PH-2528 (RP-106) -- Scaling Characteristics of Cooling Tower Water

Deposition of undesirable materials occurs frequently on heat exchanger surfaces, resulting in an increase in the overall resistance to heat transmission. Technically speaking, the heat exchanger is fouled. Potential foulants may be crystalline salts, suspended solids, corrosion products, microorganisms, or a combination of these, depending on the deposition mechanism that is controlling. The most common deposit from cooling tower water is inverse solubility salts such as calcium carbonate, magnesium hydroxide, etc. The rate of deposition is a complex function of the temperature of the heated surface, flow velocity, water quality and surface conditions. Water quality is generally described by the various constituents in the cooling water, such as calcium hardness, magnesium hardness, alkalinity, silica, pH and bulk temperature.Different aspects of the deposition process and its characterization are described well by Taborek, et al. (16). A good review of the effects of variables is provided by Suitor, et al. (15).

Due to the complexity of the deposition process, a generalized predictive correlation for the design of heat transfer equipment has not been established. It was the decision of TC 8.5 to begin this study of fouling by focusing on surface temperature as it effects the deposition of salts from cooling tower water. The effects of changes in water quality were also observed. The lower range of surface temperature that was investigated, the bulk temperature, heat fluxes and velocity corresponded to those normally found in the condensers of air-conditioning units.

In the design of condensers for air-conditioning systems, a minimum waterside fouling factor of 0.00025 hr-ft-F/Btu is added to that assumed during rating tests (ARI Standard 550-77). This standard also permits the assumption that a fouling factor of 0.00025 hr-ft-F/Btu exists during the initial rating tests. The basis of this value is not documented nor are the conditions for which it applies. This prompted TC S.5 (Liquid-to-Refrigerant Heat Exchangers) to begin a program of systematic evaluation of condenser water fouling as applied to the air-conditioning and refrigeration industry. The study was initiated by a seminar on this topic at the 1975 Winter meeting in Atlantic City and a publication resulted from this seminar (13). A forum on this topic was held at the Seattle meeting in 1976. The present experimental program GRP-l06 began in July, 1976. This paper is a report on the first half of this study.

Citation: ASHRAE Transactions, Volume 85, Part 1, Philadelphia, PA



Product Code(s): D-PH-2528