what is backflush in a heat exchanger...describe?
Back-flush systems will clean a plate and frame heat exchanger while the unit remains on-line. Daily back-flushing in the heat exchanger virtually eliminates plate fouling and the need to open the heat exchanger for periodic manual cleaning
Edited Backflushing is to reverse the direction of water flow through the unit, usually through the tubes of a shell and tube heat exchanger (the most common type). This is done to reduce fouling which is the build-up of contamination (dirt) clinging to the inside walls of those tubes when non-treated water is used such as river or lake water. Fouling will both impair flow and reduce heat transfer. Special purpose transfer (4-way) valves are commonly employed to affect the flow reversal. When the backflush is employed, the exiting water will be dirty until the loose material has been removed while the unit stays in service. The flow is often reversed back and forth several times. High tube velocities tend to run cleaner by sweeping the material along, but there are less desirable long term effects of high velocity. When I put in a back flush design I provided for opening or bypassing of the water flow control valve to achieve the temporary fast rinse. The performance of the heat exchanger is monitored and when back flushing is no longer effective, a system shutdown for exposed -tube (or exposed plate and frame surace) cleaning must be employed. There have been auxiliary cleaning systems where a short brush is inserted into each tube and a cage is attached to the ends of each tube. When the flow is reversed the brushes traverse the tubes, automatically cleaning the inside walls. These eventually fail if algae of other aquatic growths are present.
Heat exchangers, specifically those using water as a cooling medium generally receive water coming from a lake, river or the sea. Water from these sources is Screened to remove large pieces of debris like seaweed, lumps of wood and other debris. It is then filtered to remove as much silt and sand and fine debris as possible but, it is not all sifted out. During operations, there is always some build up of silt and sand.. etc that eventually causes deterioration in the Heat Transfer Rate and efficiency of the exchanger. Exchangers of this type are generally built having a standby unit. The piping arrangement is such that, a fouled exchanger can be isolated and 'Back Flushed' by operating the valving to change an Outlet to an Inlet and Inlet to an Outlet. This will flush out most of the debris build-up and so increase the Heat exchange rate near to it's normal level when put back into service. It will eventually become necessary to shut down, isolate, depressure and drain where required, open up the exchanger and perform an Inspection upon which the maintenance required will be decided. A thorough clean-out of both Shell and Tube sides of the exchanger and a new or re-tubed bundle may also be needed...etc.
