Static Velocity and Total Pressure Explained

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February 19, 2024

static velocity and total pressure measurement

In this article we explain the fundamental concepts of static pressure (Sp), velocity pressure (Vp), and total pressure (Tp) in the realm of HVAC systems. Understanding these pressure components is essential for designing, maintaining, and optimizing air conditioning ducts. In this article, we will explore the definitions of static, velocity, and total pressure, their significance in HVAC engineering, and practical insights on the instrument commonly used to measure them.

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

Table of Contents

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air above a specific point on the Earth’s surface. It is the pressure exerted by the Earth’s atmosphere on objects within it. At sea level atmospheric pressure is approximately 14.7 psia (pounds per square inch absolute) or 1,013 millibars.

Atmospheric pressure decreases with altitude. HVAC systems in high-altitude locations may require adjustments to account for lower atmospheric pressure, affecting factors such as air density and combustion processes.

Using a U-tube manometer with both ends open to the atmosphere, the liquid level on both sides of the tube rest at the same height. This is because both ends of the tube are open and exposed to the same atmospheric pressure.

Static Pressure

SP is the pressure exerted by the air in all directions within the ductwork when it is not in motion. It is a measure of the potential energy of the air as it remains static or stationary within the system.

Static pressure is the force exerted by the air on the walls of the ducts and other components when the air is not flowing. It is typically measured in units such as inches of water column (inWC) or pascals (Pa) using an inclined manometer as shown in image.

Air encounters resistance as it moves through ducts, filters, coils, and other components. This resistance generates static pressure. When choosing a fan for an HVAC system, consideration must be given to the static pressure the fan must overcome to deliver the required air to the space.

The layout and design of the ductwork, including bends, transitions, and fittings, can contribute to an increase in pressure.

High static pressure can lead to increased energy consumption, as the system must work harder to overcome resistance.

(Sp) affects the airflow within the ducts. Excessive pressure may result in reduced airflow, impacting the system’s ability to deliver conditioned air to the desired spaces.

Velocity Pressure

The Velocity pressure is the pressure component associated with the movement of air within a duct.

Consider a scenario where a fan is propelling air through a duct system; during this process, two distinct pressures come into play – velocity pressure and static pressure. The combination of these pressures is termed total pressure. Velocity pressure, arising from the movement of air, can be quantified by determining the variance between total pressure and static pressure, with pressure units denoted in inches of water (W.G.) or pascals (Pa).

Direct measurement of velocity pressure is not done; instead, the most straightforward method to ascertain the airflow velocity is by assessing the velocity pressure within the duct using a pitot tube assembly linked to a differential-P sensor, comprising both a static and total pressure probe.

The pitot tube can be inserted into the airflow to measure both static and dynamic pressures. The dynamic pressure measured by the pitot tube represents the velocity pressure.

Total Pressure

A total pressure probe, positioned in line with the airflow, detects both the duct’s velocity pressure and static pressure, resulting in the total pressure. Meanwhile, a static pressure probe, oriented perpendicular to the airflow, exclusively measures static pressure. The disparity between the total pressure reading and the static pressure reading represents the velocity pressure.

Devices used to Measure Pressure

In our example we used an inclined manometer, but there are many other ways to measure pressure, velocity and air flow. Manometers or pressure gauges are commonly used to measure static pressure within the ductwork.

Inclined Manometer

A manometer is a straightforward and widely used instrument for measuring pressure. There are different types of manometers, including inclined, vertical, and digital versions.

Inclined Manometer

To measure static pressure using a manometer, connect the instrument to pressure taps on the duct—one at the high-pressure point and one at the low-pressure point. The difference in fluid levels in the manometer provides the static pressure reading.

Manometer

Digital Pressure Gauge

Digital pressure gauges are modern instruments that provide accurate and quick static pressure measurements. These gauges typically have a display screen that shows the pressure readings directly.

Connect the gauge to the pressure taps on the duct, and it will display the static pressure.

Pitot Tube and Differential Pressure Sensor

A pitot tube can be used in conjunction with a differential pressure sensor to measure pressure. The pitot tube is inserted into the duct, and the differential pressure sensor measures the pressure differences.

Pitot Tube

The static pressure is then determined by subtracting the velocity pressure from the total pressure measured by the pitot tube.

Capture Hoods

Capture hoods are devices designed to measure both air volume and static pressure at diffuser and grille locations. They consist of a hood that covers the diffuser or grille, and a connected manometer or pressure gauge provides the static pressure reading.

