Open Cooling Tower Forced Draft and Induced Draft Equipment Layout Guide COMPRESSED SYSTEM COMPONENTS INGERSOLL-
TABLE OF CONTENTS Introduction.2 Forced Draft Unit Layout Single Units...3-5 Multiple Units and Large Installations...5-6 Induced Draft Unit Layout Single Units...7-8 Multiple Units and Large Installations..8-9-10 Expansion to Existing Systems 10 Other Layout Criteria.11 Summary.11 INTRODUCTION The location of a cooling tower is an important consideration when reviewing system design. Evaporative cooling equipment requires large quantities of air. Adequate spacing around a unit must be provided if it is to perform properly. This technical manual was written by engineers to provide recommended layout criteria for typical equipment installations. The spacing requirements given are considered to be minimum distances and should be increased when the installation allows. Air Recirculation Recirculation occurs when some of the hot moist discharge air leaving the cooler flows back into the fresh air inlet. It can be just a small portion of the discharge air or, in severe cases, large amount of leaving air. The heat-laden discharge air is saturated and typically at 20º F or so higher wet bulb temperature than the ambient wet bulb. Therefore, any recirculation increases the entering wet bulb temperature, and this, of course, reduces the capacity of the cooler. A small increase in the entering wet bulb temperature means a significant reduction in capacity. For example, an increase in inlet wet bulb temperature of only 2º F, say from 78º F to 80º F, will reduce the cooler capacity 16%. This means an increase in the leaving water temperature of 1.5º F. We have seen recirculation cause 5º to 6º increases and even more in the inlet wet bulb temperature. In these cases capacity is reduced over 50% with subsequent increases in the leaving water temperature of 4.5º or more. What to do about it? How do you solve or prevent this potentially serious problem? There is not one simple answer that will always work; but by following a few basic location guidelines before installing the cooler, the problem of recirculation can be avoided. The objective is to select or design the best layout or location to provide the required amount of fresh air at the lowest possible entering wet bulb temperature. 2
FORCED DRAFT UNIT LAYOUT Single Unit Installations The best place for a cooler is on a roof by itself. However, when this is not possible, the layout guidelines must be followed to provide a satisfactory installation. The first item to consider is the position of the unit with respect to other structures. The top of the cooler must be higher than any adjacent walls, buildings or other structures. When the top of the unit is lower than the surrounding structures, (Figure 1), recirculation can be a major problem. If the unit is on the windward side as shown in Figure 1, the discharge air will be forced against the building and then spread in all directions, including downward, toward the fan inlets. There are two simple methods to correct this problem. The first is to elevate the unit on structural steel so that the top is higher than the wall, as shown in Figure 3. When the wind comes from the opposite direction, the resulting negative pressure area created by the wind passing over the building will cause the discharge air to be forced back into the inlets per Figure 2. Even if neither of these conditions occurs, sometimes the presence of much taller structures can inhibit the dissipation of the hot moist discharge air. 3
The second is to install a tapered discharge hood (Figure 4) which discharges above the height of the structure. The discharge hood offers the added advantage of increasing the discharge air velocity, which works to minimize the possibility of recirculation even more. However, the addition of a discharge hood normally requires the next larger size fan motor. from above. This downward draw is why it is so important to provide the correct D1 dimension in order to prevent recirculation. When a cooler is selected with air inlets on two sides, care must be taken to analyze each air inlet side independently. For example, with a cooler which measures 8 feet wide from air inlet to air inlet, use Table 1 to determine the minimum distance D1 between one air inlet side and its facing wall. Repeat the procedure for the opposite fan side when appropriate. The distances for D1 in the Tables have been developed using a formula based on years of successful experience that assumes all the air is fed in from the ends at less than 600 F.P.M. As can be seen, elevating a unit on structural steel will allow the clearance to be reduced. This is because the end area is effectively increased by the amount the unit is elevated. When a cooler is located near a wall, it is best for the air inlet to face away from the wall as shown in Figure 5. If this is not possible and air inlets must face the wall as per Figure 6, then the distance D1 between the wall and the unit should be in accordance with either Table 1 or 2. Table 1 covers smaller coolers which are defined as units up to 5 feet wide with air inlets on one side and up to 8 feet wide with air inlet on two sides. Table 2 covers larger units up to 10 feet wide. For instance, a large unit 12 feet long mounted at grade must be at least 7 feet away from the wall. The fans will draw air in through the space between the unit and the wall and also down 4
