What Causes Scale in Heat Exchangers
The accumulation of unwanted material on the internal surfaces of a heat exchanger tubes is commonly referred to as scale. Deposits come from within source water and if left untreated, will harden onto surfaces compromising the system’s ability to operate and reducing energy efficiency. As scale accumulates onto surfaces it insulates heat exchanger tubes increasing the system’s resistance to heat transfer. Even modest layers of scale will increase the energy required to heat or cool water and degrade chiller plant equipment. Regular heat exchanger cleaning is essential to remove scale and prevent overheating, internal tube failure, and increased energy consumption – all which increase operational costs. In the worst-case scenario unchecked fouling can eat through the walls of tubes and lead to total replacement of plant equipment.
The Chemical Composition of Scale
The most common form of scale, by far, is calcium carbonate (CaCO3). Calcium scale forms when the calcium and carbonate within source water can no longer be dissolved. This causes the heavier, undissolved minerals to fall out of suspension and form a continuous deposit layer on the surface of the heat exchanger. As fouling causes internal temperatures to rise, the solubility of minerals generally decreases compounding the problem. What we call hard water describes the amount of minerals found within a certain volume of water and is measured in parts per million (PPM). Magnesium, silica, and calcium are the most common minerals impacting the quality of source water. Heat within the system evaporates pure water and leaves an increasing concentration of minerals that eventually combine to form calcium carbonate scale. Until more recent advancements in technology, plant operators who were stuck with poor quality source water had no way to prevent fouling and the required manual cleaning using chemical treatments. However, new solutions such as the Helios Automatic Tube Cleaning System offer a more advanced process for preventing scale accumulation.
Controlling Scale in Heat Exchangers with Chemical Treatment
The traditional approach to scale prevention has focused on modifying the solubility of calcium carbonate with chemical threshold inhibitors. By establishing the threshold solubility operators can pinpoint the concentration at which scale begins to form. Chemical treatments are then used to control the composition of source water keeping it below the margin for scale to form.
Preventing Scale in Heat with Automated Tube Cleaning Systems
With advances in technology, leading chiller plant operators are migrating toward a more preventative approach that uses a permanently installed maintenance system. The Helios Tube Cleaning System continuously cleans heat exchangers by circulating sponge balls through the heat exchanger tubes at programmed intervals, preventing residue, deposits, and bio-film from accumulating in the tubes. The sponge balls are slightly larger in diameter than the tubes, and wipe the tubes clean as they pass through. The balls are then trapped on the outlet side of the heat exchanger where they are collected, transported, and stored until the next cleaning cycle begins.
Determining when Heat Exchangers Require Cleaning
Measuring heat exchanger performance begins with measuring the difference between the heat going into (approach temperature) and coming out of the heat exchanger. The metric used is referred to as Delta T (ΔT) and is a measure of heat transfer efficiency. When a heat exchanger fails to operate within a certain ΔT range this is an indicator that scale has accumulated reducing efficiency.
Traditionally operators had to maintain a daily operational log for each piece of equipment to monitor system performance. However, with emerging IoT applications, this process is frequently automated with an emphasis placed on preventative maintenance.
What Is Helios Step-By-Step Procedure for Cleaning a Heat Exchanger?
A primary benefit of an automated tube cleaning system is the elimination of manual cleaning. This in and of itself represents a significant cost savings. However, continual cleaning keeps water cooler heat exchangers operating at peak efficiency dramatically reducing energy costs and a facilities’ demand draw from the grid. This is significant because According to the US Department of Energy (DOE), chillers can combine to use approximately 20% of the total electric power generated in North America. Furthermore, the DOE estimates that chillers can expend up to 30% in additional energy usage due to various operational inefficiencies. These acknowledged inefficiencies cost companies and building facilities billions of dollars annually.
So, how do automated tube cleaning systems work?
The Helios Tube Cleaning System continuously cleans heat exchangers by circulating sponge balls through the heat exchanger tubes preventing scale from accumulating. The sponge balls are slightly larger in diameter than the tubes and wipe the tubes clean as they pass through.
1. At programmed intervals, the Controller commands the injection valves to open and pump to start, and the balls are injected into the heat exchanger inlet line.
2. Normal cooling water flow transports the balls through the heat exchanger tubes and into the Ball Trap.
3. The Controller then prompts the collection valves to open and pump to start, and the balls are returned to the Collector, where they are held until the next injection/collection.
4. Between cycles, the sponge balls are stored in the Collector and all valves are closed.