| Division/Refrigeration Division/Techno Tips/News Letter |
| EFFICIENCY : |
1.Air cooling unit is very efficient compared to Bunker coil. Running time for compressor is very less for plants with air cooling units than with bunker coils. This is because bunker-coil runs with less suction pressure (20-25 psig) than with air cooling unit (30-35 psig.) thereby reducing the compressor capacity. This saving directly reduce the power till substantial.
2.If air colling units are used, the size of the compressor can also be reduced - Instead of 7-1/2" x 7-1/2" compressor used in 2000 mt. Cold storage with bunker coil, 6-1/2"x6-1/2" compressor-can be used for a 2000 mt. Plant with air cooling unit. |
| GAS SAVING : |
| Since the bunker coil used very high volume, huge quantity of ammonia gas required to run the plant where as air cooling unit required very low requirement of gas. For a 2000 kg- Of gas where as a plant with air cooling unit require only 900 kg. of gas. This leads to a substantial saving in the capital as well as running cost. |
| SPACE SAVING : |
| Bunker coil occupies huge space in the cold storage where- as air cooling unit occupy very little space. More products can be stored inside the cold storage there-by increasing the revenue by using air cooling units. |
| UNIFORM TEMPERATURE : |
| Air Cooling units increase the air circulation inside the cold storage and there- by ensuring uniform temperature throughout the cold storage. When bunker coil is used, the temperature of the ground floor is much higher than the top floor temp. To overcome this, the plant has to be run continuously to bring the ground floor temp. to the required level there-by increasing the running time of the compressor. |
| QUALITY OF THE PRODUCT : |
The suction pressure of the plant is low for a refrigeration plant with bunker coil than a plant with air cooling units. The weight loss of the products increase when the suction becomes low. By using air cooling units the quality of the product is better than that of plant with bunker coil.
COMPARING ALUMINIUM FINS VS. GALVANIZED STEEL FINS AMMONIA AIR COOLING UNITS
Air-cooling Units used in ammonia system have traditionally been made using galvanized (zinc coated) carbon steel. There are other metals which are compatible with ammonia including stainless Steel, and aluminium. Designers and installer of industrial ammonia Air-cooling units for low temperature applications must be concerned with the weight, performance, and reliability of the equipment being specified. Additionally, there may be requirements for corrosion resistance, cleanbility, and defrosting characteristics which need to be considered ammonia and aluminum are very compatible and have been used in ammonia heat exchangers for several decades. The properties of Aluminium make it an ideal metal to use in ammonia refrigeration applications. This article will describe those properties and compare them to galvanized steel, another commonly used combination of metal used for ammonia heat exchangers to clearly show " The Aluminum Advantage". Table 1 |
| Properties of Various Metals |
| Metal |
Density |
lbm/cuft Thermal Conductivity Btu/sqft h F ft |
Specific Heat Capacity Btu/lbm F |
| Aluminium |
165 |
117 |
0.215 |
| Carbon |
489 |
26 |
0.108 |
| Zin |
440 |
65 |
0.092 |
|
| COMPARISON OF PROPERTIES |
Table 1 compares several properties of aluminium to those of carbon steel and zinc. Galvanized steel is obtained by dipping carbon steel in a bath of molten zinc, hence these two base metals are shown in the table. The Density of the metal directly affects the weight of the heat exchanger, and when multiplied by the specific Heal Capacity the product indicates the amount of energy required to heat up and cool down the heat exchanger during a deforst cycle. Designers and installers of industrial ammonia Air-cooling units for low temperature applications must be concerned with the weight, performance, and reliability of the equipment being specified. Additionally, there may be requirements for corrosion resistance, cleanbility, and defrosting characteristics which need to be considered ammonia and aluminum are very compatible and have been used in ammonia heat exchangers for several decades. The Thermal Conductivity of the metal affects the thermal performance of the heat exchanger, and the speed and effectiveness of defrost. |
| ADVANTAGE : WEIGHT |
