Title: Components of HVAC Found in MCO
Course: 2016FA-MCO-110-401 > Section: Module 4: Facilities Maintenance and HVAC

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Components of HVAC Found in MCO

Cooling Towers

A cooling tower is defined by the Cooling Technology Institute as, “…a heat rejection device, which extracts waste heat to the atmosphere through the cooling of a water stream to a lower temperature” [13]. The term cooling tower is sometimes used to refer to either a closed or open circuit heat rejection system. A cooling tower requires specific maintenance including water treatment in order to prevent bacterial growth. Left untreated bacteria that can cause Legionnaire’s[1] disease could be accidentally distributed by wind or condensation [14].

Cooling towers are characterized by how they move the air, for instance mechanical draft cooling towers use fans requiring power to force air through the tower. Natural draft cooling towers use the natural tendency of the rising exhaust air to be buoyant to move the air.

Open System Cooling Towers

There are two main types of open system cooling towers, the crossflow and the counterflow. Evaporation in a crossflow system takes place as air flows perpendicular to water running over a heat transfer surface. Fill is a term used to describe the air and water interface used as a heat transfer in many open system cooling towers. A fill can vary in how it is designed but it exists to provide a lot of surface area for heat exchange. In counterflow, evaporation takes place as air flows parallel to, but in the opposite direction of water running over a heat transfer surface [14].

Closed System Cooling Towers

In a closed system cooling tower the water that is used to remove the waste heat is contained in a heat exchange coil. The coil keeps the water clean and free of contamination by preventing outside air and contaminants from affecting the water. Air never touches the water that is being cooled. In this type of system there are two water circuits, the water in the heat exchange coil system and also a water circuit that serves as spray water to go over the coil. A closed system offers less maintenance, but typically is more expensive [15].

The water that has been cooled by a cooling tower can be used for generating electricity, manufacturing, or for air conditioning [13].

Cooling tower systems must maintain a minimum level of water at all times to prevent severe damage to equipment in the system. There are times that the water level drops due to evaporation or bleed. In these cases make-up water must be added to bring the water level up to minimum level requirements. The evaporation that occurs means that the cleanest water from the system is lost, leaving behind more concentrated contamination in the cooling water that must be treated to remove solids and living organisms.

Cooling Tower Maintenance

Cooling towers can lose efficiency if not properly maintained. In addition to the regular mechanical maintenance that you would expect, some other common things that cooling towers should be regularly inspected for include scaling, corrosion, and biological growth. Scaling occurs when minerals from the water used in the cooling tower deposit on the tower’s components and build up over time. Corrosion also occurs because of water, but can be mitigated through the use of PVC, stainless steel, and other corrosion resistant materials incorporated into the tower’s design. Biological growth would include bacteria that could cause the aforementioned Legionnaires’ disease, or algae which can build up enough to restrict water flow and lower efficiency. Water treatment (the use of chemicals in the water) and the regular bleeding or discharge of water in order to clear it of contaminants are also important when maintaining a cooling tower. The result of this bleeding is also known as blow down. Blow down is water that is discharged from the system in order to control the concentration of salt and other impurities in the circulating water. You can think of the process resulting in blow down as being similar to bleeding the brakes on a car to remove air (a contaminant) from the brake lines; bleed and the resulting blow down removes contaminants from the water in a cooling tower system [16].

Direct Expansion

A direct expansion (DX) system is a type of air conditioning system that uses a refrigerant[2] to reject heat and directly cool air. They get their name from the role the refrigerant plays in the system since it expands in air cooling coils.

DX systems come in different configurations that are usually identified by the type of compressor used and how they are set up. For example there are reciprocating or scroll compressor units that are available as package units (components are together on the same skid) or available as a split system (meaning the components can be located remotely - away from each other). These typically are made up of a condenser, compressor, and an evaporator [17].

Compressor, Condenser, and Evaporator

In a refrigeration system the compressor, condenser, and evaporator are often used together in a cycle to provide cooling, this system can be referred to as a compression refrigeration system.

Refrigerant is compressed in the compressor, causing a rise in pressure. Then a condenser (a machine or apparatus used to convert a gas into a liquid) condenses the gaseous refrigerant to a liquid. Next the evaporator, the component of an HVAC system in which heat is absorbed by the refrigerant, draws the heat out of the area that is to be cooled. The now vaporized refrigerant returns to the compressor to start the process over again. [18]

Dry Cooler

A dry cooler is used in situations where the outside air or ambient[3] air is used in conjunction with large fans and air to water heat exchangers to reduce heat inside of a facility. With a dry cooler system there is no refrigerant used. The dry cooler system often is used to supply air cooled water to CRAC units in data centers [19].

Chiller Plant

A “…chiller can be water-cooled, air-cooled or evaporatively cooled. The compressor types typically are reciprocating, scroll, screw or centrifugal. The evaporator can be remote from the condensing section on air-cooled units” [20].

Economizer

A device that is used in order to reduce energy usage, or to economize on energy costs, the economizer is a heat exchanger. They are often called either air-side or water-side economizers, depending on where they are located in an HVAC system. When the air supplied to a mission critical facility must be kept at a particular humidity, the water-side economizer is usually chosen, to reduce electrical costs, since it permits minimal outside air into the facility [21].

Thermal Wheel

A thermal wheel, or rotary heat exchanger, is used to reduce the load on a chiller. It is a wheel that is made up of a material that absorbs heat and is rotated. As the wheel turns it absorbs heat from exhaust air during half a rotation and then releases it to the fresh air stream during the second half of the rotation [22].

CRAC

A CRAC (computer room air conditioning unit) is often found in a data center or in other data communications rooms. A CRAC can be set up to work with different building cooling systems like DX, air cooled, water cooled, or chilled water. A CRAC is typically just like the air conditioners found in most homes. It can have a DX refrigeration cycle built in as well as a compressor. Traditional CRAC units can be only on or off and usually does not have the ability to adjust the volume of airflow [23].

CRAH

A CRAH (computer room air handler) typically uses water instead of a refrigerant. A CRAH works like a chilled water air handling unit and is usually supplied with chilled water from a chiller. “CRAHs can have VFDs[4] that modulate fan speed to maintain a set static pressure either under floor or in overhead ducts [23].”

[1] Legionnaires’ disease is a form of pneumonia most often contracted from inhaling bacteria. [32]

[2]Refrigerant – A substance used for cooling or refrigeration that enables the transfer of heat [30]

[3] Ambient air – the air surrounding a particular site or building. (Ambient air cooling is cooling that makes use of naturally cool air surrounding a facility in order to reduce heat inside it.) [31]

[4] VFDs – A variable frequency drive is a system that when used in an HVAC application is attached to a fan to save energy by supplying only the amount of air that is needed [33].

Objectives met:
- C4
- 4.5