Friday, 18 March 2016

Four Pipe HVAC System

Four Pipe HVAC System:

The system's piping consists of four insulated pipes, two supply and two return lines. One set is dedicated to chilled water, kept between 60 degrees F and 40 degrees F. Another set of pipes is dedicated to hot water, generally kept between 150 degrees F and 200 degrees F. The pipes run to air handlers, which use the chilled or hot water to change the air temperature.

Air Handlers:

  • Air handlers in a four-pipe HVAC system can be custom designed to meet a wide variety of heat or cooling demands. They also usually are versatile in that they can be kept in mechanical rooms, on the roof of the building, or with smaller units, in the space above ceilings.

Boilers and Chillers:

  • The water in the system runs through two separate systems in order to change the temperature. Cold water is brought down in temperature through chillers, which can be located on the ground or the roof of the building. Chillers come in a wide variety of types to fit different budget and efficiency needs. Water is heated up using a boiler. The boiler is kept either inside or outside. As with the chillers, a number of boiler types are available, for different efficiency and budget needs.

Advantages Compared to Two-Pipe HVAC:

  • The four-pipe HVAC system has a number of advantages over a two-pipe system. Four-pipe systems have separate heating and cooling fan coil units and separate pipes for heating and cooling. This means that hot or chilled water is always available, so the system can immediately change over from heating to cooling mode. Two-pipe systems have to be manually switched over, which is not only inconvenient but time-consuming. Four-pipe systems also can cool some rooms while heating others, offering great flexibility in a building with a variety of heating and cooling needs.

Disadvantages Compared to Two-Pipe HVAC:

  • Four-pipe HVAC systems have a number of disadvantages compared to a two-pipe system. They are more expensive to install and maintain and have twice as many valves, coils, controls and pipes to maintain. They also are twice as prone to congestion due to the increased piping.
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Propeller Fans

Propeller Fan:

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. 
The propeller fan sometimes called as PANEL FAN, is the most commonly used of all fans. It can be found in Industrial, Commercial, Institutional and Residential applications. It can exhaust hot and contaminated air or corrosive gases from factories, welding shops, foundries, furnace rooms, laboratories, laundries, stores or residential attics or windows.

Sometimes several propeller fans are installed in the walls of a building operating in parallel and exhausting the air.




 Propeller dynamics, like those of aircraft wings, can be modelled by either or both Bernoulli's principle and Newton's third law. A marine propeller of this type is sometimes colloquially known as a screw propeller or screw, however there is a different class of propellers known as cycloidal propellers - they are characterized by the higher propulsive efficiency averaging 0.72 compared to the screw propellers average of 0.6 and the ability to throw thrust in any direction at any time. Their disadvantages are higher mechanical complexity and higher cost
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Types of Fans used in Construction.

Generally there are 6 types of fans used in Mechanical Construction.

1. Axial Flow Fans:

There are four types of axial-flow fans. Listed in the increasing order of static pressure.
They are:
a. Propeller fans(PFs).
b. Tubeaxial fans(TAFs).
c. Vane axial fans(VAFs).
d. Two Stage Axial Flow fans.

2. Centrifugal Fans:

There are six types of centrifugal fan wheels in common use. Listed in the order of decreasing efficiency, They are:
a. Centrifugal fans with AirFoil(AF)blades
b. Centrifugal fan with Backward Curved(BC) blades.
c. Centrifugal fan with Backward Inclined(BI) blades.
d. Centrifugal fan with Radial Tip(RT) blades.
e. Centrifugal fan with Forward Curved(FC) blades.
f. Centrifugal fan with Radial blades(RBs).

3. Axial Centrifugal Fans.4. Roof Ventilators.5. Cross Flow Blowers.6. Vortex Regenerative Blowers.





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About

HVAC is the technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR, (heating,ventilating and air-conditioning & Refrigeration) or ventilating is dropped as in HACR (such as the designation of HACR-rated circuit breakers). HVAC is important in the design of medium to large industrial and office buildings such as skyscrapers, onboard vessels, and in marine environments such as aquariums, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors. Ventilating or ventilation (the V in HVAC) is the process of "exchanging" or replacing air in any space to provide high indoor air quality which involves temperature control, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide. Ventilation removes unpleasant smells and excessive moisture, introduces outside air, keeps interior building air circulating, and prevents stagnation of the interior air. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types.