Magnehelic Gauge

A Magnehelic gauge is a type of pressure gauge commonly used to measure air and gas pressures. It is easy to use and can be installed directly on the duct. The gauge displays the pressure difference between the duct and the ambient air.

Magnehelic Gauge

Balometer

A balometer is a specialized instrument used for measuring air volume and velocity, but it often includes a pressure measurement component. It is particularly useful for assessing airflow at diffusers and grilles.

Airflow Hood

When measuring static pressure, it’s important to follow these general steps:

– Identify suitable pressure tap locations on the duct.

– Ensure proper sealing of the instrument to prevent air leaks.

– Take measurements at various points to assess uniformity and identify potential issues.

– Compare readings with design specifications to evaluate system performance.

Always follow manufacturer guidelines and safety precautions when using any measuring instrument in an HVAC system. Regular static pressure measurements help HVAC professionals maintain and optimize system performance.

Air Velocity

Air velocity is the speed at which air moves through a duct or HVAC system, typically measured in feet per minute (fpm) or meters per second (m/s). Controlling air velocity is essential for maintaining comfort and efficiency. Low velocity may lead to inadequate air distribution, while excessively high velocity can result in noise, pressure drops, and increased energy consumption. Engineers need to optimize air velocity to achieve the desired airflow without causing discomfort or system inefficiencies.

In summary, understanding and carefully managing air pressure and velocity are critical for designing HVAC systems that deliver optimal performance, comfort, and energy efficiency in the construction projects you work on, such as commercial buildings, hospitals, hotels, and other facilities.

Static, Velocity, and Total Pressure Explained

BTU Meter

By

MEP Academy Instructor

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February 10, 2024

0

In this article we’ll discuss BTU Meters, also known as Energy Meters, which is a device used to measure the amount of thermal energy (heat) consumed or transferred in a system. These meters are commonly employed in various applications where accurate measurement and billing of heating or cooling energy are essential. Here’s how a BTU meter works and some applications for their use.

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

Table of Contents

How a BTU Meter Works

Temperature Sensors

BTU meters typically consist of two temperature sensors, one placed at the inlet and the other at the outlet of a heat exchange system, which could be a chiller, boiler, air handler or the heating hot water and chilled water service lines feeding one of many campus or district buildings. These sensors measure the temperature of the fluid before and after it undergoes the heating or cooling process.

BTU Meter measuring energy consumption in a Heating Hot Water System

Flow Sensor

A flow sensor measures the rate of flow of the fluid within the system. This is essential for determining the volume of fluid passing through the heat exchange system.

Heat Calculation

Using the temperature difference (ΔT) between the inlet and outlet, along with the flow rate, the BTU meter calculates the amount of thermal energy transferred or consumed. The specific heat capacity of the fluid is often considered in this calculation.

The formula for calculating the energy consumed is Q = GPM x 500 x Delta-T

Q represents the total amount of energy in BTU’s, while GPM is the volume of water flowing through the pipes, 500 is a constant, and the Delta-T measures the difference between the supply and return temperatures.

Display and Data Transmission

The calculated energy value is then displayed on the meter’s interface, and/or is transmitted to a building automation system.

Applications of BTU Meters

Heating and Cooling Systems in Buildings

BTU meters commonly used in residential, commercial, and industrial buildings measure the energy consumption of heating and cooling systems. This includes applications such as central heating, air conditioning, and district heating or cooling networks.

District Heating and Cooling Networks

In large-scale district heating or cooling systems, BTU meters are essential for accurately measuring and billing customers based on their actual energy consumption. This is common in urban areas where a centralized plant supplies heating or cooling to multiple buildings.

BTU Meters measuring energy consumption at each building on a large Campus

Industrial Processes

BTU meters find applications in various industrial processes where precise measurement of thermal energy is critical. Industries such as manufacturing, chemical processing, and food production use these meters to monitor and manage energy consumption.

Renewable Energy Systems

BTU meters used in solar thermal systems, geothermal systems, and other renewable energy applications to quantify the thermal energy harvested or produced. This information is crucial for assessing the efficiency and performance of these systems.

HVAC Systems

Heating, ventilation, and air conditioning (HVAC) systems in commercial buildings often utilize BTU meters to monitor the energy used for space heating and cooling. This helps in optimizing system efficiency and identifying opportunities for energy savings.

