What is AHU?

 An air handling unit, commonly called an AHU, is the composition of elements mounted in large, accessible box-shaped units called modules, which house the appropriate ventilation requirements for purifying, air-conditioning or renewing the indoor air in a building or premises.

They are usually installed on the roof of buildings and, through ducts, the air is circulated to reach each of the rooms in the building in question.

 

 

Main functions of an AHU

In addition to managing the proper ventilation of the interior with outside air, the AHU performs other functions:

• Filtration and control of the quality of the air that will reach the interior, thanks to the air purification filters, and depending on the retention of these filters, the air will be clean.
• Control of the air temperature that regulates the air conditioning system in cold or hot, so that the thermal sensation in the interior is the desired one.
• Relative humidity monitoring for greater indoor comfort.

For its part, the places for which the AHU is intended are those in which the flow of people is very large and accumulates many people at the same time and whose natural ventilation is limited: hotel dining rooms, function rooms, restaurants, convention halls... It is also a suitable option for those spaces with very high hygiene requirements: laboratories, clean rooms or operating theatres, among others. An AHU can also be used to ventilate places where air conditioning is provided by radiators or underfloor heating, for example.

What does an AHU consist of?

 

• Air intake: air handling units collect air from outside, which is treated and distributed throughout the rooms; and/or indoor air that is "recycled".
• Filter: depending on the air purity requirements, the filter applied will have a higher or lower particle, viruses, bacteria, odours, and other air pollutants retention.
• Fan: this is an electromechanical system that powers the air to expel it from the AHU to the ducts that distribute the air throughout the rooms.
• Heat exchangers: devices that transfer temperature between two fluids, in this case, coolant and air, separated by a solid barrier.
• Cooling coil: the air passing through this module is cooled. In this process, water droplets can be generated, which are collected in a condensate tray thanks to the built-in droplet separator.
• Silencer: coatings that considerably reduce the sound level of the installation.
• Plenums: empty spaces in which the air flow is homogenised.

Energy efficiency of AHUs

The ultimate aim of an air handling unit is energy efficiency and this is mandatory since 2016 by the European Ecodesign Regulation 1235/2014.

By having heat recovery units, the AHU reduces the use of energy required in air conditioning, as in the exchanger, the indoor and outdoor air is mixed, so that when the air reaches the coil the temperature contrast is lower, therefore, the climatic contribution is also lower and energy consumption is also reduced.

Likewise, the variable regulation of the equipment means that the fans can work according to the flow rate needs, reducing their consumption.

Types of refrigerant used in refrigeration system

 Types of refrigerant used in refrigeration system

Refrigerants are divided into groups according to their chemical composition. Following the discovery that some of these chemical compounds may be harmful to the environment, they are being replaced with more environmentally friendly alternatives (see Figure 5.2). The process is not easy, and although there are alternatives to old refrigerants, the new ones are usually not flawless.
In the following section, different groups of refrigerants are discussed, some examples are given and their fields of application are described.

CFC = ChloroFluoroCarbons

Chlorofluorocarbons are refrigerants that contain chlorine. They have been banned since the beginning of the 90's because of their negative environmental impacts. Examples of CFCs are R11, R12 and R115. The conversion of equipment and systems using CFCs has not yet been completed. On the contrary, the illegal market for this type of refrigerants flourishes worldwide, and it is estimated that no more than 50% of CFC systems worldwide have been upgraded.

HCFC = HydroChloroFluoroCarbons

The slow phase-out of CFCs shows it is a costly process. However, and more importantly, it also shows the problems and indecisiveness surrounding the availability of HCFCs, which were officially indicated as temporary (until 2030) substitutes for CFCs. The hasty actions of the European Union that culminated in the ban of HCFCs, immediately for refrigeration and soon (2004 at the latest) for air conditioning, has upset the industry's programs and plans.

