29 September 2020
Mark Donoghue, Director at DHD Cooling, outlines how optimising performance in virtually any system can come down to a few simple measures.
Keep it clean
It probably comes as no surprise but heat exchangers rely upon their heat transfer surfaces. These surfaces are used in a few different ways, but in all cases the function of a heat transfer surface is to create the best conditions for heat to travel from one fluid to another.
The definition of a fluid in physics can be described as the following:
A fluid is a substance that continually deforms (flows) under an applied shear stress, or external force. Fluids are a phase of matter and include liquids, gases and plasmas.
Transferring heat from a car engine, for instance, tends to rely upon a coolant being circulated around the key engine components through a series of tubes and channels where it picks up heat created by the engine, which is passed through a radiator where the heat is removed by the air passing through the tightly packed fin materials in the radiator.
This type of heat exchanger surface is often referred to as a finned tube bundle, or coil. The fluid inside the tubes in the heat exchanger are in contact with the internal walls of the finned tubes, and air on the outside comes into contact with the external extended finned surface. The heat in the fluid is transferred through the materials of the heat exchanger to the external surface of the finned tubes where air takes the heat away.
The two fluids on either side of the exchanger can be anything and there are many types of heat exchanger design available to cater for the many different applications where heat exchange is required. These include things like: Steam to Air; Steam to Water; Water to Water; Oil to Water; Gas to Water; Gas to Air; Water to Air; and Air to Air.
With heat exchangers of this type the key to good thermal heat transfer is maintaining clean heat exchange surfaces.
Any contamination of the heat transfer surfaces will act as a barrier between the heat transfer fluids, and the heat transfer materials, resulting in a reduction in performance.
In evaporative cooling towers the heat transfer surfaces are designed to maximise the surface area of the circulating water, so that heat can be removed by bringing air into contact with the water for evaporation to occur. There are two main types of heat transfer surface in evaporative cooling towers, categorised as splash or film fills. The type of fill used is usually selected depending on the quality (cleanliness, or fouling potential) of the water, which is why knowing the circulating water quality is a critical factor in cooling tower design and sometimes overlooked.
The purpose of selecting the right fill material, is to prevent the untimely build-up of contamination on the heat exchanger surfaces, although in certain applications it is impossible to completely stop the build-up of impurities, which can be introduced into a system, through both the circulating water and the environment.
Where film and splash fills differ from coils is that the heat exchange occurs between the surface of the water and the cooling air, not through (or across) a heat exchange material, so light fouling will not prevent the air and water from coming into contact, and heat exchange will still occur, often without affecting performance. Performance does become affected if the fouling becomes bad enough to restrict the movement of the air and water through the heat exchange media.
Maintain optimum fluid flows
Heat transfer design assumes an even distribution of fluid flows across the complete surfaces of the heat exchanger. Through testing and experience, designers then apply corrections to the mathematical formulas to allow them to guarantee heat exchanger performance. Then the real-world kicks in!
Pipework modifications to try to overcome internal pressure issues
Fluid flows can be affected by many different things, in piping circuits additional valves and equipment can increase the pressure drop through piping, affecting flows and pressures, internal fouling or changes in the properties of a fluid can all affect performance.
External fouling, wind, weather and environment as well as uneven distribution of air due to other equipment, or restrictions to air flow, will reduce airflow through a cooling system.
In cooling towers, poor distribution due to low flow or maldistribution of water flow can lead to dry areas of the heat exchange media, which results in the cooling air not coming into contact with the circulating water, and heat rejection not occurring in these areas. The effect of this can be catastrophic in terms of heat rejection.
Ensure mechanical components are in good working order
Cooling systems function by continually moving the heat transfer fluids around, the amount of fluid is critical to the temperatures of the system, where the energy released through fluid heat exchange is a function of mass and change in temperature.
These two variables are inversely proportional when it comes to heat transfer, where a reduction in flow (mass of cooling fluid) will lead to an increase in the temperature difference of the fluid as a result, and vice versa.
It is critical to the transfer of heat that the fluids involved are sufficiently and safely moved in order for the system to function as designed, any variation from design can completely change the performance of a system, both positively and negatively.
Fans and pumps are designed to overcome system pressure to move fluids, changes in system pressure can lead to operating conditions that adversely affect these systems.
Pump cavitation can destroy impellers, and poor airflow conditions for fans can result in mechanical vibration that can damage rotating machinery over time.
Evaluate and service
Throughout the lifetime of a plant or process, things can change and often do, differing demands on a facility may increase or reduce cooling demands, changes in water quality, environmental conditions, and the infrastructure connected to and around the equipment can change.
Even if there have not been any changes, evaluating your systems will allow you to consider whether your cooling plant is running at optimum efficiency, and the impact in the performance of your plant.
In the UK, each wasted kW over the course of a year costs a business around £1000. We have been to sites where demand has changed so much that the cooling systems are too big for what is required, but are still run at full capacity, using more energy than needed.
Air inlet filtration
Air inlet filters can reduce the amount of fouling on heat exchange surfaces extending cleaning intervals but even with filters, cleaning must be one of the critical services performed on heat exchangers.
Maintaining fluid flow must be the key function of any activity regarding heat exchangers, cleaning and mechanical maintenance make up the bulk of the activities to ensure stable long-term operation and should not be overlooked in order to ensure optimum long-term performance.
So, if you can’t clean it anymore, then things are only going to get worse!