If the standard D1 distance shown in the Tables is too great for the available space, the use of tapered discharge hood (Figure 7) can reduce the distance. The tapered discharge hood should be at least 3 feet tall with an exit air velocity of 1200 to 1500 F.P.M. The use of a discharge hood will allow the distances given in Tables 1 and 2 to be reduced by 20%. However, the minimum distance can never be less than: Small Units: Up to 5 Feet Wide Single Fan Sided = 4 Feet Up to 8 Feet Wide Double Fan Sided = 4 Feet (Each Side) Large Units: Up to 10 Feet Wide Single Fan Sided = 6 Feet Sometime other pieces of equipment such as receivers, compressors, piping, etc. are placed in front of the fan inlet. These should not be any closer than the above minimum dimensions. Closer placement can possibly create imbalances in the air flow which has an adverse affect on the fan performance. In installation of three or more coolers where it is necessary for the fan inlets of two units to face each other (Figure 11), then the minimum distance D2 between fan inlets must be per Tables 3 or 4 (next page). Multiple Unit & Large Installations When more than one cooler is installed at the same location, recirculation becomes a bigger problem. The larger the size of the installation, the greater the potential for recirculation simply because of the larger quantities of air being handled. The following guidelines, however, will provide for satisfactory and efficient operation. When dealing with two unit installations, the coolers should be placed either back-to-back as shown in Figure 8 (the preferred position) or endto-end as shown in Figures 9 and 10. The only difference in these latter two layouts is the additional space required when connection ends face each other. 5
Table 3 covers small units up to 5 feet wide with air inlets on one side and units up to 8 feet wide with air inlets on two sides. When coolers with air inlets on two sides are selected, both fan inlets must be considered. Table 4 is for larger units up to 10 feet wide with air inlets on one side. These tables are based on formulas which assume all the air flows to the units from the ends at velocities of less than 600 F.P.M. This criteria has been proven through years of successful experience with evaporative cooling installations. increase is dependent on the number of units, type of installation and various other factors. The surrounding area for instance, plays an important part in the design of a large installation. Locating a large installation in a valley, low area or between buildings will increase the chances that the discharge air will not dissipate and thereby raise the local wet bulb temperatures. Another criterion to consider that is particularly important when dealing with larger multiple unit installations is prevailing winds. Prevailing wind conditions generally change with the season, so the concern is with the wind direction during the hottest part of the year. It is better to locate the cooler so that the air inlets are nearly perpendicular to the prevailing wind direction (Figure 12). It is not always possible to exactly achieve this. The object is to orient the units to keep the wind from blowing any of the discharge air into the fan inlets. If there is not enough room to meet the minimums given in the Tables, then the use of tapered discharge hoods may provide a good solution. These hoods should be designed as previously described, i.e. minimum of 3 feet tall with an exit air velocity of 1200 to 1500 F.P.M. The distances in the tables can be reduced 20%, however, the spacing between fan inlets even with discharge hoods, cannot be less than the minimums below: Small Units: Up to 5 Feet Wide Single Fan Sided = 6 Feet Up to 8 Feet Wide Double Fan Sided = 6 Feet (Each Side) Large Units: Up to 10 Feet Wide Single Fan Sided = 10 Feet Very large multiple unit installations can create their own environment. Under certain weather and atmospheric conditions the large quantities of discharge air will cause the wet bulb temperature in the immediate area to be higher than local design data. It is important for best operation in this type of installation that the minimum distances between units, especially those where fan inlets face each other, be increased beyond those dimensions shown in the tables. The amount of 6
INDUCED DRAFT UNIT LAYOUT Single Unit Installations The best place to locate any cooler is on a roof by itself. However, when this is not possible, correct layout guidelines must be followed to provide a satisfactory installation. The first item to consider is the position of the unit with respect to other structures. The top of the cooler must be equal to or higher than any adjacent walls, buildings or other structures. When the top of the unit is lower than the surrounding structures (Figure 1), recirculation can be a major problem. If the units on the windward side, as shown in Figure 1, the discharge air will be forced against the building and then spread in all directions, including downward, toward the air inlets. The condition shown in Figures 1 and 2 can be corrected by elevating the unit on structural steel so that the top is higher than the adjacent structures, as shown in Figure 3. Fan cowl extensions can also be provided to elevate the fan discharge of the cooler to the proper height, as shown in Figure 4. When the wind comes from the opposite direction, the resulting negative pressure area created by the wind passing over the building will cause the discharge air to be forced back into the inlets, as shown in Figure 2. Even if neither of these conditions occurs, the presence of much taller structures can potentially inhibit the dissipation of the hot moist discharge air. 7