The very low density of aluminum compared to steel and zinc results in a much lighter heat exchanger. A galvanized steel ammonia air cooling unit coil will weigh 2.5 times more than an aluminum air cooling unit having the same dimensions. The higher Thermal Conductivity of aluminum results in better thermal performance compared to galvanized steel. When the effect of increased performance with aluminum is taken into account, the galvanized steel version will weigh 2.5 times more than an aluminum air cooling unit coil having the same cooling capacity. The lighter weight of Aluminum air cooling unit reduces structural requirements for buildings where units are ceiling hung, an especially important feature in high seismic areas. Rigging and handling are also made easier with lighter weight aluminum air cooling units. It is easy to visualize the advantages of mounting a 600 kg aluminum air cooling unit compared to 1500 kg. steel air cooling unit in a room with a 50 foot ceiling. The superior cooling capacity of aluminum air cooling units allows the designer the choice between
(a) selecting an air cooling unit having fewer rows and/or wider fins spacing for lower first cost, or (b) using the same size unit (same rows and fin spacing) and use to operate the plant at higher suction pressure with resulting reduced operating costs, compared to galvanized steel. |
| ADVANTAGE : DEFROST |
| The much higher Thermal Conductivity of aluminum (4 times higher than steel) will also result in faster, more effective defrosts. Aluminum air cooling simply defrost faster than better than galvanized steel coils. Also, a substantial amount of energy is expanded during defrost to heat the mass of metal in refrigeration air cooling unit up to the melting point of ice (32 deg.F), then to cool the metal back down to operating temperature. When the Density of the metal is multiplied by the Thermal Conductivity the resulting products indicates the amount of energy required to heat or cool a heat exchanger of a given volume by one degree. Based on this analysis, a galvanized steel air cooling unit will require 70% more energy than the same size aluminum air cooling unit to heat up and cool down during every defrost cycle. This component of defrost energy become very significant at lower temperature (i.e. in freezers). Using aluminum air cooling units can produced significant saving in operating costs over the course of a year, especially at freezer temperatures. |
| ADVANTAGE : CORROSION RESISTANCE |
| Corrosion of heat exchanger by contact with, or proximity to food stuffs is a concern in food processing facilities. All food stuffs are mildly acidic. Aluminum is more corrosion resistant than galvanized steel when exposed to acetic and citric acids (Dairy Products, beverages, fermentation, blood). Aluminum is more corrosion resistant than galvanized steel in the presence of sodium chloride (preservation of meats and vegetables), and sulphur dioxide (grape storage). Neither galvanized steel more aluminum is recommended for exposure to nitrites (cured and smoked meats). Stainless steel is the suggested material to use in the presence of nitrites. Generally speaking, aluminum is a better metal to use than galvanized steel where there is concern about corrosion due to contact with most food stuffs. |
| ADVANTAGE : CLEANIBILTY |
Cleanbility of equipment in food processing facilities, including air cooling units, has become an increasingly important issue. bacteria contaminates must be removed with regular cleaning and locations where they can accumulate need to be minimized. The smooth, hard surface of aluminum air cooling unit are idea for effective cleaning in food processing equipment and facilities. The rough, porous surfaces of galvanized steel air cooling units may present difficult cleaning problems for operators in facilities where bacteria-free operation is critical. Another issue of importance is the effect of various cleaners on aluminum and galvanized steel. There are four basic types of cleaners used in the food processing industry.
1. Acidic
2. Strongly Alkaline
3. Mildly Alkaline
4. Chlorine Based
Both aluminum and galvanized steel are attacked by Acidic, Strongly Alkaline, and Chlorine based cleaners. These types of cleaners are not recommended for use on any ammonia air cooling units. A foaming type Mildly Alkaline cleaners (such as "Based-511" manufactured by Great Western Chemical) is recommended for both aluminum and galvanized steel surface. It caustic soda (Strong Alkaline) must be used for clean up, then galvanized steel is preferred over aluminum, since zinc has higher resistance than aluminum to mildly alkaline solutions. |
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