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Providing air conditioning in the summer and heating in the winter, HVAC is an essential part of any building. Heat or cold air is supplied to the different rooms or offices by ducting and is normally thermostatically controlled.
The following sections examine some modern methods to improve the efficiency of HVAC systems. The first section provides an overview about how a typical HVAC system operates and the major components.
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Components of a Typical HVAC System
An HVAC system is made up of a number of components.
- Air Handling Unit (AHU)
- Humidifiers and De-humidifiers
- Volume control and Fire/Smoke Dampers
- Air Distribution Diffusers
- Air return Grills
Air Handling Unit
The air handling unit contains heating and cooling coils, the air circulating fans, and the supply and return filters.
Depending on the requirement of hot air or air conditioning, hot water or chilled water can be supplied to the coils. Alternatively, a heat pump can be used with an evaporator/condenser supplying the hot or cold medium.
Excessive noise due to the air being circulated through the ducting is both annoying and distracting to the workforce. To mitigate the noise, attenuators (baffles or silencers) are fitted to the ducting.
The fans or blowers draw the air through heating/cooling coils, circulating it through the supply and return ducting
Suction and Discharge Filters
Suction filters are on the return ducting into the AHU; these remove any dust smoke or other undesirable particles or fumes before the air enters the heating/cooling coils and fan chambers. Discharge filters remove any remaining particles before the air is recirculated again.
Air conditioning can be a very dry method of keeping a building cold. In the seventies, I was flown out to Dubai for a final interview for an engineer’s job at the Dubai Aluminum Smelter and was offered the job, subject to a medical. However, the rheumatoid arthritis that was to bother me for the rest of my life was just kicking in and the smelter’s doctor thought the heat would exacerbate this.
The other medical problem was my daughter's asthma. She was only a few years old but the doc felt that the dryness of the essential air conditioning would bring on more frequent asthma attacks. I didn't take the job and flew back to Inverness the next day, where on arrival home my wife informed me that an offshore oil concrete platform construction yard, away up in the Western Highlands of Scotland wanted me to start as Project Engineer immediately; but that’s another story!
Back to the present and the use of humidifiers; these spray water into a dry hot or cold air stream using very fine sprays.
De-humidifiers on the other hand take the wetness out of the air stream by passing the wet air over cooling coils and storing the condensed water in a tray underneath.
The actual condition of the air supply to the rooms and offices of a building is very important in preventing sick building syndrome with high humidity promoting the growth of molds, mildew and mites, and with low humidity causing throat irritation and dry skin. It is very important to get the humidity of the workplace at the optimum level. Most building regulations recommend this to be maintained between 30% and 50%.
High and low humidity can cause problems not only to employees working in the environment, but also the building’s structure: steel, composites, and wood. However, in the HVAC system, the humidity in the air can be controlled by a humidistat or de-humidistat. The dehumidifier can have antibacterial/ultraviolet treatment against Legionnaires' Disease and the growth of other bacteria/organisms in the condensed water storage tray.
Diffusers and Grilles
Diffusers, being designed to slow the discharge velocity of the air, are normally located in the room’s ceilings. This is to ensure the air is distributed evenly and in the required direction into the rooms and offices of the buildings. These are set into the ceiling; the louvers are automatically adjustable or fixed, directing the air into the return ducting. The return ducting is run back to the discharge plenum, through the return filter, then back into the air handling unit, completing the system circuit. Several examples of efficient diffusers and grilles available from www.atlantasupplies.com are shown below; please click on images to enlarge.
1. Volume control dampers are incorporated into the ducting to control the quantity and velocity of the air flow. Fire and smoke dampers and can also be installed that shut automatically in the event of a fire. These dampers need to be reset manually.
2. The heating and cooling coils can be supplied by hot or chilled water; or a heat pump can also be used. This is explained in more detail in the next section.
3. Ambient air from outside the building can be added and mixed with the circulating air from time to time to freshen the hot or cold air supply to the rooms or offices.
A typical flow diagram for a building's HVAC system is shown in the sketch below. Please note that two types of heating/cooling coils are shown inside the AHU; normally only one type is used unless the system is being operated in very cold winter locations. In this case the heat pump can be accompanied by a hot water coil for winter use.
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Modification of an Existing HVAC System to Improve Efficiency
In the US, 40% of the total energy used is consumed by buildings, and HVAC consumes a substantial percentage of this energy.
It is therefore in companies’ interest to decrease this energy use, and one way to do this is to improve the efficiency of the HVAC plant.
Before any modification is commenced, maintenance records should be checked to ascertain that filter replacement, heating/cooling coil cleaning ,and all volume control damper maintenance has been attended to regularly.
The state of the existing ducting should be closely examined to ensure there are no leaks in the system. Flanged joints and flexible ducting are notorious for leaks; gaskets should be replaced where required and any leaking flexi-ducting should be replaced.
In a large HVAC system, a duct leakage of 10% can end up as a 30% increase in energy consumption.
Insulation is also very important; loss of heat or coldness through steel ducts is costly! Therefore, all ducting should be inspected for breakdown of insulation, with it being noted but not repaired at this stage as new components may be fitted to the ducting. Modern ducts that can be purchased pre-insulated with the latest high efficiency insulation should be considered in an upgrade.
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Use of Highly Efficient HVAC Components
Designers of HVAC systems are famous (or infamous?) for being a bit heavy-handed when estimating the heating and ventilation requirements of buildings. This is presumably to ensure supply despite air leakage from ducting and components. Sometimes this is justified as extensions are frequently added, so the existing HVAC system will cope.
