Solution to the Loss of Efficiency and Productivity in Closed Water Circuits
For Air Conditioning and Industrial Refrigeration Systems
Fouling – Dirt Sediments
As buildings get older, loss of efficiency and output worsens in closed water systems, both in the cold water and condensing water cycles of industrial cooling and air conditioning systems.
Sludge that is caused by corrosion and bacterial activity blackens the water and causes sediments to develop on all parts of the system. The sediments stick to all sections of the cooling system, clogging it and drastically decreasing its energy efficiency. This phenomenon is known in industry circles as fouling.
How Fouling Affects Productivity and Efficiency
Fouling is very common in both the Cooler (Evaporator) and Condenser of air conditioning systems. It causes a decrease in their heat transfer coefficient. The graph below shows the effects of pipe sediment thickness on the unit’s heat transfer coefficient:
In addition to sediments, the water contains germs and bacteria, such as Legionella, that endanger the system operators and maintenance teams who clean the filters and fix cracks and leaks. Since these water systems are almost completely closed off, they are barely visible and offer extremely limited access, so maintenance personnel cannot identify effectively the reasons for the drop of cooling capacity over time. Furthermore, unlike condensers that can be opened and cleaned relatively easily, detailed cleaning of closed systems is practically impossible. and the common cleaning method is the stirring of chemicals. Fouling is unstable and it becomes greater over time. Partial cleaning does not solve the problem and simply provides an even better platform for additional sediment to stick to and become thicker.
Closed Water Circuit Components Affected by the Loss of Efficiency
The loss of efficiency in the closed water circuits affects the following components:
- Chiller (evaporator) tubes
- Fan-Coil and filters
- Main pipes and control equipment
- Closed condensing systems
The following pages describe the problems identified in these components:
Loss of Output in the Evaporator Tubes
The Evaporator Tubes are cooled by the Refrigerant Gas that runs via the shell side of the Evaporator. The layer that develops as a result of the sludge, corrosion and bacteria on the internal walls of the tubes serves as insulation that hinders heat transfer. As the sediments thicken the temperature of the vaporization drops, which results in a decrease in cooling output down to two digit percentages in the radical cases. The following graph describes the dependence of the evaporation temperature on the thickness of the dirt build-up (shown as the Fouling Factor) in a typical 550 TR Evaporator. The Fouling Factor is the common way to simulate heat exchanger fouling in the design stage.
Loss of Efficiency in Fan-Coil Tubes
The cooled water from the Evaporator is distributed to the end units in the building. The water flows through the copper tubes while air from the fans is blown on them. However, when the inner walls of the tubes are lined with sludge, the heat transition is impeded, causing an increase in the temperature and humidity of the cooled air. In order to overcome the performance deficit of the Fan-Coils, cooler water must be supplied by the Evaporator resulting in a marked increase in the chiller’s electricity consumption. In addition, as a result of the sediments the pressure drop increases and the cool water flow-rate decreases. Clogs within the Fan-Coil filters also cause flow-rate decline. The combination between the layer of insulation that is created by the sediments and the decrease in water supply causes a loss in output that can be between single digit to double digit percentages in extreme cases.
Loss of Efficiency in the Main Pipeline System and Control Equipment
The cooled water is pumped by the recirculation pumps via the building’s main pipeline system and distributed to the different floors and rooms on each floor. When the quality of the water is affected and contains sludge, corrosion and bacteria, the control valves, filters and the pipeline itself begin to deteriorate. This phenomenon also causes loss of output and marked energy waste, as follows:
- When the control valves aren’t working properly the water flow is impeded and the water distribution is not implemented as planned.
- Clogged filters hamper the water supply reducing the cooling efficiency.
- The narrowing of the main and/or floor pipeline causes a decrease in the water supply and inconsistencies in the supply distribution between the various floors and rooms. Consequently, the cooling distribution between the floors and rooms is affected, resulting in supplying colder water than planned in order to reach the desired temperature in the problematic areas.
- In buildings where the consumers pay per cooled water consumption there is an additional problem: the flow rate gauges become stuck with sludge and charging the consumers becomes inaccurate.