The HCFCs contain less chlorine than CFCs, which means a lower ODP (see section 5.3). Examples of hydrochlorofluorocarbons include R22, R123 and R124 (see Figure 5.3).

HFC = HydroFluoroCarbons

The hydrofluorocarbons are refrigerants that contain no chlorine and are not harmful to the ozone layer (ODP = 0, see section 5.3). However, their impact on global warming is very large compared with traditional refrigerants. The most common HFC refrigerants available since the ban on HCFCs are presented in Table 5.1 (see also Figure 5.4):

Table 5.1 The most common refrigerants among halogenated hydrocarbons.

Some comments on the refrigerants presented in the table are given below:

• R32 and R125 are seldom used as single refrigerants, but only in mixtures with particularly favorable thermodynamic properties.
• R245c and R245fa are used almost exclusively in the United States and in a rather experimental way.
• R404A has been developed as an alternative to R502 for refrigerators and freezers.
• R134a was the first HFC introduced in refrigeration and air conditioning with great success, because it requires almost no changes in the equipment designed for R22. However, it offers a very limited efficiency, about 40% lower than that obtained with R22. Consequently, the manufacturer has two choices: either to accept a substantial reduction in the thermal capacity in a given system, or to increase its dimensions (and cost) to achieve the same capacity. For this reason, R134a is used mainly in large systems (over 250 kW) that can afford the higher costs.
• R407C is, like R134a, thermodynamically similar to R22 and works as a "drop in" refrigerant. However, unlike R134a, which is a pure compound, R407C has a glide of 7 K, making it barely usable in small residential (household) equipment. There are two reasons to justify such a limitation: residential equipment is more subject than other equipment to sudden accidental losses, and it is usually serviced on site. In the event of a sudden leakage, a 7K glide may result in changes in the proportions of the mixture, because the relative losses of its most volatile components will be disproportionately high. If a standard refill is used, there is no guarantee that the new refrigerant mixture has the same proportions as it had before the leakage. Due to its high glide, this refrigerant is used only in medium-capacity systems (50-250 kW), which are usually serviced by skilled personnel.
• R410A has very attractive thermodynamic properties, higher energy efficiency than R22, no glide and hence no problem with the mixture remaining after charge loss and refill. However, it has an operating pressure almost double that of R22, and therefore requires a redesign of the whole system with larger compressors, expansion valves, etc.
• R507A is used successfully in industrial and commercial refrigeration.
• R508B is less frequently used in low temperature cycles. R507A and R508B have favorable thermodynamic properties and no problems with temperature glides, because they are azeotropic mixtures.

FC = FluoroCarbons

Fluorocarbons (Figure 5.5) contain no chlorine and are not harmful to the ozone layer. However, they are extremely stable, and they have a high GWP (cf. section 5.3). R218 is an example of a fluorocarbon, and FCs are also present in the mixtures R403 and R408.

HC = HydroCarbons

Hydrocarbons are a very limited solution to the environmental problems associated with refrigerants. They are harmless to the ozone layer (ODP = 0) and have hardly any direct green house effect (GWP<5), but they are highly flammable. The use of HCs as refrigerants is confined to Europe, because many other countries elsewhere have banned the use of flammable gas in the presence of the public. According to the standards ISO 55149 and EN 378.2000, this should apply also in Europe. However, the standard IEC 355.2.20 allows the use of HCs in household refrigerators with refrigerant charges up to 150 g.

This standard has opened the way for some European refrigerator manufacturers to produce household refrigerators with flammable isobutene, R600a.

These have been accepted enthusiastically by environmentalists, and have achieved great success in the market.

NH3 = Ammonia

Ammonia, R717, is an attractive refrigerant alternative. It has been used in refrigeration systems since 1840 and in vapor compression since 1860. In terms of its properties, it should be considered a high-class refrigerant. Furthermore, its ODP and GWP are 0. However, although it is a selfalerting gas, i.e. leaks can easily be detected by the smell, ammonia is very hazardous even at low concentrations because the smell often causes panic. This is the main reason why ammonia was withdrawn from applications for use by unskilled people and retained only for industrial applications.