- Electric Motors
Air circulating fan electric drive motors are a classic example of overrating, so the first thing to check is the rating plate against the design requirement in the original specification. This will tell you if you can downgrade the motor output.
If the system has been operating for a long time, 10-15 years for example, fitting variable speed motors is a worthwhile investment. This will allow the speed of the fan to be altered to suit changing conditions in the system. This has been reported to achieve savings in energy of up to 50% and, if the existing switchboard and wiring are retained, even more savings can be made, ensuring a short pay-back period.
- Air Filters
Replacing standard disposal air filters rated at MERV < 4 with energy efficient pleated air filters, at MERV11 for example, can make for savings in the fan drive motor’s energy use of 2-4%. However, remember that disposable filters are used because they are cheaper to buy and maintain; the permanent filter is more expensive to buy and will need regular maintenance of stripping and cleaning.
Note: MERV is a rating for air filters where the efficiency increases as the value increases.
- Fabrication of Ducting
The fabrication of HVAC ducting should incorporate energy efficient measures such as turning vanes at 90° bends and smooth transitions between ducting and components of different sizes or sections. Galvanic corrosion between galvanized steel and stainless steel should be prevented by isolating them from each other; this will save on duct/component replacement costs.
- Diffusers and Grilles
The supply and extraction of the air to the rooms/offices can be made more efficient by using modern diffusers and grilles and improving airflow control. These ensure that the optimum amount of air is admitted and extracted from the rooms, promoting more uniform room temperatures.
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Heating and Cooling Methods to Upgrade HVAC
As we have seen earlier, the hot or cold air is supplied to the building by the fans that draw the air through coils in the air handling unit.
Large commercial buildings use hot water as supplied by a furnace to circulate through the coils producing hot air and cold water from chillers being used for the air conditioning. Both these methods have been in use for some time, but modern appliances have seen efficiency rise by 20%, therefore the modern furnaces and chillers should be examined.
One of the biggest energy efficiency savings in heating and cooling can be achieved by using a heat pump.
In the hot air mode, refrigerant gas is circulated around the coils of the heat pump condenser located in the AHU. When the fans draw the air over the coils. the refrigerant condenses to a liquid, giving up its heat to the air. The fans then discharge the hot air to the building.
In the air conditioning mode the system is reversed by a reversing valve; the coils in the AHU act as an evaporator, with liquid refrigerant being circulated through the coils this time.
The air is drawn over the coils by the fans, evaporating the refrigerant and converting it back to a gas. This removes the heat from the air before being discharged by the fans to the building.
A layout of the components of a heat pump used in HVAC is shown below.
This system can be made even more efficient by the use of geothermal energy in the form of a geothermal heat pump, which is suitable for large buildings provided there is the ground-space to locate the buried pipe loops. Vertical loops take up less space, but these are more costly to install than their horizontal counterparts.
Briefly, a geothermal heat pump can be applied to the building’s HVAC system in summer or winter. During the winter months, the heat pump uses the heat from the underground loops to evaporate the refrigerant. This heat from the refrigerant gas is transferred to the air that is then used to heat the building. In the summertime the heat pump is used to cool the air by dissipating the heat from the building back into the ground through the geothermal loops.
Heat pump components for use in commercial buildings are continuously being upgraded. For instance, modern screw or scroll compressors are much more efficient than the standard reciprocating ones developed only a few years ago.
Refrigerants are becoming more environmentally friendly that originally reduced their efficiency, but even these modern refrigerants are becoming more energy efficient. When I was an engineer at sea, CO2 was the popular refrigerant (but very energy inefficient) until Freon came on the market. But this was subsequently banned and phased out due to the discovery of the “ozone hole." There was a swing back to CO2, but today’s modern popular, although ozone-depleting, refrigerant is R22.
Modern HVAC management control systems can also improve the efficiency of the system. The control system constantly monitors the temperatures and pressures of the air at strategic points throughout the HVAC system.
- Ducting static air pressure monitoring and control through automatic adjustment of circulating fans/dampers. (Also detects air leaks)
- Checks on air condition for humidity and filtration operation
- Electronic Programmable Thermostats -These automatically control pre-set, comfortable temperatures both during working hours and when building is unoccupied, reducing energy and increasing overall plant efficiency.
- Zone sensors - These are sensors for control of air quality and quantity supplied to the different zones or areas of the building, e.g. offices, rest rooms, conference rooms, executive suites, canteens, and cafeterias.
HVAC provides a very important service to commercial buildings, but is responsible for a sizable chunk of industries overall energy consumption. Modern machinery and components along with electronic HVAC management control systems are becoming more efficient to combat this. However, basic maintenance such as checking ducting and components for air leaks, changing or cleaning air filters, and keeping windows and doors shut can also increase the HVAC plant efficiency.
Compromising air quality to lower costs should not be contemplated as employees will work more efficiently and willingly in a healthy and pleasant atmosphere.
- Descoenergy: HVAC Major Energy Users
- Airconditioning and heating: Controlling Humidity Levels
- Willie Scott- Anti-vibration supports; HVAC Fabrication; Geothermal Heat Pumps
- Schneider-electric: HVAC energy efficient electric motors
- WGBD: High Performance HVAC
- Atlanta Supply: Commercial wall and ceiling vent products