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T-Method of Duct Design
The T-Strategy for duct measuring is an as of late evolved Duct plan enhancement strategy that incorporates framework beginning expenses and working expenses, energy costs, active times, yearly acceleration, loan costs, and so forth. Manual strategies and conditions might be found in Part 32 of the ASHRAE 1989 Essentials Handbook, however the technique is best utilized with the
legitimate PC programming.
The various of the technologies available to fabricate a duct system can be broken down into a few different categories. The different categories are defined by what equipment and techniques are used to construct the duct system, which are described below. The different methods and techniques are referred to as:
Semi-Self-Heating Fitting
The semi-self heating fitting provides an opening in the sheet metal duct system, which allows for a heat or air to be warmed. The semi-self heating fitting is sometimes referred to as a thermal trap, because it requires that a heat source or air flow be present for the duct to take on heat.
Methods & Equipment:
Solid Rotary Punch Fitting
The solid rotary punch fit is a flat punch with the shape of a round cylinder. The box of the rotary punch fits in the duct opening. The interior of the box is filled with a pvc flange or similar solid material and the pvc flange allows the duct to provide air flow while the box of the rotary punch is heated. A burner is attached to a nearby furnace to provide heat to the hole in the rotary punch. The rotary punch will usually be centered in the duct to provide a consistent heating surface, and usually will have the same heating surface on each side.
Modified Roll Forming (MRF)
The modified roll forming method is a semi-self heating fit through a duct opening. This method involves using a circular shaped sheet metal duct system as the source of heat and an air flow. A short duct with a circular hole is inserted into the duct system. This duct hole is referred to as the heat source.
Methods & Equipment:
Wire Rail System
A wire rail system has an inside surface with flat rails and an outside surface with round rails. The round rail section of the rail system is a ground or embedded heating surface. Inside the insulated rail, the flat rail section provides the heating surface.
Vent Tee System
The vent tee system is a fitting that provides a ground or embedded heating surface for heating or cooling the inside of duct systems. The vent tee allows the heating to reach the inside of a duct system, while preventing air from escaping.
T-Method of Duct Design in HVAC and Refrigeration
Low-pass T-method is a preferred duct design for HVAC and refrigeration. Low-pass duct design is used for exhaust discharge from condenser (HPV). Low-pass duct design is the best method to reduce cool air leakage into the ambient environment. Due to reduced air leakage into the environment, the temperature of the ambient environment is kept cooler. Low-pass duct design can reduce the heating of the building.
Reverse Wall Pressure:
The height of the entire exhaust duct is maintained as higher than the temperature difference between the incoming and exhaust ducts. In this method, the pressure of the incoming air is much higher than the pressure of the exhaust air. It is called reverse wall pressure. The temperature difference of the outgoing and incoming air are not maintained and it is called reverse wall pressure. As the temperature difference in the exterior becomes higher, the pressure is maintained and the emissivity (the ability of a material to emit light at a specific temperature) of the building can be maintained at an optimum level. The emissivity is the amount of light that can pass through a material) of the material is reduced. If the emissivity of the material is reduced to an optimum level, the energy generated from the interior is increased.
High Emissivity:
High emissivity of the materials can be maintained when the air inlet temperature of the building is much lower than the emissivity of the material. If the building is not located in a direct path of the wind, a considerable reduction of the incoming air’s emissivity is required to maintain the emissivity of the building and it will result in the cooling effect of the building. Emissivity is directly proportional to the temperature difference in the material. The higher the emissivity of the material, the higher the temperature difference in the material.
Selecting a Low-Pass Duct Design
The Low-Pass Duct design is the most effective in terms of total cost of ownership and energy efficiency. However, before you decide on the HVAC and refrigeration duct design, you must consider a few factors.
The exhaust air duct should be of a flat profile and must be parallel to the vertical structure of the building. Make sure that the plenum is designed to be above the floor line. If the plenum is at a lower height than the floor line, you might not be able to achieve the HVAC and refrigeration duct design.
The Design of Exhaust Air Ducts
Condenser:
As the Condenser exhaust air goes through the stack of ducts and air pass through the first phase to condenser, the condenser loses energy, but the heat from the exhaust air is trapped in the condenser. The condenser heat should be released to the environment in the first place. The exhaust air coming from the condenser should be allowed to flow through the first phase of the condenser and not forced into the second phase. The air from the first phase of the condenser should be allowed to flow out through the bottom of the air duct. The air is forced into the second phase of the condenser and its trapped in the condenser without the exhaust air flow. The wasted energy will accumulate and reduce the efficiency of the condenser. It will increase the demand on the compressor for more energy.