It is also quite common in commercial refrigeration, although safety regulations require that it be used with a secondary distribution loop. Obviously, this secondary loop reduces the efficiency.

CO2 = Carbon Dioxide

R744, carbon dioxide, has several attractive characteristics: non-flammable, does not cause ozone depletion, very low toxicity index (safety A1), available in large quantities, and low cost. However, it also has a low efficiency and a high operating pressure (approximately 10 times higher than R134a). For the two latter reasons, efforts are needed to improve its refrigeration cycle and related technology, particularly heat exchangers and expansion devices. A major forthcoming CO2 application seems to be air conditioning in the automotive industry. Heat pumps could also benefit from CO2 due to the higher temperature that can be obtained even at very low ambient temperatures.

Summary Table

PSYCHROMETRIC CHART USE

DOWNLOAD A PSYCHROMETRIC CHART FROM PARAMETER GENERATION & CONTROL

Parameter Generation and Control now offers website visitors a free, downloadable PDF of the psychrometric chart. If carrying out heat load or cooling load calculations with a humidity control room or humidity chamber, turn to the psychrometric chart as an initial resource to understand the relationship between the different variables in air. For more information on our chamber and control room product and service offerings, contact us to request a quote today.

WHAT IS A PSYCHROMETRIC CHART?

A psychrometric chart represents the psychrometric properties of air. With this chart, engineers can better assess psychrometric processes and find practical solutions. While this chart looks complicated and even intimidating, it’s actually quite helpful and simple to understand once you grasp the basic properties of air. If you know two parameters of air where the lines will cross each other, the psychrometric chart can do the rest of the work for you.

 

THE BENEFITS OF USING A PSYCHROMETRIC CHART CORRECTLY

A psychrometric chart prevents engineers from spending time on tedious mathematical formulas. While there are online calculators and applications to help make calculations, using the chart correctly provides engineers with a more accurate reading as long as you know two parameters of air. Knowing how to read a psychrometric chart is a wise skill for engineers to have in the event that technology fails or isn’t available.

 

WHAT ARE THE PARTS OF A PSYCHROMETRIC CHART?

A psychrometric chart consists of eight standard parts, including:

• Temperatures
• • Dry Bulb – This is the temperature reading found on a typical thermometer. You can find a psychrometric chart that offers these temperature ranges:
    • Low temperatures that range from -20 degrees FDB to 50 degrees FDB
    • Normal temperatures that range from 20 degrees FDB to 100 degrees FDB
    • High temperatures that range from 60 degrees FDB to 250 degrees FDB
  • Wet Bulb – This is a typical thermometer’s standard reading if the sensing bulb is covered with a wet wick or sock and exposed to air flow.
  • Dew Point – At this temperature, moisture starts condensing from the air.
• Specific Volume & Density – Specific volume is measured in cubic feet per pound. This refers to the amount of space air occupies per pound of weight.
• Enthalpy – This is the measurement of heat energy. Enthalpy is measured by Btu (British thermal unit) per pound of dry air. 
• Sensible Heat Ratio – This is the total sensible heat flow divided by the total heat flow.
• Sensible Heat Flow – 60(specific heat of air in Btu/lb ºF (0.24 at 72ºF))(density of air in lb/ft³)(air flow in ft³/min)(| supply air temperature – conditioned room temperature |) 
• Latent Heat Flow – 60(latent heat of vaporization of water in Btu/lb (970 at sea level))(density of air in lb/ft³)(air flow in ft³/min)(humidity ratio difference in lb water/lb dry air)
• Moisture Content – Also known as the humidity ratio, this is the total weight of water vapor per pound of dry air.
• Relative Humidity – This refers to the percentage of water vapor per pound of dry air in relation to how much the air can hold at its current temperature.
• Vapor Pressure – Vapor pressure is measured in inches of mercury and represents the pressure exerted by water vapor in air.
• Standard Air Dot – This dot marks the measurement for standard air. Standard air is typically 70 degrees Fahrenheit with a relative humidity of 54% and 60 gr/lb of specific humidity.