Utilities and Billing

Utilities use BTU meters for accurate billing based on actual energy consumption. This is particularly important in situations where tenants or customers are individually responsible for their energy costs.

Energy Efficiency Assessments

BTU meters used in energy audits and assessments to evaluate the efficiency of heating and cooling systems. Measuring actual energy consumptions generates recommendations for energy efficiency improvements.

In summary, BTU meters play a crucial role in measuring and managing thermal energy in various applications, providing accurate data for billing, optimizing system performance, and promoting energy efficiency.

BTU Meters, also known as Energy Meters

Closed Circuit vs Open Circuit Cooling Towers

By

MEP Academy Instructor

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January 30, 2024

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learn the differences between closed-circuit vs open-circuit cooling towers

In this article we’ll explain the differences between a Closed-Circuit and an Open-Circuit Cooling Tower and the advantages and disadvantage of each. How do you know which type to use for a project? They both provide heat rejection but in slightly different ways. These systems are often referred to as open and closed loop systems. These are the two commonly used HVAC cooling tower system designs.

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

Closed Circuit Cooling Tower versus Open Circuit Cooling Tower Comparison Diagram

Table of Contents

Closed-Loop vs Open-Loop Operation

In a closed-circuit cooling tower, the process fluid, which could be water, or a water-glycol mixture is circulated within a closed loop piping system. There are two separate water sources, one external within a closed loop, and the second one that circulates water from the tower basin over the heat exchanger. There is never any contact between the water in the enclosed loop and the water circulated within the tower.

The water in the tower basin is pumped up and sprayed over the coil that is fed from the closed-loop piping while a fan forces air over the wet coil. This increases heat transfer through the coil while minimizing the evaporation of water from the basin. The cooled water in the closed-loop coil returns to the building to absorb more heat.

Vertical Water Source Heat Pump in a Closed Circuit Cooling Tower System

Return air brings heat from the space over the indoor coil. The refrigerant cycle moves that heat to the water-cooled condenser coil, where the water circulated from the cooling tower picks up that heat. The heat is pumped to the cooling towers closed-loop heat exchanger coil where water is sprayed over it as air is induced or forced over the coil.

Closed-Circuit Cooling Tower serving Vertical Water Source Heat Pumps in a Hotel

Open-Circuit Cooling Tower

In an open circuit cooling tower, the water is directly exposed to the outside air. Water enters the top of the tower and is sprayed over the fill or heat transfer media. The exchange of heat occurs directly between the water and the entering ambient air. The water that is circulated to the chiller’s condenser and the air within the tower touch each other. This increases the chance of contaminants being scrubbed out of the air and into the cooling tower basin.

The water in the basin is then returned to the facility or the condenser side of a water-cooled chiller, which could foul the chillers condenser coil if the water is not properly treated. The makeup water to the tower can also introduce contaminants to the process water.

Induced-Draft vs Forced Draft

There are several tower configurations including induced draft and forced draft. The fans for an Induced draft tower are located at the top of the tower and induces air into the tower. The fans for a forced-draft tower are located at the bottom of the tower and forces air into the tower and over the coil.

Forced Draft vs Induced Draft Fluid Coolers (Closed Circuit Towers)

Closed Circuit Cooling Tower Advantages and Disadvantages

Closed circuit cooling towers also known as Evaporative Fluid Coolers play a crucial role in the operation of water source heat pump systems in the HVAC industry. It’s important to use a closed loop system because the water is circulated into the coils of all the water-source heat pumps scattered throughout the building. Sending water from an open tower into all these remote coils could create a maintenance nightmare along with a reduction in efficiency for these units.

Fluid Cooler with Horizontal Water-Cooled Heat Pumps

Advantages of a Closed-Circuit Cooling Tower

Reduced risk of contamination since the process fluid is isolated from external elements.
Water conservation as the process fluid is recirculated within a closed loop.
Less susceptibility to scaling and fouling due to the closed nature of the system.
Reduced equipment maintenance.
Reduced water treatment cost due to lower volume of water circulated from the basin.
Sensible heat rejection with pumps off, saving additional energy when conditions are right.

Disadvantages of a Closed-Circuit Cooling Tower

Typically, higher initial cost due to the need for additional equipment like pumps and heat exchangers.
Requires maintenance of the closed loop system to prevent corrosion and ensure fluid quality.

Applications

Closed circuit cooling towers are often preferred in applications where water quality is critical, and there’s a need to minimize the risk of contamination. This makes them suitable for water-cooled heat pump applications and those industries with strict quality standards, such as laboratories or data centers.

