The use of building cooling systems in the hotel industry has greatly increased management's physical plant responsibility. For a hotel building it undoubtedly has some type of air-cooling system, even if it is only needed for a month or two during the entire year. Customers expect and demand a comfortable environment. Air cooling reduces the seasonal nature of many lodging and foodservice businesses.
The heart of the HVAC (heating, ventilation and air-conditioning) system is the air-conditioning component. It is most costly component to install. It is greatly affected by building construction techniques and materials. Its air quality output can have very good or very poor effects on employees and customers. Its operating cost can be very high. Its maintenance requirements are generally the highest of all electromechanical systems within the building.
There are three terms that describe the systems used to cool air: air-conditioning, air-cooling and refrigerated air.
Air-conditioningAny treatment of the environment air within a building is air conditioning. Technically, a fan placed in a room to circulate air meets the air-conditioning definition. Filtering air is conditioning air; heating is conditioning the air; cooling, humidifying, or dehumidifying are kinds of air-conditioning. Thus, the process of changing any one of several properties of the air can be correctly classified as air-conditioning.
Air-coolingAir-cooling is any process that reduces air temperature. There are several air temperature reduction techniques. A fan blowing air over a bucket of ice water can be called air-cooling. Evaporative coolers or swamp coolers are air-cooling techniques. The evaporative device lowers air temperature and increases the moisture content in the air.
Refrigerated AirRefrigerated air is produced by a mechanical refrigeration cycle. The primary refrigeration systems are vapour compression and absorption cycle. The primary refrigeration systems are vapour compression and absorption cycles. The refrigerated air system is capable of providing cool comfortable air at all times. It is also most costly air-cooling system to install and operate.
CLASSIFICATION• Refrigerant circulating system
• Directly cools cooling coils
• Chilled water circulating system
• Refrigeration system cools chill water, which cools cooling coils
• Self-contained system
• Added on in establishments which originally did not have AC plants
• Not located in ventilation system (window unit)
AIR CONDITIONING SYSTEMSAIR-COOLING SYSTEMS
A very common air-cooling system in hotels is the evaporative cooler. Evaporative coolers require two energy resources:
1. Electricity to operate fan
When water is absorbed by air, the air temperature is reduced and the enthalpy is increased causing the percent relative humidity to increase. The total cooling energy requirement for this system is generally much less than the energy required to operate a refrigerated-air system.
The evaporative cooling process also filters air. However, evaporative cooling systems do not work when air is moist, high outside relative humidity. Air with medium to high relative humidity cannot absorb enough additional moisture to be cooled.
Evaporative coolers are installed in kitchens, laundries or dry storage areas. In some cases, the evaporative cooler may be used only about one or two months each year, with refrigerated-air systems used during the remaining cooling period. The double cooling system is expensive to install, but energy saving can be very large during this period, large enough to recover the cost of the system in less than 2 years.
Refrigerated-air cooling system makes available cool and dehumidified air throughout the cooling season. Currently, it is the most economical way to remove moisture from air. Air moisture condenses from air when it strikes an evaporator of the refrigeration cycle and releases its latent heat of vaporization. Moist air has a high dew point. The evaporator must be below the dew-point temperature to reduce the air's dew point, thus dehumidifying the air.
Depending on the cooling energy and initial cost requirements, two choices of the refrigerated-air system:
1. Vapour compression refrigeration
2. Absorption cooling system
Vapour Compression Refrigeration SystemsThe vapour compression refrigeration system was discussed earlier. The evaporator does the cooling in the cycle. Energy is required to operate the compressor and auxiliary components, such as fans, control systems and water spray filters' and humidifiers. Two types of vapour refrigeration systems are used: direct expansion (DX) and chilled water.
Direct Expansion (DX) Refrigerated Air SystemThis system cools ventilation air directly at the evaporator, can be used for most cooling requirements. Each guestroom or building section could separate mechanical refrigeration systems. It is possible to cool an entire large building by this process. In a medium sized hotel e.g. 300 guestrooms, at least 10 and as many as 300 individual systems would have to be installed and maintained. Maintenance cost could be very high, and operating cost could also be very high if occupancy remained at a high level during the entire cooling season.
The system is basically very simple. The evaporator is in the room or near the area that is to be cooled. A fan is used to blow air over the evaporator. The air is cooled and is blown into the area to absorb the heat load. The system can cool fresh air, circulated air or a combination.
Disadvantage of this system is that, as the evaporator is directly cooling air, any refrigerant leak at the evaporator allows the refrigerant gas to contaminate the air in the room.
The DX system is very efficient for small areas and is least costly refrigerated-air system to install.
Chilled-Water Compression SystemA common system for medium to large-sized buildings is the chilled-water compression system. This system uses indirect evaporators. The basic vapour compression system is used to produce chilled water. Pumps, to areas that are to be cooled, circulate the chilled water. Air is then blown over chilled-water indirect evaporators.
These systems are relatively efficient when operated at 80 percent or higher heat loads, resulting in maximum cooling. Under these conditions, the compressor starts and operates for long time periods. System efficiency decreases when compressors are operated in short on-off cycles and when operating less than 50 percent of the time.
|chilled water circulation system|
Absorption Cooling SystemsNormally, absorption-cooling systems using lithium bromide absorption refrigeration are used in large or medium sized buildings, but now are installed in many smaller buildings also, such as a hotel of 80 guestrooms. Installation and maintenance costs for absorption cooling are moderate, energy operating cost are low if city steam is available, when compared to the vapour compression system operating costs.
|absorption cooling system|
In all cases, the absorption cooling system produces chilled water. If city steam is not available, a heat plant is generally used to produce steam for the generator. The same steam boiler can be used during the heating months to heat the building, produce domestic hot water and generate kitchen steam and hot water.
ACTIVE SOLAR SPACE COOLINGSolar space cooling is quite costly to implement. It is best to use a solar system that serves more than just the cooling needs of a house to maximize the return on investment and not leave the system idle when cooling is not required. Significant space heating and/or water heating can be accomplished with the same equipment used for the solar cooling system.
The technologies that are being developed for gas cooling systems are the same ones being developed for active solar space cooling systems. Desiccant cooling systems and advanced absorption systems are the primary technologies that are used. High temperature liquid collectors are typically used in these systems.
Desiccant SystemA moisture absorbing material (desiccant) is located in the air stream going into the living space. As the air passes through the desiccant, which is usually located on a wheel that slowly rotates into the air stream, moisture is removed from the air, dropping the humidity level in the air stream to the point that an evaporative cooler can then cool the air. The heat generated by the solar collectors as it rotates out of the air stream dries the desiccant.
Absorption Air ConditioningHeat from solar collectors separates a low boiling refrigerant in a generator that receives the pressurized refrigerant from an absorber. Solar heat can also be used in the evaporation stage of the cycle.
Air-conditioning system componentsAir-conditioning system components include:
1. Filters. Removes impurities from air to provide clean air for guests, clients, members and employees. Fresh outside air, re-circulated air, smoky air should be filtered.
a) Dry filters
b) Oil-treated filters
c) Water-spray filters
d) Electronic filters
e) Grease filters
f) Pollution control filters
2. Pre-heaters and re-heaters. Partially heats air or adds sensible heat to air, usually to reduce its relative humidity.
3. Pre-coolers. Reduces the temperature of air or removes sensible heat from air.
4. Dehumidifiers. Removes moisture from air or removes latent heat from air, usually an evaporator of a refrigeration cycle.
5. Humidifiers. Adds water to air or increases the latent heat content of air. Usually an evaporator cooler or water spray device.
6. Fans. Required for ventilation and air flow.
(a) Centrifugal fans
(b) Axial-flow fans
7. System air supply.