Hydraulics & Pneumatics September 2022

HYDRAULICS “I am looking for an air-cooled heat exchanger manufacturer who will give me an inexpensive oil cooler.” We were recently approached by a hydraulic equipment manufacturer for a quote on an air-cooled oil cooler. When we asked him for details about his application, his answer was: “Just give me any inexpensive oil cooler. Actually, I don’t need one as I am using a large tank of 750 litres. I just need to complete the bill of materials to satisfy the customer.” For us, this is not an uncommon response. On a little prodding he disclosed that his hydraulic system used a 75 HP motor coupled to a gear pump delivering 160 LPM oil flow. The equipment had a continuous duty cycle. If the heat produced during the work cycle is more than the heat dissipated from the tank surface a heat exchanger is required. In continuous duty systems, if the wasteful energy that is converted into heat cannot be dissipated effectively by radiation naturally from the surface of the tank, we do need an external oil cooler. The amount of heat that a tank can dissipate with respect to the temperature difference between the oil temperature & the temperature of the ambient air can be estimated with the help of the table below. Figure 1 is based on the surface area of mild steel tanks with a gap of 150 mm space underneath & free air circulation all around. Heat Dissipation for any other tank capacity or surface area can be calculated by the formula: Heat dissipated IN KW = U*A*(To- Ta)/1000 U= 11 to 15 W/(m 2 ·degC). Value of U is taken as 14 for Figure 1. A= Surface area including bottom in sq m To= Oil Temperature in degrees C Ta= Ambient Air Temperature in degrees C We can see that the heat dissipation from the tank surface increases as the temperature difference between the oil & ambient air increases. If the heat generated in the hydraulic system is more than the heat dissipation capacity of the tank the oil temperature will keep rising, until the heat generated equals the heat dissipation capacity of the tank. An overheating hydraulic system will result in any or all the following: Increased power consumption Lower production speeds Higher breakdowns Higher maintenance More oil spills Leading to catastrophic irreversible failure. In the least it will result in service visits to the customer. In this case the 24 HYDRAULICS & PNEUMATICS September 2022 www.hpmag.co.uk By Vineet Taneja, CEO, Ace Automation Engineers. The case for air-cooled heat exchangers HEAT RADIATING CAPACITY OF OIL TANK TANK CAPACITY SURFACE AREA TEMPERATURE DIFFERENCE in °C (OIL TEMPERATURE - AMBIENT AIR TEMPERATURE) 10 15 20 25 30 35 40 45 Litres Metre Sq. Kcal/hr KW Kcal/hr KW Kcal/hr KW Kcal/hr KW Kcal/hr KW Kcal/hr KW Kcal/hr KW Kcal/hr KW 40 1 120 0.14 181 0.21 241 0.28 301 0.35 361 0.42 421 0.49 482 0.56 542 0.63 55 1.2 144 0.17 217 0.25 289 0.34 361 0.42 433 0.50 506 0.59 578 0.67 650 0.76 75 1.3 157 0.18 235 0.27 313 0.36 391 0.46 470 0.55 548 0.64 626 0.73 704 0.82 110 1.5 181 0.21 271 0.32 361 0.42 452 0.53 542 0.63 632 0.74 722 0.84 813 0.95 150 2.3 277 0.32 415 0.48 554 0.64 692 0.81 831 0.97 969 1.13 1108 1.29 1246 1.45 200 2.7 325 0.38 488 0.57 650 0.76 813 0.95 975 1.13 1138 1.32 1300 1.51 1463 1.70 225 2.9 349 0.41 524 0.61 698 0.81 873 1.02 1047 1.22 1222 1.42 1397 1.62 1571 1.83 300 3.7 445 0.52 668 0.78 891 1.04 1114 1.30 1336 1.55 1559 1.81 1782 2.07 2005 2.33 380 4.4 530 0.62 795 0.92 1060 1.23 1324 1.54 1589 1.85 1854 2.16 2119 2.46 2384 2.77 450 4.9 590 0.69 885 1.03 1180 1.37 1475 1.72 1770 2.06 2065 2.40 2360 2.74 2655 3.09 570 5.2 626 0.73 939 1.09 1252 1.46 1565 1.82 1878 2.18 2191 2.55 2504 2.91 2817 3.28 750 6.5 783 0.91 1174 1.37 1565 1.82 1957 2.28 2348 2.73 2739 3.19 3130 3.64 3522 4.10 Below Figure 1: Heat radiating capacity of reservoirs.