilities. However, managing chemical fume exhaust, creating high-velocity plumes, and handling 100% outside air can result in significant energy consumption. By implementing a VAV lab system and providing proper training for lab users to close their sashes when not in use, it is possible to maintain a safe environment while also reducing overall energy usage.

VAV Laboratory Fume Hood Control

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

Space Pressurization

Chemical Fume Hood Flow

Lab Airflow Valves

Fume Hood Exhaust Valves

General Airflow Exhaust Valves

Supply Airflow Valves

Sash Position

Fume Hood Monitor

Duct Pressure and Bypass Air

Lab Exhaust Fan

Summary

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