First Phase of Condenser Exhaust Air:
The pressure drop from the condenser to the first phase of the air duct is minimal because the space to condense the exhaust air is small in the first phase.
Condenser Exhaust Air Flow:
The second phase of the air duct is designed for energy recovery, so as the pressure drop from the first phase of the condenser to the second phase of the air duct increases, the heat energy is released from the condensed air which flows out of the duct. The collected heat energy is then used to heat the air in the building. If the first phase of the air duct is not used to get heat from the exhaust air, then there is no extra heat energy in the exhaust air that flows into the second phase of the condenser.
But if the first phase of the condenser is made efficient, the heat from the condenser will flow into the first phase of the air duct which can be used to heat the air in the building.
However, there is a limitation in the design of the air duct. If the air duct has more than one transition (from the first phase to the second phase), the airflow system can be adversely affected.
If the second phase of the air duct is not operated in a way that the heat energy collected from the condenser can be circulated throughout the air duct, then there is no additional heat energy available to heat the building (which decreases the energy efficiency of the system).
Airspace Design:
Plenum Space:
Plenum space is a maximum of 1/5th of the height of the building.
Duct Space:
Duct space should be designed in such a way that the total heat energy collected in the duct is 1/10th of the total heat energy that is released by the system.
Preheat Heating or Cold Exchange System:
The Preheat Heating or Cold Exchange System is necessary for the installation of heat recovery systems. A preheat system requires warm air to heat up the outdoor air before entering the building through the duct. This system allows the hot outside air to cool down before entering the building. It cools the outdoor air prior to being forced into the building. The fresh outside air is then pressurized and flows through the ducts in a way that the outside air is warmed and the heat inside the building is carried to the point that the hot outside air has cooled down and the hot air has been cooled.
Ducts are not only the only way to move air in and around a building, they also define the physical boundaries of spaces, whether that’s within the walls of a room, or through the mechanical and electrical systems that control heating and cooling.
But from a design perspective, there’s another option when it comes to building HVAC duct systems: air handling methods.
Air handling methods are used by duct engineers to define how air moves from one place to another in HVAC systems. For example, with traditional duct systems, it was common to route air into a system through a single duct and then through a filter to keep dust out of the air before it reached the interior of the room.
Today, however, modern duct designs can route air through multiple ducts into a single room. These newer designs can eliminate up to 90% of dust with the simple elimination of the filter. It’s a much better option than simply eliminating a filter, especially for older systems, which often lacked the capabilities of modern ducts.
You can see that we can eliminate most of the dust problem with air handling methods, and the use of an aerator in the duct can help eliminate all the dust as well.
As air handling methods become more common, they can help provide a more efficient HVAC system that delivers heat more effectively.
However, it’s important to understand that duct design and air handling methods are different things. With a traditional duct system, the air flow that the duct delivers to the interior of a room is generally continuous, with ducts and filters on either side of a room. That makes sense, because all we need is a continuous flow of air to heat or cool the room.
However, in today’s modern duct systems, air can be handled by multiple ducts, sometimes in multiple rooms. It’s this ability to work efficiently that makes air handling methods an important choice for the design of HVAC duct systems.
A Quick Note About Choice of Air Handling Method
Choosing between different air handling methods will depend on a few factors, including:
How many rooms you have?
Does your system need to work within a defined space, or can you work with natural airflow?
How many filters do you want to keep out of the air?
Types of Air Handling Methods
There are four basic types of air handling method: coved, panelized, gasketed, and pozzolanic.
The term coved refers to a duct system in which the ducts in each room are enclosed, usually with a layer of plastic wrap. This helps to reduce dust.
Panelized systems will often be pozzolanic systems, as they will often be rigid in nature and include ventilation pockets to allow air to move through the system when it’s needed.
Gasketed duct systems are usually less expensive, but they have a higher rejection rate of dust. Gasketed duct systems are typically used in areas where dust is less of a problem.
Then there’s pozzolanic systems, which are pozzolanoic gravel/stone blocks that are inserted into the duct system to collect dust. These are typically more expensive, but they’re better at handling dust than the gasketed systems.
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