TREATMENTS AND FLUIDS
between those media. Aluminium and copper are good materials for this purpose as they have high heat conductivity.
Standard liquid-to-air heat exchangers are made with copper tubes and aluminium fins. A weakness in this design is the joint between the copper and aluminium. As long as the fins are tightly joined to the copper tube, without gaps or interference of organic layers or corrosive products, the heat transfer will be optimal. Pollution on the fin surface will also influence the heat transfer of a heat exchanger and the airflow through it.
The joint between the copper tubes and aluminium fins is one of the more corrosion sensitive parts of an air conditioning unit. With aluminium being less noble than copper it will be sacrificed in the presence of electrical conducting fluids. These fluids will always be present due to pollution and moisture from the environment . The accelerated corrosion due to the presence of different metals is called galvanic corrosion and is one of the main problems in copper-aluminium heat exchangers.
An example of this galvanic corrosion is given in figure 2. The joint that existed between copper and aluminium is now replaced by a copper aluminium oxide joint. The heat conductivity of aluminium oxide is much lower than that of aluminium. Therefore, the heat transfer between copper tubes and aluminium fins is significantly decreased.
If pollution on the fins limits the airflow through the heat exchanger, the temperature of the air that is passing over the aluminium fins will increase (the same kW in less kg of air). This will cause the temperature difference between the liquid/gas in the copper tube and the air passing over the fins to decrease. A smaller temperature difference will result in reduced heat transfer. The only way the system can cope with this loss of heat transfer is the undesirable increase of the condensing pressure and temperature .
In the design phase engineers must take into account the effect corrosion and pollution will have during the lifetime of a cooling installation. Corrosion can of course be controlled by selecting the right materials but also the type of heat exchangers is important. The use of indirect adiabatic cooling on a heat exchangers will create massive galvanic corrosion due to the amount of moisture that is brought into the heat exchanger. Protecting heat exchangers from corrosion and accumulating pollution is essential for cooling installation capacity and energy
52 June 2017
consumption. The options available in the market to realize this can be divided into three parts; metal optimization, pre-coated metals and post-coated metals.
Metal optimisation consists of looking into different metals or alloys to reduce the risk of corrosion. Using copper fins instead of aluminium is a good example of this. The resulting copper tube-copper fin heat exchangers will not suffer from extreme galvanic corrosion anymore. Apart from the price and the weight the disadvantage is that in industrial environments sulphurous and nitrogen contamination will create high amounts of metal loss.
Heat transfer will not directly be affected but the lifetime of the heat exchangers will significantly decrease.
Precoated aluminium is often offered as a “better than nothing” solution against corrosion in heat exchangers. The ease of application makes this a cheap and tempting solution . In this case the aluminium fin material receives a thin coating layer before being cut and stamped to fit. The result is that during cutting and stamping the fins, the protective layer is damaged creating hundred and thousands of meters of unprotected cutting edges in every single coil. Next to this one must take into account the fact that the protective layer will be between the copper tube and alu fin which reduces the heat transfer already without any corrosion being present.
Post coating is a technique where corrosion protective coatings are applied to heat exchangers after full assembly. If the right coating and the right procedures are applied the metals will be sealed off from the environment without reducing heat transfer. Disadvantage is that it requires special coatings and application can only be done by specialized companies.
Applying these special heat conducting coatings on the complete heat exchanging surface of coil is difficult and time consuming. This creates extra challenges with respect to pricing and delivery times compared to other solutions that can often be produced/ supplied by heat exchanger manufactures themselves. Even though post coatings are preferably applied as a preventive measure before installation, it is possible to use them as corrective measure if choices made in the design phase turn out to be insufficient.
Fluid laun revolutio transfer
A
global leader in safety critical frost protection chemistry and rheology has launched a first-of-its-kind heat transfer fluid that could revolutionise food and drink production globally, and for the first time provided a commercial solution to the long-standing efficiency-versus-toxicity challenge.
The Kilfrost ALV Plus fluid offers a viable alternative to monoethylene glycol (MEG) and monopropylene glycol (MPG) as it is certified as NSF safe for incidental contact with food and beverage, and offers premium performance.
www.acr-news.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120
