Direct injection of water from a cooling tower into a closed chilled water loop has been used to reduce refrigeration chiller operation, save energy and reduce costs. This practice has led to severe corrosion and fouling problems in the I-WAC systems of many commercial buildings. The applicable mechanisms are microbiological fouling, microbiologically influenced corrosion (MIC) and suspended solids deposition. This paper discusses the effects of direct free cooling condenser water operations on corrosion and microbiological control in condenser and chilled water systems, and describes some case histories of problems that have resulted from free cooling operations. Suggestions are offered for operating practices to minimize these problems.
INTRODUCTION
In a typical commercial HVAC design chilled water is generated in the evaporator section of a refrigeration unit and is circulated around the facility in a closed loop. Heat from the refrigeration unit is exhausted to a cooling tower via the condenser section. Under certain ambient conditions, the cool water from the cooling tower has a low enough temperature to satisfy building chilled water needs. As an approach to energy reduction, operation of a refrigeration unit can be avoided by using cooling tower discharge as the source of chilled water. This can be done either indirectly, by use of a heat exchanger, or directly by injection of the cooling tower water into the chilled water loop.
Cooling tower systems are open to the air and operate under wnditions of continuous water loss (evaporation and blowdown) and replacement (makeup). These systems are contaminated to some extent with bacteria and suspended material scrubbed from the air. Typical chilled water loops, on the other hand, are closed systems with very little intentional water loss and makeup. These systems are treated with high concentrations of corrosion inhibitors and usually remain relatively free of bacterial contamination.
The direct injection of cooling tower water into a closed chilled water loop has led to significant problems in many facilities. Under the appropriate ambient conditions, water from the cooling tower loop is circulated through the chilled water loop. Once the cooling tower cannot meet chilled water temperature needs, the chilled water loop is isolated from the cooling tower loop and the refrigeration chiller is operated. Under these conditions, the chilled water loop is again closed but is filled with condenser water containing bacteria, suspended solids and low inhibitor levels. These represent poor water quality conditions for a closed loop. Corrosion rates can be high, microbiological growth can be rapid and suspended solids can accumulate as deposits in the system.
DESCRIPTION OF SYSTEMS
A conventional HVAC system design consists of completely separate condenser water and chilled water loops as shown in Figure 1. In this system design, there is no mixing between the condenser and chilled water loops.
Conventional System; Separate Condenser and Chilled Loops
A direct injection system is shown in Figure 2. If the ambient temperature is low enough (e.g. less than 10°C; 5OT), the chiller is turned off and water from the cold water basin of the cooling tower is diverted directly into the chilled water loop…
Keywords: materials and corrosion, Pipeline Corrosion, Production Chemistry, flowline corrosion, oilfield chemistry, riser corrosion, water system, Upstream Oil & Gas, free cooling system, operation
Subjects: Production Chemistry, Metallurgy and Biology, Pipelines, Flowlines and Risers, Materials and corrosion, Well Integrity, Subsurface corrosion (tubing, casing, completion equipment, conductor)
1998. NACE International