Open Circuit Cooling Tower Advantages and Disadvantages

Advantages of an Open-Circuit Cooling Tower

Generally lower initial cost as there is no need for additional closed loop equipment.
Simplicity in design and operation.
Large range of capacities and configurations.
Energy efficiency.

Disadvantages of an Open-Circuit Cooling Tower

Greater susceptibility to contamination from external elements like dust, debris, and biological growth.
More water consumption as water is continuously replenished and not recirculated.
The requirement for water treatment.
Extra level of maintenance.
Reduced efficiency due to the scale and/or fouling of the Chillers Condenser coils if water treatment is not properly maintained.
Complicated design when tower is installed below the piping system or pump.

Applications

Open circuit cooling towers are commonly used in the HVAC industry where water quality is less critical, and the focus is on cost-effective cooling. They are found in all types of buildings spanning the HVAC industry. The addition of a heat exchanger between the cooling tower and the chiller can add a layer of protection against the fouling of the chiller’s condenser tubes, but at an added cost. This doesn’t eliminate the need for water treatment of the tower, it just shifts the tower basin water from contacting the chillers coil to the heat exchanger.

Open versus Closed Circuit Cooling Towers

Thermal Energy Storage

By

MEP Academy Instructor

-

January 6, 2024

0

Thermal energy storage systems including chilled water and ice storage systems TES

In this article we’ll cover the basics of thermal energy storage systems. Thermal energy storage can be accomplished by changing the temperature or phase of a medium to store energy. This allows the generation of energy at a time different from its use to optimize the varying cost of energy based on the time of use rates, demand charges and real-time pricing. Utility incentives could also be available to reduce the upfront cost of installation.

By running chillers at night when the electrical rates are less than daytime rates, the operational cost of the facility can be reduced.

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

Table of Contents

Thermal Energy Storage (TES) Strategies

There are two basic Thermal Energy Storage (TES) Strategies, latent heat systems and sensible heat systems.

Chilled Water Thermal Stratification (Sensible Heat)

Stratification is used within the tank as a strategy for thermal layering of the stored water. Colder water is denser and will settle toward the bottom of the tank, while the warmer water will naturally seek to rise to the top.

Stratification within a Chilled Water Storage Tank

As water enters and leaves the tank it’s important to make sure not to disturb or mix the stratified layers. This is done with the use of diffusers within the tank on the inlet and outlet piping.

Charging the Tank

When charging the tank, the warm water is taken from the top of the tank and sent to the chiller, while the chilled water is returned to the tank near the bottom.

Tank Size Requirements

Chilled water storage tanks require a large footprint to store the large volume of water required for these systems. Approximately 15 ft3/ton-hour is required for a 15F (8.3C) temperature difference. The greater the delta-t of the water, the smaller the tank can be. Tanks can store millions of gallons of water or much smaller amounts.

There are dozens of various layouts for thermal energy storage system, but we’ll cover the basic theory for its use.

In the image above there is the typical primary chilled water loop that produces the chilled water. Then there is the condenser water loop that uses a cooling tower to reject the heat to the atmosphere. A secondary loop that feeds chilled water to the air handler coils. And the last piece is to add in the thermal energy storage tank tied into the primary chilled water loop.

The system can run using just the chillers, or the chiller could be run at night to charge the storage tank when electrical rates are cheaper. The three way valve will close forcing the chilled water to go through the tank. While during the day when the electrical rates are higher, the chilled water can be pulled from the tank in a full storage system, and sent to the air handler coils without the use of the chillers. Partial storage systems use the stored chilled water to supplement the main chiller equipment when they have reached their full capacity and additional cooling is required.

Ice Storage Systems (Latent Heat)

Latent heat transfer strategies are more complex. There are several strategies for producing ice, one of which is to circulate a glycol solution through coils submerged within the tank. Ice then accumulates on the outside of the coil within the tank.

Ice Storage System using Glycol in Primary chilled Water Loop

In this image we have the same water loops as the chilled water storage tank system. There is the primary chilled water loop, condenser water loop, and secondary chilled water loop. The difference with this system is that a glycol solution will circulate through the system in order to produce ice on the coils within the tanks. Glycol prevents the water from freezing. A heat exchanger will separate the primary and secondary loops. The three way valve and control sequence will control the flow of water to and from the tank..