Single / Multiple Unit Installations Induced draft, counterflow design units may have air inlets located on all four sides of the unit. When it is located near a wall or other structure that blocks fresh air from entering the unit, consideration must be given to the clearance distance between the air inlets of the unit and this blockage. In this type of layout, air will be drawn in through the space between the unit and the wall or other structure as well as down from above. Therefore. It is important to provide adequate space in front of each air inlet to insure proper air flow and prevent air recirculation. When more than one induced draft counterflow unit is installed at the same location, the potential for recirculation becomes a greater concern. For installations with two or more coolers, the units may be placed in a variety of locations depending on the site conditions and available space. Recommended distances have been developed for various cases of induced draft counterflow layout. These distances have been developed to ensure that the units are provided with adequate airflow and that recirculation is minimized. Space must also be provided for piping, removal of access panel and for maintenance of the mechanical equipment. Therefore, the data presented in Tables 1 and 2 show the minimum dimensions D1 through D8 required for a variety of installation cases. See the following figures that illustrate these various cases. 8
9
Either orientation will operate efficiently given the proper space between units. However, when units are installed with connection ends facing each other, the minimum dimension between units should not be less than 4 feet to accommodate necessary piping. In cases where external motors are located between two belt drive model units, the minimum distance between units should be increased to 6 feet to provide for motor accessibility. Very large multiple unit installations can create their own environment. Under certain weather and atmospheric conditions, the large quantities of discharge air will cause the wet bulb temperature in the immediate area to be higher than ambient. It is important for best operation that the minimum distances between units be increased beyond those dimensions shown in Tables 6 and 7. The amount of increase is dependent on the number of units, type of installation and various other factors. The surrounding area, for instance, plays an important part in the design of a large installation. Locating a large installation in a valley, low area or between buildings will increase the chances that the discharge air will not dissipate and thereby raise the ambient wet bulb temperature. Another criterion to consider, that is particularly important when dealing with larger multiple unit installations is prevailing winds. Prevailing wind conditions generally change with the season, so the concern is with the wind direction during the hottest part of the year. It is better to locate the cooler so that the prevailing wind is oriented as shown in Figure 28. Expansions to Existing Systems Expansions or additions to existing systems present the same type of problems as multiple unit installations plus some additional ones. Since in an expansion the new cooler may not be identical to the old one, it is important to examine the height of the new and old units. Whenever possible, the tops of all units should be at the same level to avoid recirculation from one unit to another. If the heights are different, structural steel should be used to get air discharges at the same level, as in Figure 31, or the units should be spaced farther apart than normally recommended. 10
OTHER LAYOUT CRITERIA So far, in our discussion of locating cooler our concern has been to provide adequate fresh air without recirculation. However, there are three other criteria which will be discussed briefly which also must be considered in determining the final layout. They are access for maintenance, noise, and certain piping requirements. Provide Space for Maintenance When a cooler is located in close proximity to anything, there are minimum clearances required for periodic maintenance. Proper access must be provided for: 1) Adjustment and replacement of drive belts. 2) Lubrication of motors and bearings. 3) Cleaning of water distribution system. 4) Access to the water sump for cleaning and pump maintenance. Minimum dimensions for service are shown in Figures 32 and 33 and apply to all installations, single units, multiple units, etc. A cooler which is located so that periodic routine maintenance can be accomplished easily will get proper care. A unit that is hard to service will not get timely and correct maintenance which will reduce its performance and life. Also, in addition to the periodic maintenance items, unit drawings must be reviewed to ensure there is room for any future major repair work. This might include fan motor or pump replacement, fan replacement, or fan shaft replacement. Noise and Layout Another consideration is the orientation of the unit with respect to any neighbors or offices. When a unit is located in a residential area or possibly close to general offices, noise should be considered. If noise is a concern, there are several solutions. 1) Face fans away from potential problem area. 2) Avoid amphitheatre type locations. 3) Provide natural sound barriers such as trees, shrubs or a small hill between unit and neighbors. 4) Consult manufacturer for possible alternate selection of a quieter unit. 5) In extreme cases, install a wall-type sound barrier or have manufacturer provide a factory-built attenuation package. SUMMARY To produce full capacity, a cooler requires large quantities of air at the lowest possible entering wet bulb temperature. In this technical manual we have tried to provide the necessary guidelines to insure this goal is obtained, thus preventing the capacity robbing effects of recirculation of the hot discharge air. No installation will be 100% free, 100% of the time from some small amounts of recirculation; but following these recommendations will provide installations with practical layouts, and trouble-free operation. 11