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Duct Sizing in HVAC
Duct sizing in HVAC systems has been an issue for a long time. A typical ratio between heating and cooling air flow is 60:40. Thus a heating system normally has 15 to 20 times the pressure than a cooling system. However, recent developments in modern heating and air conditioning systems have changed this ratio to 20:70. As a result, several ducts (called ductwork) have to be filled with air and forced into a system.
In addition, as air pressure decreases with altitude, ductwork sizes should also decrease to accommodate lower temperatures. Many builders are moving towards larger ductwork dimensions to reduce the size and thus cost of heating and cooling systems.
Depending on the type of piping and connections used in the system, it can be very expensive to cut, join, and calculate the correct duct size.
The scientific process of measuring proper size
After figuring out the ductwork size to be used, engineers often go back to the drawing board to determine how to cut and join all the various piping components. Today's materials offer a number of materials that can be used to make some basic piping connections, but as new materials are developed, they are designed with more complex geometries.
Engineers must then go through a thorough test to ensure that the pipes and fittings are properly installed. Each new addition to the design must be thoroughly tested. Once that is done, the engineers must then design the piping itself.
If a duct is too small, the cool air will condense and turn into a block of ice within minutes.
How to size A/C ducts in a hot attic?
The only way to correct this problem is to size the air supply to the upper unit properly. Make sure to factor in the 2-inch difference between the large duct and the small duct.
Duct size in HVAC systems is very important. It is important to ensure proper ductwork layout is used. Large buildings with more than 20,000 square feet often have a large amount of ductwork. The age of a building and changes in how products are used can also affect the need for ductwork.
Ductwork is manufactured with a number of ductwork makers, but the bottom line is always quality. Look for a ductwork company that has been in business for at least 10 years. If a building was built in the 1980s, chances are it has aged and needs some ductwork upgrades. Ductwork projects tend to be long term, so it's important to choose a company that is knowledgeable about maintenance and repair.
Hvac duct design criteria
The ductwork air ducts should be separated by at least 5 feet so that all supplies can be vented separately. They should also be bolted to the wall or ceiling, and be mounted with air-assist bolts. The service manger is responsible for maintaining the ductwork. It should be checked at least annually and also, if necessary, replaced if required.
Enclosed HVAC ducting is preferred because it creates a sealed duct, preventing air leakage and frost/freeze buildup. However, it is only applicable for installed systems.
Exterior air conditioning ductwork may not need this kind of seal. A ductile iron or flexible ductwork system may need some type of internal seal, but only if they are installed properly. Remember: if the external seal isn’t sealed, frost can build up in the interior of the ductwork and may leak into the HVAC system.
Topography of the residence
Before you begin the design process, survey the area and ensure that all the terrain is flat and free of debris. Using a tape measure, measure and mark every duct. A flat surface is important as you will be bending and bending HVAC ducts in such a way that any imperfections on the surface will be magnified and will slow down the installation process.
HVAC duct design criteria can be based on the cooling and heating requirements of the individual loads, the ceiling height, the dimensions of the ductwork, the presence of electrical wires, wires for communication with outside systems, and other construction-related characteristics.
Typically, the AC ductwork of a home or commercial building will have an area, or ductwork panel, that fits into a plastic box to contain the HVAC wiring and tubing.
Ductwork can then be separated into the components necessary for heating and cooling with insulation, which provides thermal isolation, and tape, which provides a smooth surface to guide the wires to the appropriate connectors.
To make the wiring and tubing fit into the box, the box may be made out of metal or fiberglass.
In many homes, you will see the HVAC ductwork running under the floor.
Usually, the floors are cork planks, laminate, or some combination of these materials, which can be surfaced with insulating materials like foam and/or carpet.
Hvac duct design criteria can be the overall structure of the duct or its constituent sections.
These must meet the following criteria:
Pressure drop is a measure of the loss due to the performance of the hvac duct.
The design of hvac ducts is affected by the design of the building envelope and the use of seals and/or pressure release devices on doors and windows.
In larger structures, air is passed through ducts below the ground.
This improves indoor air quality, reduces heating costs, and reduces operating expenses by reducing wear and tear on the duct system.
In such cases, a duct system is typically contained in a "ground-plane structure".
On the surface of the building, hvac ducts may be connected to the central heating system via pipes, ductwork or fire barriers.
In buildings without ground-plane structures, such as those with pitched roofs or flat roofs, air is passed through hvac ducts or air conditioning ducts.
The air is passed through the building to the heat pump, in which the air moves.
In buildings with only a ground-plane structure, the hvac ducts are attached to the hot-water piping which is connected to the heat pump.
In either case, the hvac ducts are on the ground-plane structure of the building, in the areas where the demand for air changes.
In apartment buildings, where heat and hot water distribution are not centralised, the heat-pump must produce enough heat to compensate for the air needed by the air conditioning system.
This is usually done through a combination of forced and/or distributed heating.
An underground air duct for an air conditioning system was historically the only means to transfer a heat load.
There are two types of heating systems:
In a forced system, the heat is supplied to a building in the winter by a source other than the building itself.
In an air conditioning system, the heat is supplied to a building in the summer by a source other than the building itself.
Both systems use an air-source heat pump.
(Note that these terms are often used interchangeably.
In fact, there are several types of heat pumps, including both heat pumps, which transfer heat from a source directly to a building, and air-source heat pumps, which transfer heat from the air to a building.
In each case, a heated water vessel is transferred into the building in the winter and moved out in the summer.)
Air conditioning requires a heat source (the heat pump) and a cooling source (the air conditioner).
If these two heat sources are separated, the building's temperature and humidity will increase as the air outside the building moves toward a heat source and toward a cooler air conditioner.
The answer to this problem is the zone cooling system.
The zone cooling system is basically a system of outdoor pipes which move hot (and in the summer cooler) air into and out of the building.
Heat is supplied from an air source, such as the sun or an evaporative cooler.
Under certain conditions (e.g., if the evaporative cooler is very large) heat is supplied from an air source other than the building.
The heat and cold exchanges are regulated by the heat pump.
However, this solution is not suitable for extreme heat, such as can be found during the summer in the desert Southwest of the United States.
This solution is often used where a building receives less than 75 °F (24 °C) during the winter and 95 °F (35 °C) or more during the summer.
A closed-circuit pneumatic air conditioning system provides on demand cooling, allowing the unit to respond to temperature differences between the inside of the building and the outdoors.
A pneumatic system's primary benefit is its ability to perform exactly like the outdoor cooling towers that most buildings of any size use.
The open-channel design, which uses a small pumping unit to supply air into the building and a compressor to make the air hotter or cooler as needed, is frequently criticized as being expensive to install, more prone to damaging leaks, and unable to deliver very cold air.
The large capacity (up to 300 liters/sec) pumps are impractical for high-rise buildings with many floors.
They can be quite expensive, and require skilled operators for use indoors.
Access Previous parts of this course, if you have missed them, by clicking Below...
How to Calculate External Static Pressure (ESP) with worked Example
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