中央空调冷却塔英文文献翻译 第4页
desirable as long as the condensing temperature does not fall below the chiller manufacturer’s recommendations.
Since most modern towers use two- or three-speed fans, a near optimal control scheme can be developed as follows (Braun and Diderrich, 1990):
• Tower fans should be sequenced to maintain a constant approach during part load operation to minimize chiller/cooling tower energy use.
• The product of range and condensing water flow rate, or the heat energy rejected, should be used to determine the sequencing of the tower fans.
• Develop a simple relationship between tower capacity and tower fan sequencing.
De Saulles and Pearson (1997) found that savings for a setpoint control versus the near optimal control for a cooling tower were very similar. Their control scheme called for the tower to produce water at the lowest setpoint possible, but not less than the chiller manufacturer would allow, and to compare that operation to the savings obtained using near optimal control as described above. They found that the level of savings that could be achieved was dependent on the load profile and the method of optimization. Their simulations showed 2.5 to 6.5% energy savings for the single setpoint method while the near optimal control yielded savings of 3 to 8%. Use of variable speed fans would increase the savings only in most tower installations. It is more economical to operate multiple cooling tower fans at the same speed than to operate one at maximum before starting the next fan. Variable speed fans should be used when possible in cooling towers.
The system designer should ensure that any newly installed cooling tower is tested according to ASME Standard PTC 23 (ASME 1986) or CTI Standard ATC-105. These field tests ensure that the tower is performing as designed and can meet the heat rejection requirements for the connected chiller or refrigeration load.
Selection Criteria
The criteria listed in Table 4.2.6 are usually known a priori by the designer. If not known explicitly, then commonly accepted values can be used. These criteria are used to determine the tower capacity needed to reject the heat load at design conditions. Other considerations besides the tower’s capacity include economics, servicing, environmental considerations, and aesthetics. Many of these factors are interrelated, but, if possible, they should all be evaluated when selecting a particular tower design.
Because economics is an important part of the selection process, two methods are commonly used — life-cycle costing and payback analysis. These procedures compare equipment on the basis of owning, operation, and maintenance costs. Other criteria can also affect final selection of a cooling tower design: building codes, structural considerations, serviceability, availability of qualified service personnel, and operational flexibility for changing loads. In addition, noise from towers can become a sensitive environmental issue. If local building code sound limits are an issue, sound attenuators at the air intakes and the tower fan exit should be considered. Aesthetics can be a problem with modern architectural buildings or on sites with limited land space. Several tower manufacturers can erect custom units that can completely mask the cooling tower and its operation.
Applications[1]
Unlike chillers, pumps, and air handlers, the cooling tower must be installed in an open space with careful consideration of factors that might cause recirculation (recapture of a portion of warm and humid exhaust air by the same tower) or restrict air flow. A poor tower siting situation might lead to recirculation, a problem not restricted to wet cooling towers. Similar recirculation can occur with air-cooled condensing equipment as well. With cooling tower recirculation, performance is adversely affected by the increase in entering wet-bulb temperature. The primary causes of recirculation are poor siting of the tower adjacent to structures, inadequate exhaust air velocity, or insufficient separation between the exhaust and intake of the tower.
Multiple tower installations are susceptible to interference — when the exhaust air from one tower is drawn into a tower located downwind. Symptoms similar to the recirculation phenomenon then plague the downwind tower. For recirculation, interference, or physically blocking air-flow to the tower the result is larger approach and range which contribute to higher condensing pressure at the chiller. Both recirculation and interference can be avoided through careful planning and layout.
Another important consideration when siting a cooling tower installation is the effect of
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