Ice storage systems take less room for storage than chilled water systems. This is because of ices greater capacity to store energy per unit area. The storage volume ranges from 2 to 4 ft3/ton-hour for ice systems, compared to 15 ft3/ton-hour for a chilled water.

The application for energy storage systems varies by industry, and can include district cooling, data centers, combustion turbine plants, and the use of hot water TES systems.

Utilities structure their rates for electrical power to coincide with their need to reduce loads during peak periods. Producing ice or chilled water at night can also increase the efficiency of the overall system. This occurs because ambient temperatures are cooler at night, this is especially true with the use of air-cooled chillers.

Carbon Footprint Offset an Advantages of Using TES

Carbon offsets need to be analyzed for each location and the type of energy provided locally.

A thermal energy storage (TES) system has the potential to reduce the carbon footprint of a facility. The extent of carbon footprint savings depends on factors such as the energy source, system efficiency, and the overall energy management strategy. Here are several ways in which a thermal energy storage system can help mitigate the carbon footprint:

Load Shifting

TES systems allow for the storage of excess energy during periods of lower demand or when renewable energy sources are abundant. This stored energy can then be used during peak demand periods. This reduces the need for conventional, often carbon-intensive, energy sources to meet high demand.

Integration with Renewable Energy

TES systems can be effectively integrated with renewable energy sources such as solar or wind. Excess energy generated during peak renewable production periods can be stored for use during periods when renewable energy production is lower or during peak demand times, reducing reliance on fossil fuels.

Optimized Chiller Operation

TES can optimize the operation of chillers. Chillers can be operated during off-peak hours when electricity demand is lower or when energy from renewable sources is more readily available. This helps reduce the carbon intensity associated with electricity generation.

Thermal Energy Storage System (Charging of Storage Tank)

Reduced Grid Strain

By allowing for load shifting and avoiding simultaneous high-demand periods on the electrical grid, TES systems contribute to grid stability and reduce the need for additional power plants to be brought online during peak times. This, in turn, can reduce overall carbon emissions.

Energy Efficiency Improvements

TES systems can enhance the overall energy efficiency of a facility. By storing and using energy more efficiently, there is a reduction in the total energy consumption. This can lead to lower carbon emissions associated with energy production.

Avoided Use of Backup Generators

TES systems can help avoid the need for backup generators, which are often powered by fossil fuels.

Carbon Offsetting

In some cases, a TES systems can be part of a broader strategy for carbon offsetting and sustainability initiatives. By actively managing energy consumption and reducing reliance on carbon-intensive energy sources, organizations can align with environmental goals.

It’s important to note that the effectiveness of a TES system in reducing carbon footprint depends on the specific design, application, and operational strategies implemented. Additionally, the carbon intensity of the electricity grid in a particular region plays a role in determining the environmental impact. Overall, integrating TES systems intelligently with energy management practices can contribute to more sustainable and environmentally friendly operations.

Thermal Energy Storage (TES) Systems and how they work

Atmospheric Pressure

Static Pressure

Velocity Pressure

Total Pressure

Devices used to Measure Pressure

Inclined Manometer

Digital Pressure Gauge

Pitot Tube and Differential Pressure Sensor

Capture Hoods

Magnehelic Gauge

Balometer

Air Velocity

How a BTU Meter Works

Temperature Sensors

Flow Sensor

Heat Calculation

Display and Data Transmission

Applications of BTU Meters

Heating and Cooling Systems in Buildings

District Heating and Cooling Networks

Industrial Processes

Renewable Energy Systems

HVAC Systems

Utilities and Billing

Energy Efficiency Assessments

Closed-Loop vs Open-Loop Operation

Open-Circuit Cooling Tower

Induced-Draft vs Forced Draft

Closed Circuit Cooling Tower Advantages and Disadvantages

Advantages of a Closed-Circuit Cooling Tower

Disadvantages of a Closed-Circuit Cooling Tower

Applications

Open Circuit Cooling Tower Advantages and Disadvantages

Advantages of an Open-Circuit Cooling Tower

Disadvantages of an Open-Circuit Cooling Tower

Applications

Thermal Energy Storage (TES) Strategies

Chilled Water Thermal Stratification (Sensible Heat)

Charging the Tank

Tank Size Requirements

Ice Storage Systems (Latent Heat)

Carbon Footprint Offset an Advantages of Using TES

Load Shifting

Integration with Renewable Energy

Optimized Chiller Operation

Reduced Grid Strain

Energy Efficiency Improvements

Avoided Use of Backup Generators

Carbon Offsetting

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