 

HOW TO READ A PSYCHROMETRIC CHART

A psychrometric chart can easily be read by following these steps:

Step 1: Locate the dry bulb temperature. This will be measured in degrees Fahrenheit or Celsius and will be along the bottom axis. Also identify the vertical line for each temperature.

Step 2: Locate the humidity ratio, sometimes labeled as a mixing ratio. This will be along the right vertical axis. Humidity ratio units are grains of moisture per pound of dry air or grams of moisture per kilogram of dry air.

Step 3: Located the left-most curved line. This refers to the saturation curve where relative humidity is 100%. 

Step 4: Locate the interior curved lines, which represent percentage levels of relativity humidity.

Step 5: Locate the dew point. This is a vertical line on the right side of the chart. These lines traverse the chart as horizontal lines.

Step 6: On the other side of the dew point’s vertical line is the vapor pressure scale. Vapor pressure lines also traverse the chart as horizontal lines.

Step 7: On all outer sides of the chart, you’ll see scales representing enthalpy. With a ruler, you can match the scales across the chart. 

Step 8: Find the second set of diagonal lines which identify wet bulb temperature. Though these lines are close to the enthalpy lines, they’re not actually parallel. 

When using Airtable’s PDF psychrometric chart, there are some formational aspects to be aware of. First, the properties of air indicated in the chart are calculated at standard atmospheric pressure. For other pressures, relevant corrections have to be applied.

Also note that the relative humidity lines are the curves extending from the lower left to the upper right portion of the chart. The relative humidity curves indicate different values of humidity measured in percentage. The value of relative humidity reduces from left to right.

CENTRAL HYDRONIC AIR CONDITIONING SYSTEMS

     Central hydronic air conditioning systems are also called central air conditioning systems. In a central hydronic air conditioning system, air is cooled or heated by coils filled with chilled or hot water distributed from a central cooling or heating plant. It is mostly applied to large-area buildings with many zones of conditioned space or to separate buildings. Water has a far greater heat capacity than air. The following is a comparison of these two media for carrying heat energy at 68°F (20°C)

The heat capacity per cubic foot (meter) of water is 3466 times greater than that of air. Transporting heating and cooling energy from a central plant to remote air-handling units in fan rooms is far more efficient using water than conditioned air in a large air conditioning project. However, an additional water system lowers the evaporating temperature of the refrigerating system and makes a small- or medium-size project more complicated and expensive. A central hydronic system of a high-rise office building, the NBC Tower in Chicago, is illustrated in Fig. 1.1. A central hydronic air conditioning system consists of an air system, a water system, a central heating/cooling plant, and a control system. 

Air System

are the air-handling units, supply/return ductwork, fan-powered boxes, space diffusion devices, and exhaust systems. An air-handling unit (AHU) usually consists of supply fan(s), filter(s), a cooling coil, a heating coil, a mixing box, and other accessories. It is the primary equipment of the air system. An AHU conditions the outdoor/recirculating air, supplies the conditioned air to the conditioned space, and extracts the returned air from the space through ductwork and space diffusion devices. A fan-powered variable-air-volume (VAV) box, often abbreviated as fan-powered box, employs a small fan with or without a heating coil. It draws the return air from the ceiling plenum, mixes it with the conditioned air from the air-handling unit, and supplies the mixture to the conditioned space. Space diffusion devices include slot diffusers mounted in the suspended ceiling; their purpose is to distribute the conditioned air evenly over the entire space according to requirements. The return air enters the ceiling plenum through many scattered return slots. Exhaust systems have exhaust fan(s) and ductwork to exhaust air from the lavatories, mechanical rooms, and electrical rooms. The NBC Tower in Chicago is a 37-story high-rise office complex constructed in the late 1980s. It has a total air conditioned area of about 900,000 ft2 (83,600 m2 ). Of this, 256,840 ft2 (23,870 m2 ) is used by NBC studios and other departments, and 626,670 ft2 (58,240 m2 ) is rental offices located on upper floors. Special air conditioning systems are employed for NBC studios and departments at the lower level. For the rental office floors, four air-handling units are located on the 21st floor. Outdoor air either is mixed with the recirculating air or enters directly into the air-handling unit as shown in Fig. 1.2. The mixture is filtrated at the filter and is then cooled and dehumidified at the cooling coil during cooling season. After that, the conditioned air is supplied to the typical floor through the supply fan, the riser, and the supply duct; and to the conditioned space through the fan-powered box and slot diffusers

