ASHRAE Journal - May 2009 - 68

ASHRAE 19 This system also cools and partially dehumidifies the ventilation air that is supplied directly to the unit ventilator. It should be pointed out, however, that the main drawback of doing any latent cooling at the terminal unit is the gradual build-up of odor particles on the surface of the coil and the inability to control the odor within the room. Additionally, due to the fact that there would be a multiplicity of these units in a school building, the service requirements will be higher and the quality of air filtration will be lower than desirable, less than can be obtained in a central system. 50 59–2009 YE A R S JOURNAL 1 System First Costs 1.00 0.95 1.05 0.90 1.05 0.85 0.87 1.13 1.15 0.800 0.700 2 Operating Costs 1.00 0.90 0.90 0.85 0.95 0.90 0.93 0.93 0.94 0.940 0.900 3 First Costs and Operating Costs 1.000 0.925 0.975 0.875 1.000 0.875 0.900 1.030 1.060 0.870 0.800 A. B. C. D. E. F. G. H. I. J. K. All-Air Cooling with Room Reheat All-Air Dual-Duct, Low-Velocity All-Air Dual-Duct, High-Velocity All-Air Single Duct, Variable Volume All-Air Dual-Duct, Variable-Temperature-Volume All-Air Multi-Zone, Single Duct All-Water Room Fan Coil With Wall Outside Air Room Fan Coil with Partial P.A.* (Four-Pipe) Room Fan Coil With Full P.A. (Four-Pipe) Roof Mounted Multi-Zone, Gas-Fired Direct Expansion Unitary Package/Gas-Fired Roof-Mounted Units Another type of air-conditioning system is the roof-mounted, multizone factory packaged heating and cooling unit, utilizing a gas-fired heat exchanger for the hot deck, and direct-expansion air-cooled condensing section for the cold deck. Some of the advantages of this type of system are that these units are easy to set in place and reduce installation manhours. Also, the units can be added as additional areas to the building are added. However, there are some problems with this type of system that are inherently built into it. One of these is maintaining the hot deck temperature over an extended period of time without failure to either the controls or to the heat exchanger itself. In order to maintain an even hot deck temperature, it is necessary to cycle the heat exchangers at a fairly rapid rate. This causes undue stress on the heat exchanger, as well as cold spots, thus reducing the life of the heat exchanger. Another disadvantage of 68 ASHRAE Journal Note: 1. These costs do not allow for depreciation, taxes or interest costs. 2. Costs of all systems referenced to all-air with room reheat. * P.A. – Primary Air Table 3 Economic comparisons of air-conditioning systems for new school buildings. these systems in the northern climates is the fact that they are roof mounted, and proper servicing can become a problem. The ability of these units to bring in 100% outside air and maintain it under all conditions is equivalent to the capability of the classroom unit ventilator. Thus, it is not possible to maintain positive control over the outside air to the room, with a resultant odor build-up in the space. Inasmuch as the roof-mounted unit is an all-air unit, special provision will have to be made for those buildings having large perimeter areas of glass. That is, either an auxiliary radiation system will have to be installed to serve the perimeter glass, or duct work will have to be extended out into the perimeter distribution system, blowing air up along the glass. a s h r a e. o rg However, in moderate climates these auxiliary perimeter systems need not be included. A roof-top unit, like the unit ventilator, will increase the total maintenance costs of the building in comparison to the central system. Also, there will be no chance to take advantage of any diversity in shifting of loads within the building. In some areas, careful consideration should be given to the demand charges that multiple systems will impose upon the utility bill. The direct expansion refrigeration cycle offers some disadvantages on the refrigerant control side; these are that at low partial loads it is difficult to maintain a cold deck temperature. There are some ways that this can be overcome, such as hot gas bypasses. May 2009

ASHRAE Journal - May 2009

Table of Contents for the Digital Edition of ASHRAE Journal - May 2009