SECTION V SPECIALIZED TOWER USAGE AND MODIFICATIONS
K. Free Cooling
All air conditioning systems, and many process-es, require much colder water than a cooling tower is capable of producing at summertime design at-mospheric conditions. In those cases, a chiller of some type is designed into the cooling water circuit to provide water at an acceptably depressed tem-perature.
Shown in Figure 124 are the typical water cir-cuits for a tower-chiller-load combination. In the diagrammed case, the load is assumed to be air conditioning. Note that the chilled water pump (CHWP) circuit delivers water to the “load” at a temperature unachievable by the cooling tower under summertime conditions. The chilled water, somewhat elevated in temperature, then returns to the chiller where its load is transferred to the con-denser water pump (CWP) circuit.
Note also that the load delivered to the condens-er watcondens-er circuit and cooling towcondens-er is somewhat higher than the actual load imposed by the air con-ditioning system or process. This is because of the added work necessary to achieve the chilling process. In the case of the refrigerant compression system shown, the load is increased by a factor of 1.25 (15000/12000 Btu/hr/ton), representing the added heat of compression. If the chiller were of the absorption type, the load would be increased by a factor of 2.5, representing the added heat of steam condensing.
The temperatures to and from the load, indi-cated for the chilled water circuit, are typical of those required in order to achieve both cooling and dehumidiication in an air conditioning system operating in the summertime. Cold water tempera-tures required for a process are normally expected to be somewhat higher, more on the order of 60°F to 75°F. The required cold water temperature level takes on signiicant importance in the proper appli-cation of a tower on a “free cooling” cycle.
The opportunity for free cooling begins to occur in the fall of the year, carrying through to spring. It occurs because of the normal reduction in a cooling tower’s cold water temperature brought about by a depression in wet-bulb temperature and/or load.
(Fig. 25 or 107) In most localities, there will come a period during the course of the year when the ambient wet-bulb temperature will have suficiently depressed to permit the cooling tower to produce a cold water temperature previously achievable only by the chiller. With a properly arranged system, the cooling tower water can then be re-circuited to di-rectly serve the load; thereby obviating the use of the most energy-intensive piece of equipment in the cooling water system — the chiller.
Several arrangements by which free cooling can be accomplished are possible, all of which are well documented in manufacturers’ literature. The two most basic methods will be discussed herein, as follows:
Figure 124 — Schematic of water circuitry in typical air conditioning system.
1. Direct Free Cooling: By adding valved by-passes and interconnecting piping to the basic system indicated in Figure 124, the system shown in Figure 125 is achieved. Water from the cooling tower lows directly to the load and back to the tower, by-passing the chiller completely. Several notable points, which may require some explana-tion, are apparent in this diagram:
a. Since the previous two water circuits are now common, one of the circulating water pumps must be by-passed. Although good energy management would seem to suggest use of the typically lower power chilled water pump, proper winter operation of the cooling tower (Sect. I-H) dictates that the higher low capability of the condenser water pump be utilized.
b. The water temperature going to the load has increased considerably becoming more in line with the temperature that might be expected for a process. This is because off-season air conditioning loads include drastically less de-humidiication than is required in summertime.
Furthermore, the higher temperature water
will easily accommodate any residual cooling load, and will do so at a temperature much more compatible with the heating required in many zones of the building.
c. The water temperature rise across the load has reduced. This happens for two basic rea-sons: 1) The cooling load will have reduced die to seasonal factors, and 2) the increased quantity of water delivered to the load by virtue of utilizing the condenser water pump permits less temperature rise. (In a process applica-tion, the level of relative water temperatures might be expected to remain somewhat more constant year-round.)
d. The percent of total load normally contributed by the chiller (compressor) is no longer im-posed upon the tower.
e. The use of some means of iltration is sug-gested. This is because the raw cooling tower water circuit is now in a position to “contami-nate” the relatively “clean” chilled water circuit.
Because of its ability to defeat contamination at the cooling tower basin, which is the prima-ry source, by-pass iltration is recommended.
(Sect. VI-E)
Figure 125 — Schematic of water circuitry in “direct” free cooling system.
Figure 126 — Schematic of water circuitry in “indirect” free cooling system utilizing plate-type heat exchanger.
f. To prevent basin freezing during periods of winter shutdown, some means of basin heat-ing is suggested. (Sect. VI-D-5)
g. Either two-speed cooling tower fan motors or AVP fans are recommended. Not only does this assist in the prevention of freezing during wintertime operation (Sect. I-F-1), but it also affords a means of improving energy use and controlling cold water temperatures (Sect.
V-F) as ambient reduces.
2. Indirect Free Cooling: By the inclusion of a simple plate-type heat exchanger, free cooling can be accomplished with total separation of the two water circuits (Fig. 126), which precludes
the water quality control problems inherent in a direct-connected system and is seen by most operators as a distinct advantage. Offsetting this is the disadvantage of fewer available hours of operation on the free cooling cycle during the course of a year. This is because of the need for a reasonable temperature differential between the incoming cooling tower water and the leaving
“chilled” water at the heat exchanger. Since the cooling tower must produce colder water, it must wait for a further-reduced wet-bulb temperature, and the time interval can sometimes represent a signiicant number of operating hours.