Water System 

        The water system includes chilled and hot water systems, chilled and hot water pumps, condenser water system, and condenser water pumps. The purpose of the water system is (1) to transport chilled water and hot water from the central plant to the air-handling units, fan-coil units, and fan powered boxes and (2) to transport the condenser water from the cooling tower, well water, or other sources to the condenser inside the central plant.

    In Figs. 1.1 and 1.2, the chilled water is cooled in three centrifugal chillers and then is distributed to the cooling coils of various air-handling units located on the 21st floor. The temperature of the chilled water leaving the coil increases after absorbing heat from the airstream flowing over the coil. Chilled water is then returned to the centrifugal chillers for recooling through the chilled water pumps.

    After the condenser water has been cooled in the cooling tower, it flows back to the condenser of the centrifugal chillers on lower level 3. The temperature of the condenser water again rises owing to the absorption of the condensing heat from the refrigerant in the condenser. After that, the condenser water is pumped to the cooling towers by the condenser water pump.

Central Plant 

    The refrigeration system in a central plant is usually in the form of a chiller package. Chiller packages cool the chilled water and act as a cold source in the central hydronic system. The boiler plant, consisting of boilers and accessories, is the heat source of the heating system. Either hot water is heated or steam is generated in the boilers.

    In the NBC Tower, the refrigeration system has three centrifugal chillers located in lower level 3 of the basement. Three cooling towers are on the roof of the building. Chilled water cools from 58 to 42°F (14.4 to 5.6°C) in the evaporator when the refrigerant is evaporated. The refrigerant is then.

    compressed to the condensing pressure in the centrifugal compressor and is condensed in liquid form in the condenser, ready for evaporation in the evaporator. There is no boiler in the central plant of the NBC Tower. To compensate heat loss in the perimeter zone, heat energy is provided by the warm plenum air and the electric heating coils in the fan powered boxes.

Control System 

        Modern air conditioning control systems for the air and water systems and for the central plant consist of electronic sensors, microprocessor-operated and -controlled modules that can analyze and perform calculations from both digital and analog input signals, i.e., in the form of a continuous variable. Control systems using digital signals compatible with the microprocessor are called direct digital control (DDC) systems. Outputs from the control modules often actuate dampers, valves, and relays by means of pneumatic actuators in large buildings and by means of electric actuators for small projects.

CLASSIFICATION OF AIR CONDITIONING SYSTEMS ACCORDING TO CONSTRUCTION AND OPERATING CHARACTERISTICS

Clean-Room Air Conditioning Systems

     Clean-room or clean-space air conditioning systems serve spaces where there is a need for critical control of particulates, temperature, relative humidity, ventilation, noise, vibration, and space pressurization. In a clean-space air conditioning system, the quality of indoor environmental control directly affects the quality of the products produced in the clean space. 

    A clean-space air conditioning system consists of a recirculating air unit and a makeup air unit—both include dampers, prefilters, coils, fans, high-efficiency particulate air (HEPA) filters, ductwork, piping work, pumps, refrigeration systems, and related controls except for a humidifier in the makeup unit (refer to Chap. 30). 

