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Closed Circuit vs Open Circuit Cooling Towers

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January 30, 2024

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

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

MEP Academy Instructor

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January 6, 2024

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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.

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

Solar Water Heaters

MEP Academy Instructor

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December 29, 2023

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In this article, we’ll explain the inner workings of both Active and Passive Solar Water Heaters, examining their advantages, disadvantages, and real-world applications.

Active Solar Water Heaters are a marvel of engineering that relies on fluid circulation, advanced controls, and the tireless power of pumps to efficiently warm water for diverse applications in commercial and residential buildings. On the flip side, Passive Solar Water Heaters takes a more elegant, simplified approach, using nature’s thermosyphon principle to create a self-sustaining flow of warm water.

Solar water heaters are described by the type of solar collector and circulation system that they use.

Active Solar Water Heaters

Active solar water heaters come in two main types: direct circulation systems and indirect circulation systems. These systems harness solar energy to heat water for various applications, such as domestic hot water, space heating, or industrial processes. Let’s delve into the specifics of each type:

Direct Circulation Systems

Direct circulation systems, also known as open-loop systems, involve the direct transfer of water from the collector to the end-use application without an intermediate heat transfer fluid. This simplicity makes them suitable for regions with mild climates where freezing is not a concern.

Direct Circulation Solar Water Heating Diagram

Indirect Circulation Systems

Indirect circulation systems, also known as closed-loop systems, use an intermediate heat transfer fluid to transfer thermal energy from the solar collectors to the water in the storage tank. This allows them to operate in colder climates without the risk of freezing.

Passive Solar Water Heaters

A Passive Solar Water Heater operates without the need for mechanical pumps or electrical components. These systems are less expensive than Active systems but are usually not as efficient. Without the need for moving parts, these systems can be more reliable and last longer.

Thermosyphon Systems

Like an active system, a passive system relies on a solar collector to absorb sunlight. This collector is often a dark-colored, heat-absorbing material like metal or special coatings on a surface. In a passive system, the sunlight heats the water directly without the use of a separate fluid. The collector absorbs the solar energy, and this heat is transferred directly to the water circulating through or stored in the system.

Thermosyphon Principle

The core principle behind passive solar water heaters is thermosiphon. As water absorbs heat, it becomes lighter and rises. Simultaneously, colder, denser water descends to replace it. This creates a natural circulation of water through the system.

Evacuated Tube Solar Collector

The heated water typically rises from the collector to a storage tank located at a higher elevation. This tank is positioned above the collector to facilitate the thermosiphon effect. The warm water is stored in this tank until it is needed.

When hot water is required, it is drawn from the storage tank. The cold water that enters the collector to replace it completes the natural circulation loop, creating a continuous flow of warm water if there is sunlight.

Passive solar water heaters are characterized by their simplicity and reliance on natural processes. They are often used in residential and small-scale applications, providing a cost-effective and energy-efficient way to obtain hot water. While they may not be as suitable for large-scale commercial projects, the principles of passive solar design can still be applied to aspects of building construction to enhance energy efficiency and reduce reliance on traditional heating systems.

Storage Tanks and Solar Collectors

Most solar water heaters require a properly insulated storage tank. These tanks typically feature an extra outlet and inlet that are linked to the collector. In two-tank configurations, the solar water heater heats the water in advance of it entering the standard water heater. Conversely, in one-tank setups, the backup heater is integrated with the solar storage within a single tank.

Collector Types for Solar Water Heaters

Solar water heaters for residential properties usually use three different types of collectors to capture sunlight and convert it into heat for heating water. These collectors are critical components that determine the efficiency and performance of the system. Here are the main types of collectors used in solar water heaters:

Flat-Plate Collectors

Flat-plate collectors are the most common type and consist of a flat, insulated box with a transparent cover (usually glass) on top. Inside the box is a dark absorber plate, typically made of metal or other materials with high thermal conductivity.

Sunlight passes through the transparent cover and strikes the absorber plate, which absorbs the solar energy and converts it into heat. The heat is then transferred to a fluid (usually water or a heat transfer fluid) flowing through tubes attached to the absorber plate.

Flat-plate collectors are versatile and used in both residential and commercial solar water heating systems. They are suitable for moderate climates and are effective for domestic hot water applications.

Evacuated Tube Collectors

Evacuated tube collectors consist of rows of glass tubes with an outer and inner tube. The air is evacuated from the space between the tubes to create a vacuum, reducing heat loss through conduction and convection.

Like flat-plate collectors, sunlight passes through the outer glass tube and strikes an absorber within the inner tube. The absorber transfers the heat to a fluid circulating within the tube.

Evacuated tube collectors are more efficient than flat-plate collectors, especially in colder climates. The vacuum insulation minimizes heat loss, allowing them to capture solar energy even on cloudy days.

Evacuated tube collectors are commonly used in colder climates and are suitable for both residential and commercial applications.

