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Commercial HVAC Systems
Unitary, rooftop, or packaged?
The purpose of an HVAC (heating, ventilating, and air-conditioning) system is to provide and maintain environmental conditions within an area called the "conditioned space." The type of system selected is determined by the mechanical designer's knowledge of systems and the building owner's financial and functional goals.
The commercial system selected for a particular application should endeavor to provide the optimum environment for employee comfort and productivity, process function, and good indoor air quality with energy efficiency and cost-savings. Different systems will satisfy each of these objectives with different degrees of success. It is up to the designer and the owner to make the correct assessments.
In most applications, there are several choices for the type of system to use. The selection of the type of HVAC system by the designer and the building owner is a critical decision. It is the designer's responsibility to consider the various systems and select the one that will provide the best combination of initial cost, operating cost, performance, and reliability based on his understanding of the owner's needs and goals. In the selection process all factors must be analyzed, but cost of installation and operation are usually foremost.
Some of the many cost concerns include initial or installation cost, operating and maintenance cost, and equipment replacement costs. Another cost concern that may be overlooked by the designer is the cost associated with equipment failure. For example, how often might a selected system or component be expected to fail and what is the cost in loss of product and production? How long will the system be down? How will the comfort, safety and productivity of the occupants be affected by such a failure and what are these costs?
Depending on the owner's goals, each of these concerns has a different priority. Most owners do not have knowledge or understanding of the advantages and disadvantages of the different types of systems, so it is up to the designer to advise the owner which is best for each application.
Likewise, the designer usually does not usually have a
complete understanding of all the owner's financial and functional goals. For
these reasons, the best situation is when designer and owner are both involved
in the HVAC selection process.
The first step in the selection process is for the designer to ascertain and document from the owner the desired environmental conditions for the building or conditioned space.
The designer must also learn and document the restrictions placed on the system design. For example, what is the required equipment space for a particular system versus what is available? Unfortunately, it is the nature of the business that very few projects allow as much detailed evaluation of all conditions and alternatives as some would like. Therefore, the designer must also rely on common sense and subjective experience to narrow the choice of systems.
Step two in the selection process is determining the building's heating and cooling loads. For example, is the cooling load mostly sensible or latent? Is the load relatively high or low per square foot of conditioned area as compared to other similar buildings? Is the load uniformly distributed throughout the conditioned spaces, and is it relatively constant or does it vary greatly? How does the load vary with time and operating conditions?
Determining the heating and cooling loads establishes the system's capacity requirements. Cooling loads and humidity requirements are used to size air-conditioning (comfort and process cooling) systems.
In other systems, heating and/or ventilation may be the critical factors in sizing and selection. For example, a building may require a large air handling unit and duct system to provide huge quantities of outside air for ventilation or as makeup air to replace air exhausted from the building. In other buildings, in colder climates for instance, heating may be the determining factor on equipment size. The physical size of the equipment can be estimated from the heating and cooling load information alone. This information can help to reduce the choice of systems to those that will fit the space available.
There are also choices to be made depending on if the system is to be installed in a new building or an existing building. In existing buildings, for example, the HVAC system was designed for the loads when the building was built. This means if new systems are to be integrated with existing ones (in order to keep costs down or for other reasons), the new or retrofitted systems must be adaptable to existing equipment, ductwork and piping, and new equipment or systems must fit into existing spaces. If new systems are to perform properly when tied in with existing systems, the old and the new must be looked at carefully and in its entirety. The designer will need to determine how a change to one part of a system will affect another part and a how a change in one system will affect another system.
The number of choices is narrowed further to those systems that will work well on projects of a given application and size and are compatible with the building architecture.
Types Of HVAC Systems
There are four basic types of HVAC systems used in commercial buildings. They are: all-air, air and water, all-water, and unitary. Water systems are also called hydronic systems. Hydronic is the term used for heating and cooling with liquids.
All-air-type systems provide heated or cooled air to the conditioned space through a ductwork system. The basic types of all-air duct systems are: single-zone, multizone, dual or double duct, terminal reheat, constant air volume, variable air volume (VAV), and combination systems. In the typical system, cooling and heating is accomplished by the mixed air (a combination of the return and outside air) passing over a refrigerant coil (cooling) or a heat exchanger (heating).
The basic air-water system (also called air-hydronic) is a central system similar to the all-air system with chilled water coils instead of refrigerant coils for cooling (with an air-cooled condenser) and hot water coils for heating. A variation of this system is the water-air (hydronic-air) system with refrigerant coils for cooling and a water-cooled condenser.