 Space Conditioning Air Conditioning Systems 

    Space conditioning air conditioning systems are also called space air conditioning systems. They have cooling, dehumidification, heating, and filtration performed predominately by fan coils, water source heat pumps, or other devices within or above the conditioned space, or very near it. A fan coil consists of a small fan and a coil. A water-source heat pump usually consists of a fan, a finned coil to condition the air, and a water coil to reject heat to a water loop during cooling, or to extract heat from the same water loop during heating. Single or multiple fan coils are always used to serve a single conditioned room. Usually, a small console water-source heat pump is used for each control zone in the perimeter zone of a building, and a large water-source heat pump may serve several rooms with ducts in the core of the building (interior zone, refer to Chap. 28). 

    Space air conditioning systems normally have only short supply ducts within the conditioned space, and there are no return ducts except the large core water-source heat pumps. The pressure drop required for the recirculation of conditioned space air is often equal to or less than 0.6 in. water column (WC) (150 Pa). Most of the energy needed to transport return and recirculating air is saved in a space air conditioning system, compared to a unitary packaged or a central hydronic air conditioning system. Space air conditioning systems are usually employed with a dedicated (separate) outdoor ventilation air system to provide outdoor air for the occupants in the conditioned space.

     Space air conditioning systems often have comparatively higher noise level and need more periodic maintenance inside the conditioned space

Unitary Packaged Air Conditioning Systems 

    Unitary packaged air conditioning systems can be called, in brief, packaged air conditioning systems or packaged systems. These systems employ either a single, self-contained packaged unit or two split units. A single packaged unit contains fans, filters, DX coils, compressors, condensers, and other accessories. In the split system, the indoor air handler comprises controls and the air system, containing mainly fans, filters, and DX coils; and the outdoor condensing unit is the refrigeration system, composed of compressors and condensers. Rooftop packaged systems are most widely used (refer to Chap. 29). 

     Packaged air conditioning systems can be used to serve either a single room or multiple rooms. A supply duct is often installed for the distribution of conditioned air, and a DX coil is used to cool it. Other components can be added to these systems for operation of a heat pump system; i.e., a centralized system is used to reject heat during the cooling season and to condense heat for heating during the heating season. Sometimes perimeter baseboard heaters or unit heaters are added as a part of a unitary packaged system to provide heating required in the perimeter zone. 

    Packaged air conditioning systems that employ large unitary packaged units are central systems by nature because of the centralized air distributing ductwork or centralized heat rejection systems. Packaged air conditioning systems are characterized by the use of integrated, factory-assembled, and ready-to-use packaged units as the primary equipment as well as DX coils for cooling, compared to chilled water in central hydronic air conditioning systems. Modern large rooftop packaged units have many complicated components and controls which can perform similar functions to the central hydronic systems in many applications

CLASSIFICATION OF AIR CONDITIONING SYSTEMS ACCORDING TO CONSTRUCTION AND OPERATING CHARACTERISTICS

CLASSIFICATION OF AIR CONDITIONING SYSTEMS ACCORDING TO CONSTRUCTION AND OPERATING CHARACTERISTICS

Air conditioning systems can also be classified according to their construction and operating characteristics as follows.

Individual Room Air Conditioning Systems

Individual room, or simply individual air conditioning systems employ a single, self-contained room air conditioner, a packaged terminal, a separated indoor-outdoor split unit, or a heat pump. A heat pump extracts heat from a heat source and rejects heat to air or water at a higher temperature for heating. Unlike other systems, these systems normally use a totally independent unit or units in each room. Individual air conditioning systems can be classified into two categories:

 Room air conditioner (window-mounted)

 Packaged terminal air conditioner (PTAC), installed in a sleeve through the outside wall

    The major components in a factory-assembled and ready-for-use room air conditioner include the following: An evaporator fan pressurizes and supplies the conditioned air to the space. In tube and- fin coil, the refrigerant evaporates, expands directly inside the tubes, and absorbs the heat energy from the ambient air during the cooling season; it is called a direct expansion (DX) coil. When the hot refrigerant releases heat energy to the conditioned space during the heating season, it acts as a heat pump. An air filter removes airborne particulates. A compressor compresses the refrigerant from a lower evaporating pressure to a higher condensing pressure. A condenser liquefies refrigerant from hot gas to liquid and rejects heat through a coil and a condenser fan. A temperature control system senses the space air temperature (sensor) and starts or stops the compressor to control its cooling and heating capacity through a thermostat. 

