COWARD ENVIRONMENTAL SYSTEMS, INC.
500 Birchfield Drive, Suite 501, Mt. Laurel, NJ 08054
856-727-4400 -fax 856-727-4344
e-mail: cesi@cowardenvironmental.comEvaporator Temperature Control
When compared with chilled water systems, direct expansion systems have historically taken a back seat, a fact attributable to inadequate control. Chilled water is readily modulated while DX has cycled "on and off".
When a thermostat is used to open and close the liquid line solenoid valve, the space conditions vary accordingly, and a "saw tooth" control is inevitable. Compressors cannot be short cycled (they must be off for perhaps three minutes). During that off time, warm and dehumidified air is delivered to the occupied space. At least in the systems where there is a substantial quantity of outdoor air, the space is fed with raw air, the temperature and moisture content of which depend upon the vagaries of weather.
There really is no need to abandon the use of direct expansion systems for this control reason only. The answer lies in the use of "evaporator temperature modulation" devices in lieu of liquid line solenoid valve cycling. Modern day "compensated evaporator pressure regulating valves" are reliable, easy to install and perform space control functions at least equal to chilled water modulation.
When one realizes the advantage of the elimination of the added heat exchange that the chiller represents, the properly controlled direct expansion system becomes attractive and should be considered. Our experience in applying dozens of these systems allows us to offer technical help in their selection and design.
by: Kenneth W. Wicks
ASHRAE Fellow
7-99
