Hydraulics & Pneumatics January/February 2023

www.hpmag.co.uk HYDRAULICS & PNEUMATICS January/February 2023 33 has low heat dissipation per unit volume of the heat exchanger. The primary reasons for this is due to low fin density, lack of turbulence inside the tube and through the fins. Plate-fin oil coolers Plate fin coolers use a number of rectangular tubes. These rectangular tubes are fabricated by brazing together aluminum alloy plates (also known as separator plates), sidebars and inside bars. The oil to be cooled flows in through an inlet header and is then distributed through these rectangular tubes. Enclosed inside these rectangular tubes are fins called internal fins or internal turbulators. The heat is transferred from the oil to the internal fins. These fins play a very important role in the heat dissipation process. For one, there is a larger surface area available for heat transfer and secondly the fins cause turbulence in the oil thereby resulting in greater transfer of heat from the oil to the internal fins. The heat is then conducted from the internal fins to the plates. Between each pair of plates are external fins or external turbulators and the heat is in turn transferred from the plates to the external fins by conduction. A draft of cool ambient air is forced through the external fins carrying away the heat. The oil cooled in the rectangular tubes then collects in the outlet header and exits the heat exchanger through the outlet port. The external fins also serve a dual purpose of creating turbulence in the air flow and providing a larger surface area, thereby resulting in greater heat transfer. This type is often categorised as a compact heat exchanger to emphasise its relatively high heat transfer surface area to volume ratio. This and their lightweight properties make plate-fin heat exchangers an obvious choice and are widely used in many industries. They can facilitate heat transfer even when the temperature difference between the hot fluid and the cool fluid is small. The internal and external fins besides providing an extended surface area for heat transfer also serve to increase the structural integrity of the heat exchanger and allow it to withstand higher pressures and pressure fluctuations. Water-cooled oil coolers In these heat exchangers, the hot oil is passed in close proximity to water and the heat exchange takes place across a barrier between the hot oil and cool cooling water. Regarding water-cooled oil/fluid coolers, there are predominantly two types: Shell and tube-type and plate-type water cooled oil coolers. The plate type is again available in two types: Gasket plate- type water cooled oil cooler, and brazed plate-type water-cooled oil cooler. The hot and cool fluids enter the heat exchanger through respective inlet ports. After the heat exchange takes place across a barrier separating the two fluids, the fluids exit the heat exchanger through the outlet ports. For example, a shell and tube heat exchanger is comprised of a tube bundle encased in an outer housing or shell. The cool fluid usually travels through the tubes & the hot fluid moves in the shell. The heat exchange takes place across the tube surface as shown in Fig: 2. In the plate type oil coolers, the gasket take a closer look. In a water-based oil cooling system, the hot oil from the equipment is pumped through the heat exchanger. There is an exchange of heat between the hot oil and the cooling water. The cooled oil is returned back to the oil sump or reservoir. The water after it has performed its function of removing the heat from the oil, is itself now warm and has lost the ability to extract more heat efficiently. What do we do with this water? In the olden days it was simply flushed away, and more fresh water was used for cooling. With growing environmental concerns coupled with the scarcity and cost of fresh water, it needs to be recirculated. For it to be plate, as the name suggests, consists of alternating layers of gaskets and plates bolted together. The brazed-plate type is similar in construction except that the plates are brazed together in a vacuum furnace. The primary advantages of brazed plate-type over gasket plate alternatives are that the chance of intermixing of the fluids due to a damaged gasket is minimised and they can handle higher working pressures. In both these types, the hot and cool fluids flow in between alternating plates. Comparative analysis between water- and air-cooled oil coolers Now that we understand the basics, let’s Fig 2. Shell and tube-type water-cooled oil cooler. effectively re-used as a cooling media, the water needs to lose the heat that it has picked up and be cool enough to be able to remove heat efficiently from the oil. For this the warm water is then pumped to a cooling tower where an air draft is used to cool it. There are two processes involved here – the cooling of oil using water, which in turn gets heated and the cooling of hot water with ambient air. As we know, no process is 100% efficient. Assuming we have two well-designed processes, each having an overall efficiency of say 80%, we have 20% loss of efficiency in each process. Fig 3 above shows the efficiency of a water-cooled oil cooling system. It is Fig 3. Efficiency of water-based oil-cooling system.