    

    The difference between a room air conditioner and a room heat pump, and a packaged terminal air conditioner and a packaged terminal heat pump, is that a four-way reversing valve is added to all room heat pumps. Sometimes room air conditioners are separated into two split units: an outdoor condensing unit with compressor and condenser, and an indoor air handler in order to have the air handler in a more advantageous location and to reduce the compressor noise indoors. 

    Individual air conditioning systems are characterized by the use of a DX coil for a single room. This is the simplest and most direct way of cooling the air. Most of the individual systems do not employ connecting ductwork. Outdoor air is introduced through an opening or through a small air damper. Individual systems are usually used only for the perimeter zone of the building.

CLASSIFICATION OF AIR CONDITIONING SYSTEMS ACCORDING TO CONSTRUCTION AND OPERATING CHARACTERISTICS

 Evaporative-Cooling Air Conditioning Systems

    Evaporative-cooling air conditioning systems use the cooling effect of the evaporation of liquid water to cool an airstream directly or indirectly. It could be a factory-assembled packaged unit or a field-built system. When an evaporative cooler provides only a portion of the cooling effect, then it becomes a component of a central hydronic or a packaged unit system.

    An evaporative-cooling system consists of an intake chamber, filter(s), supply fan, direct-contact or indirect-contact heat exchanger, exhaust fan, water sprays, recirculating water pump, and water sump. Evaporative-cooling systems are characterized by low energy use compared with refrigeration cooling. They produce cool and humid air and are widely used in southwest arid areas in the United States.

Desiccant-Based Air Conditioning Systems

    A desiccant-based air conditioning system is a system in which latent cooling is performed by desiccant dehumidification and sensible cooling by evaporative cooling or refrigeration. Thus, a considerable part of expensive vapor compression refrigeration is replaced by inexpensive evaporative cooling. A desiccant-based air conditioning system is usually a hybrid system of dehumidification, evaporative cooling, refrigeration, and regeneration of desiccant (refer to Chap. 29).

    There are two airstreams in a desiccant-based air conditioning system: a process airstream and a regenerative airstream. Process air can be all outdoor air or a mixture of outdoor and recirculating air. Process air is also conditioned air supplied directly to the conditioned space or enclosed manufacturing process, or to the air-handling unit (AHU), packaged unit (PU), or terminal for further treatment. Regenerative airstream is a high-temperature airstream used to reactivate the desiccant.

    A desiccant-based air conditioned system consists of the following components: rotary desiccant dehumidifiers, heat pipe heat exchangers, direct or indirect evaporative coolers, DX coils and vapor compression unit or water cooling coils and chillers, fans, pumps, filters, controls, ducts, and piping.

Thermal Storage Air Conditioning Systems

    In a thermal storage air conditioning system or simply thermal storage system, the electricity-driven refrigeration compressors are operated during off-peak hours. Stored chilled water or stored ice in tanks is used to provide cooling in buildings during peak hours when high electric demand charges and electric energy rates are in effect. A thermal storage system reduces high electric demand for HVAC&R and partially or fully shifts the high electric energy rates from peak hours to off-peak hours.

    A thermal storage air conditioning system is always a central air conditioning system using chilled water as the cooling medium. In addition to the air, water, and refrigeration control systems, there are chilled-water tanks or ice storage tanks, storage circulating pumps, and controls.