All-water (all-hydronic) systems accomplish space cooling by circulating chilled water from a central refrigeration system through cooling coils in air handling units (also called terminal units or fan-coil units). The units are located in the building's conditioned spaces.
Heating is accomplished by circulating hot water
through the same (cooling/heating) coil or through a separate heating coil. When
one coil is used for cooling only, heating only, or heating and cooling at
various times, a two-pipe water distribution system is used. When two coils are
used, one for heating and one for cooling, a four-pipe water distribution system
is used. Heating may also be accomplished using electricity or steam. Straight
water heating systems will commonly use convectors, baseboard radiation, fin
tube radiation, standard fan-coil units, and unit heaters.
A unitary system is an air-conditioning unit that provides all or part of the air-conditioning functions. The components, fan(s), filter(s), controls, and the cooling apparatus (refrigerant coil, refrigerant piping, compressor(s), and condenser) are all factory-assembled into an integrated package. Components are matched and assembled at the factory to achieve specific performance objectives in accordance with industry-established increments of capacity (such as cfm of air per ton of refrigeration). These performance objectives are set by trade associations that have developed standards by which manufacturers may test and rate their equipment. These performance parameters and standards allow for the manufacture of quality-controlled, factory-tested systems.
Types of unitary systems include window-mounted air conditioners and heat pumps, through-the-wall air conditioners and heat pumps, packaged terminal air conditioners and heat pumps, packaged units, and rooftop units.
Unitary systems are used in a wide range of
applications and may or may not be used with central systems. Cooling capacity
can range from fractional tonnage for window-type units to 100 tons of
refrigeration or more for packaged units. A unitary system that uses the
refrigeration system as the primary heating source is a heat pump.
Commercial-grade unitary systems are called packaged units. Some packaged units
also have heating apparatus (e.g., natural gas heat exchanger, electric
elements, steam or hot water coils) and humidifiers. A packaged unit designed to
be placed on the roof is called a rooftop unit.
Packaged units are used in almost all types of building applications, especially in applications where performance requirements are less demanding, and relatively low initial cost and simplified installation are important. Applications include hotels, manufacturing plants, medical facilities, motels, multi-occupancy dwellings, nursing homes, office buildings, schools, shopping centers, and other buildings with limited life or limited income potential. However, packaged units are also used in applications where dedicated, high performance levels are required, such as computer rooms and laboratories.
Window-Mounted Air Conditioners and Heat Pumps
Window-mounted air conditioners and heat pumps cool or heat individual conditioned spaces. They have a low initial cost and are quick and easy to install. They are also used to supplement a central heating or cooling system or to condition selected spaces when the central system shuts down. When used with a central system, the units usually serve only part of the spaces conditioned by the central system. In such applications, both the central system and the window units are sized to cool the particular conditioned space adequately without the other operating. In other applications, where window units are added to supplement an inadequate existing system, they are selected and sized to meet the required capacity when both systems operate. Window units require outside air and cannot be used for interior rooms.
Window units are factory-assembled with
individual controls. However, when several units are used in a single space, the
controls may be interlocked to prevent simultaneous heating and cooling. For
energy management in hotels, motels and other hospitality applications, a
central on/off control system may be used to de-energize units in unoccupied
rooms. Other factors to consider when selecting window unit systems are that
window units are built to appliance standards, rather than building equipment
standards, so they may have a relatively short life and high energy usage.
Through-The-Wall Mounted Air Conditioners and Heat Pumps, PTACs and PTHPs
Through-the-wall air-conditioners, packaged terminal air conditioners (PTACs), heat pumps, and packaged terminal heat pumps (PTHPs) incorporate a complete self-contained air-cooled, direct-expansion (DX) cooling system, a heating system (gas, electric, hot water, or steam), controls, and fan in an individual package. They are designed to cool or heat individual spaces. Each space is an individual occupant-controlled zone into which cooled or heated air is discharged in response to thermostatic control to meet space requirements.
These systems are usually installed in apartments, assisted-living facilities, hospitals, hotels, motels, office buildings, and schools. Units range from appliance grade to heavy-duty commercial grade.
Unitary System Selection Guidelines
Unitary systems are selected when it is decided that a central HVAC system is too large or too expensive for a particular project, or a combination system (central and unitary) is needed for certain areas or zones to supplement the central system. For example, unitary systems are frequently used for perimeter spaces in combination with a central all-air system that serves interior building spaces. This combination will usually provide greater temperature and humidity control, air quality, and air (conditioned air and ventilation air) distribution patterns, than is possible with central or unitary units alone. As with any HVAC system, both the advantages and the disadvantages of unitary systems should be carefully examined to ensure that the system selected will perform as intended for the particular application.
Factors to consider when selecting unitary systems include: