To meet the great demand for cheaper air-conditioning installations, manufacturers produce packaged, or preassembled, units. These vary from 4000 to 30,000 Btu/ hr for window units and 9000 Btu/hr and up for commercial units. Less field labor is required to install them than for custom-designed installations.
Packaged window units operate on the complete cycle shown in Fig. 13.30, but the equipment is very compactly arranged. The condenser is air cooled and projects outside the window. The cooling coil extends inside. Both the cooling-coil fan and condenser fan usually are run by the same motor.
These packaged units require no piping, just an electric receptacle of adequate capacity. Moisture that condenses on the cooling coil runs via gutters to a small sump near the condenser fan. Many manufacturers incorporate a disk slinger to spray this water on the hot condenser coil, which vaporizes it and exhausts it to the outside. This arrangement serves a double purpose:
1. Gets rid of humidity from the room without piping.
2. Helps keep the head pressure down with some evaporative cooling.
Floor-type and ceiling-type packaged units also contain the full air-conditioning cycle (Fig. 13.30). The air-cooled packaged units are usually placed near a window to reduce the duct runs required for the air-cooled condenser.
Roof-type packaged units are available for a variety of applications. These units contain a complete cooling cycle (usually with an air-cooled condenser) and a furnace. All controls are factory prewired, and the refrigeration cycle is completely installed at the factory. Necessary ductwork, wiring, and gas piping are supplied in the field.
A combination packaged unit including an evaporative condenser is also available.
Most packaged units are standardized to handle about 400 ft3 /min of air per ton of refrigeration. This is a good average air quantity for most installations. For restaurants, bars, etc., where high latent loads are encountered, the unit handles more than enough air; also, the sensible capacity is greater than required. However, the latent capacity may be lower than desired. This will result in a somewhat higher relative humidity in the premises.
For high sensible-load jobs, such as homes, offices, etc., where occupancy is relatively low, air quantities are usually too low for the installed tonnage. Latent capacity, on the other hand, may be higher than desired. Thus, if we have a total load of 5 tons 41â„2 tons sensible and 1â„2 ton latent and we have available a 5- ton unit with a capacity of 4 tons sensible and 1 ton latent, it is obvious that during extreme weather the unit will not be able to hold the dry-bulb temperature down, but the relative humidity will be well below design value. Under such conditions, these units may be satisfactory. In the cases in which the latent capacity is too low and we have more than enough sensible capacity, we can set the thermostat below design indoor dry-bulb temperature and maintain a lower dry-bulb temperature and higher relative humidity to obtain satisfactory comfort conditions. Where we have insufficient sensible capacity, we have to leave the thermostat at the design setting.
This will automatically yield a higher dry-bulb temperature and lower relative humidity and the premises will be comfortable if the total installed capacity is not less than the total load.
As an example of the considerations involved in selecting packaged airconditioning equipment, let us consider the structure in Fig. 13.4, p. 13.46, and the load analysis in Table 13.12, p. 13.47.
If this were an old building, it would be necessary to do the following:
First-floor store (load 7.25 tons). Inasmuch as a 5-ton unit is too small, we must choose the next larger size a 71â„2-ton packaged unit. This unit has a greater capacity than needed to maintain design conditions. However, many people would like a somewhat lower temperature than 80F dry bulb, and this unit will be capable of maintaining such conditions. Also, if there are periods when more than 50 occupants will be in the store, extra capacity will be available.
First-floor office (load 0.35 tons). Ordinarily a 4000-Btu/hr window-unit would be required to do the job. But because the store unit has spare capacity, it would be advisable to arrange a duct from the store 71â„2-ton unit to cool the office.
Second-floor offices (No. 1, load 3.50 tons; No. 2, load 1.13 tons). A 5-ton unit is required for the two offices. But it will be necessary to provide a fresh-air connection to eliminate the fresh-air load from the internal load, to reduce air requirements to the rated flow of the unit. Then, the 2000 ft3 /min rating of the 5-ton packaged unit will be close enough to the 2236 ft3 /min required for the two offices.
Inasmuch as the total tonnage is slightly above that required, we will balance out at a slightly lower relative humidity than 50%, if the particular packaged unit selected is rated at a sensible capacity equal to the total sensible load.
A remote air-conditioning system would be more efficient, because it could be designed to meet the needs of the building more closely. A single air-handling unit for both floors can be arranged with the proper ductwork. However, local ordinances should be checked, since some cities have laws preventing direct-expansion systems servicing more than one floor. A chilled water system (Fig. 13.31) could be used instead.
Consideration for part load performance is very important. Remember that equipment is selected for maximum design conditions that occur only 25% of the time. Knowledge of criticality of temperature and relative humidity must be determined.
For packaged equipment, slightly undersizing the system may be more desirable to maintain radiant heat control at part load by reducing cycling. Heat wheels and desiccant dehumidification systems are also in widespread use to better maintain conditions and save energy, given code-required increases in ventilation air.
Split Systems. Split systems differ from other packaged air-conditioning equipment in that the noisy components of the system, notably the refrigerant compressor and air-cooled condenser or cooling tower fans, are located outdoors, away from the air-handling unit. This arrangement is preferred for apartment buildings, residences, hospitals, libraries, churches, and other buildings for which quiet operation is required. In these cases, the air-handler is the only component located within the occupied area. It is usually selected for low revolutions per minute to minimize fan noise. For such installations, chilled-water piping, or direct-expansion refrigerating piping must be extended from the cooling coil to the remote compressor and condensing unit, including operating controls.
Packaged Chillers. These units consist of a compressor, water chiller, condenser, and all automatic controls. The contractor has to provide the necessary power and condensing water, from either a cooling tower, city water, or some other source such as well or river. The controls are arranged to cycle the refrigeration compressor to maintain a given chilled-water temperature. The contractor need provide only insulated piping to various chilled-water air-handling units.
Packaged chillers also are available with air-cooled condensers for complete outdoor installation of the unit, or with a condenserless unit for indoor installation connected to an outdoor air-cooled condenser.
The larger packaged chillers, 50 to 1000 tons, are generally powered by centrifugal compressors. When the units are so large that they have to be shipped knocked down, they are usually assembled by the manufacturers representative on the job site, sealed, pressure-tested, evacuated, dehydrated, and charged with the proper amount of refrigerant.
