ASHRAE Terminology

represent the accumulation of dissolved minerals in the recirculating cooling water. Draw-off (or blowdown) is used principally to control the buildup of these minerals. Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems. As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals has been exceeded, they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping, and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. The use of water treatment chemicals, pretreatment such as water softening and pH adjustment and other techniques can affect the acceptable range of cycles of concentration. Concentration cycles in the majority of cooling towers usually range from 3 to 7. The water may also be filtered and dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water. In closed-loop systems, corrosion inhibitors may be used. 1. in boilers, the ratio of chlorides in the boiler water to the chlorides in the feedwater. 2. in cooling tower operation, the ratio of chlorides in the recirculating cooling tower water to the chlorides in the makeup water.

A demand response activity by which the program administrator remotely shuts down or cycles a customer's electrical equipment (e.g., air conditioner, water heater). Direct load control programs are primarily offered to residential or small commercial retail customers.

for devices whose renewal mechanism is designed to restore the performance characteristics of the device to starting conditions, dust -holding capacity per cycle is determined by feeding dust until a steady-state condition is achieved. The dust-holding capacity per cycle is averaged over at least four cycles. The result is expressed in g/cycle to the nearest gram.

for disposable and manually renewable devices, this is dust-holding capacity divided by net effective filtering area. For self-renewable devices, dust feed is continued until the steady-state condition is achieved in several cycles. Dust-holding capacity per unit area is then the average arrestance multiplied by the dust fed during the steady-state period divided by the area of media consumed during the same period. Dust-holding capacity per unit area is expressed in g/m2 (g/ft2).

property of a material which permits it to be alternately frozen and thawed through many cycles without damage.

(1) number of cycles per second through which an alternating electric current passes, in North America, frequency is generally standardized at 60 cycles per second (60 Hz). Most other countries standardize using 50 Hz. (2) the number of times a quantity (such as a sound wave) repeats itself in one second.

pressure-responsive device that cycles and/or stages condensers, cooling-tower fans, and pumps to control head pressure.

(1) output, expressed as a function of time, resulting from the application of specified input. (2) time (preferably in seconds, may also be in cycles of supply frequency) required for the output quantity to change by some agreed-on percentage of the differential output quantity in response to a step change input. Note: in measurement, the initial and final output quantities shall correspond to the test-output quantities. The response time shall be the maximum obtained including differences arising from increasing or decreasing output quantity or time phase of signal application. (3) time for a measuring sensor to reach 90% of the final value after a step change. For a measuring system that includes only one exponential time constant function, the 90% response time equals 2.3 times the time constant.

a control that cycles between two states, typically between on and off (maximum and minimum).

a modulating control that cycles a burner between the reduced input rate and OFF if the heating load is light. If a higher heating load is encountered that cannot be met with the reduced input rate, the control goes into a modulating mode where it either gradually or incrementally increases the input rate to meet the higher heating load. If, at that point, a lower heating load is encountered, the control either gradually or incrementally decreases to the reduced input rate.

in cold air or gas refrigeration cycles, a turbine in which the compressed gas expands and produces mechanical energy.

a modulating control that both cycles a controlling device between two preset conditions, could be between OPEN and CLOSED, or between ON and OFF, or between two stages or levels of capacity control.

blend comprising multiple components of different volatilities that, when used in refrigeration cycles, change volumetric composition and saturation temperatures as they evaporate (boil) or condense at constant pressure.

refers to blends comprising multiple components of different volatility that, when used in refrigeration cycles, change volumetric composition and saturation temperatures as they evaporate (boil) or condense at constant pressure. The word is derived from the Greek words zein (to boil) and tropos (to change).