Applying the dry bulb economizer - -*-- - " ---a- " -------- "-

BY WILLIAM J. COAD temperature (75 F). As fuel costs -. z - escalated faster than electric ener-

The dry bulb economizer is an gy costs, the economizer often be- energy conservation technique that came a cost burden rather than has been used effectively for many "free." This feature, coupled with years. I t is one of the few energy the fact that there is a sizable in- conservation schemes tha t pre- vestment associated with the econ- vailed even during those years of omizer (hardware, controls, and re- underpriced energy, from the mid- al estate), tended to make many de- 50s through the mid-70s. Recent signers elect to move in the direc- - LJ

analyses have shown, however, that tion of year-round mechanical cool- it is time to update our perspective ing (in some cases disguised as a on the use of the air economizer. heat pump).

For purposes of this discussion, To compare the operating and in- the dry bulb air economizer is as- vestment costs of a heat pump sumed to be that component of an against those of an economizer that air handling system consisting of "heats" from mixed air temperature outdoor air, return air, and exhaust is to use the latter as a straw man. air dampers that position to mix An economizer cycle system that outdoor air with r e tu rn air t o does not provide mixed air to the achieve a mixed temperature equal hot deck heating coil should be con- to a desired cooling supply air tem- sidered and used as the basis of perature, say 55 F, when the out- comparison. door air is below that temperature. A second problem t h a t h a s

The concept first came under some criticism and its value was questioned in the early days of com- puterized annual energy analyses and associated operating cost stud- ies. As the use of the economizer developed, it was employed with virtually all types of terminal or psychrometric control systems. This, combined with the fact that dual stream (double duct and mul- tizone) systems had emerged as the I commonest type of multiple zone controlled systems, provided some rather startling results in the com- puterized studies. In many cases, a system with an economizer cost more to operate than one without an economizer. The reason was sim- ply that all the air that was heated in the hot deck, either for control heat or basic space heat, was heated from the mixed temperature (55 F) rather than from the return air

evolved concerning the dry bulb economizer has been around for some time and does not relate to energy but to performance (al- though in many cases the perfor- mance problems were solved by the consumption of sizable quantities of energy). This problem relates to the nature of the loads served by a central chilled water plant and a fundamental difference between the operating concepts of the sys- tem designers and the system oper- ating staff.

The intent of the system designer is, generally, that when the outdoor air is a t or below 55 F, the chilled water system would be turned off, and cooling would be provided by outdoor air. When the outdoor air is above 55 F , the chilled water plant would provide the cooling. The first problem is that if, indeed, 55 Fair is

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On this page each m o n t h , t h e author shares his engineering philosophy by ex- ploring a wide variety of topics, ranging from fundamentals t o new frontiers, as they relate to building environmental systems. Mr. Coad is vice president of Charles J. R. McClure & Associates and affiliate pro- fessor of mechanical engineering at Wash- ington University, S t . Louis, Mo.

HeatingIPipinglAir Conditioning October 1982

continued from page 111 ample of a typical temperate cli- that the chilled water system would needed, t h e changeover po in t mate location, this 5 F difference be operated when the outdoor air would normally be closer to 50 F adds 259 hours of chiller operation temperature exceeded 55 F, this

mid-autumn (say, November 1) and turned i t on in mid-spring (say,

well justified in most cases. To start up and to bring these chiller plants down to temperature on a daily or hourly basis is costly and requires considerable planning and man- power. Again with St. Louis as an example, there are statistically 637 hr per year above 50 F between No- vember 1 and March 31. In most commercia l a n d in s t i t u t iona l buildings, not having the plant available creates no noticeable defi- ciency because the load time con- stant is such that the spaces could "spin through" the higher tempera- ture periods. In those spaces with more dynamic loads, the space tem- perature simply climbs, often to the point of discomfort; but few people complain about occasional exces- sive warmth for short periods in the

Serious problems have arisen, however, in cases in which there are temperature critical loads, such as electronic data processing machin- ery; work intensive spaces, such as operating rooms and athletic areas; or sensible gains high enough to provide excessive normal occupant discomfort prior to the end of the "spin through" period. In these cases, operators have found that the chilled water must be made available to provide even a minimal acceptable level of performance.

In most such cases, the tempera- ture sensitive spaces represent rela- tively small percentages of much larger systems. The operators have thus, in the interest of energy con- servation, started up the chilled wa- ter plants to serve those limited loads, leaving the larger portion of

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HeatinglPipingIAir Conditioning a October 1982

continued from page 114 these observations is that to achieve tem in such a way that all of the the connected system on econo- successful design with dry bulb air loads can be served satisfactorally mizer cycle operation. This solu- economizer systems, the design with minimum consumption. tion has created additional prob- practitioners must: The ways of accomplishing the

Configure air handling sys- last item above differ with the rela- The fixed parasitic plant bur- tems in such a way that heat or re- tive magnitudes of the critical and

dens, such as chilled water pumps, heat is never added after the out- noncritical loads. Most cooling condenser water pumps, and cool- door air is mixed with return air, loads are noncritical, and thus in ing tower fans, make the specific Provide specially designed most cases, the size of the critical power (KW per ton) quite high. tight closeoff dampers for the re- loads is small compared to the total

The chillers cannot be oper- turn air dampers in the economizer loads. In these cases, the most log- ated a t the extremely low loads im- mixing box. ical design option is first to supple-

* Minimize fan pressure rise or ment the chilled water system with Since performance, in this case, design for increased air flow (an op- an interconnected small system a t

is the paramount problem, the lat- timization problem among pres- or near t he critical loads. T h e ter problem has been solved in a sure rise, air flow, fan horsepower, smaller or supplementary systems number of ways. I t is not uncom- and cost of lower changeover tem- can usually be economically air mon to see a modification to a large perature). cooled because they only operate chiller consisting of a hot gas by- * Identify and segregate tern- during lower ambient times. The pass system to allow it to serve perature critical loads that cannot second approach is to install a small these low loads. In such cases, the "spin through" high dry bulb tem- chiller, sized to serve only the crit- energy consumed by the chiller perature periods and those that are ical winter loads, in the central generally exceeds that required for not temperature critical such that plant if the distribution system is the load by several hundred per- they can survive a n occasional arranged to operate on the reduced

modest space temperature rise. capacity winter "system" without

HeatinglPipingIAir Conditioning October 1982