Integral Collector Storage (ICS) Systems (Passive System)

ICS systems, also known as batch or breadbox collectors, integrate the solar collector and the storage tank into one unit. The collector is a black tank with a transparent cover, or dark tubes in an insulated tank. Water is heated directly in the collector, eliminating the need for separate pipes or heat exchangers. The heated water is stored in the same unit until it is used.

ICS systems are simple and cost-effective, often used in residential settings for domestic hot water applications. There should be a tempering valve that allows cold water to be mixed with the hot water coming from the tank.

They are used in open loop systems and aren’t suitable for cold climates.

The choice of collector depends on factors such as climate, available space, and the specific requirements of the solar water heating system. Each type of collector has its advantages and disadvantages, and the selection is often tailored to meet the needs of the project.

How solar water Heaters Work

How Hospital Isolation Rooms Work

HVAC Systems

MEP Academy Instructor

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December 22, 2023

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In hospital construction, isolation rooms play a crucial role in preventing the spread of infectious diseases and protecting both patients and healthcare professionals. There are positive and negative pressure room designs depending on the patient’s condition.

If the patient needs to be protected from possible contamination because of a compromised immune system, the room is put under positive pressure. If the patient has a contagious disease, then the room will be under negative pressure to prevent the spread of that disease. We’ll show you four different isolation room configurations. Here are key aspects of how a hospital isolation room typically works:

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

Positive Pressure Isolation Room

In hospital construction, positive pressure isolation rooms are designed to protect patients who are particularly susceptible to infections because of a reduced immune system.

The key characteristic of a positive pressure isolation room is that the air pressure inside the room is maintained at a higher level than that in the surrounding areas. This prevents external contaminants from entering the room.

In our example the supply air is delivered by a constant volume terminal with a reheat coil. The supply air has been calculated at 12 air changes per hour in accordance with ASHRAE Standard 170, which equates to 400 CFM (190 L/s). The toilet room will consume 60 CFM of that, while the return air will use a valve to ensure that there is always a 100 CFM difference. This additional 100 CFM creates a positive pressure within the room and ensures that nothing enters from the corridor.

Positive pressure rooms have a dedicated air supply system that provides filtered and clean air into the room. The ventilation system often includes HEPA filters to remove airborne particles and pathogens, ensuring that the air entering the room is of high quality.

The direction of airflow is carefully controlled to maintain positive pressure. Air generally flows from the isolation room towards adjacent spaces, minimizing the risk of contaminants entering the room.

Monitoring Systems

Pressure Monitoring: Continuous monitoring of air pressure differentials is essential. If the pressure drops, alarms are triggered to alert healthcare staff to take corrective action. The monitor can be connected to a building automation system.

Located in the hallway outside the room is a monitor that reveals what is happening with the pressure differential, temperature, air changes per hour, and many other metrics. The monitor will have sensors located in the room and in the corridor to track the pressure differential.

Access to positive pressure isolation rooms is restricted to authorized personnel to minimize the risk of contamination.

Anterooms

Some positive pressure rooms may have an airlock system with two sets of doors to further reduce the risk of air exchange with the external environment. You can see that the volume of air is the same, except now there is an extra door and small ante room.

Positive Pressure Hospital Isolation Room with Ante Room

We could also pressurize the Ante Room by putting a supply air grille in the room. An additional exhaust grille is added to the isolation room. The V-1 terminal valve will adjust as needed to maintain the required pressure differential.

Positive Pressure Ante Room in Isolation Room

Negative Isolation Room

A negative isolation room is also referred to as Airborne Infection Isolation (AII) Room. Negative Pressure Isolation rooms are designed with negative air pressure relative to surrounding areas to quarantine patients with infectious disease. This helps prevent small airborne contaminants from escaping the room and spreading to other parts of the facility.

Negative Isolation Patient Room in Hospital with Ante Room

High-efficiency particulate air (HEPA) filters are used to capture and filter out airborne pathogens. The ventilation system ensures a controlled flow of air to maintain the required air changes per hour (ACH) for the specific isolation room.

Exhaust valve V2 is used to pull air in from the corridor and the patient room into the Ante Room. This provides an additional barrier to contamination when entering or exiting the room. The Ante Room is provided with 10 air changes per hour to create a negative pressure. Valve V1 will maintain the proper pressure differential.

Monitoring and Alarm Systems

Continuous monitoring of temperature, humidity, and pressure differentials ensures that the isolation room is operating within the specified parameters. An alarm system alerts staff to any deviations from the desired conditions, prompting quick response and correction.

Learn How Hospital Isolation room Work

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

Active Solar Water Heaters

Direct Circulation Systems

Indirect Circulation Systems

Passive Solar Water Heaters

Thermosyphon Systems

Thermosyphon Principle

Storage Tanks and Solar Collectors

Collector Types for Solar Water Heaters

Flat-Plate Collectors

Evacuated Tube Collectors

Integral Collector Storage (ICS) Systems (Passive System)

Positive Pressure Isolation Room

Monitoring Systems

Anterooms

Negative Isolation Room

Monitoring and Alarm Systems

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