The problem of defrosting cold storage coils is one which is too often ignored at the time of making the installation. Where such has been the case, the operator often finds himself working at great disadvantage, not only because of his inability to produce the required temperature, but also because of the decreased efficiency of the mechanical equipment entailed by the reduction in back pressures necessary to coax the heat from the cold-storage rooms through the additional resistance offered to its passage by the accumulated ice. This disadvantage affects principally . the coal bill and fortunately for the operator, though unfortunately for the owner, is in the majority of cases not recognized, or, if apprehended, is not charged to the proper account.
The effect of a coating of ice on direct-expansion pipes may be shown as follows: Assuming a heat transfer of 10 B.t.u., in round numbers, per hour, per square foot per degree of difference in temperature inside and out, for a flat metallic refrigerating surface,* and an equal amount for a sheet of ice one inch thick, it follows that the heat transmission through a square foot of direct-expansion cooling surface insulated with a layer of ice one inch thick will be only one-half that of the uncoated surface. As a matter of fact, it would seem from the context that the value of 10 B.t.u. given as the heat conductivity of ice applies to plate-ice conditions under which the wetted surface of the submerged ice will transmit materially more heat than a dry surface in contact with air. This would indicate that the decrease in heat-transmitting capacity of direct-expansion surfaces in air due to a coating of ice is even more than 50 per cent. This condition will be partially offset by the fact that on account of the increasing diameter the layer of ice in the case of cylindrical surfaces such as pipes, which, together with the fact that such coatings usually present an irregular surface, further increasing the heat-absorbing area,
<Callout type="tip" title="Tip:">Direct-expansion coils can be defrosted by shutting off a portion and allowing natural melting. This reduces energy costs while maintaining some refrigeration.</Callout>
Brine Coils<br>Brine pipes may be readily defrosted by the circulation of hot brine. This may be accomplished through the main feed and return headers where the operation does not have to be performed very frequently, or as in abattoirs where the excessive amounts of moisture from the hot meats to be chilled make the accumulation of frost very rapid, or by a separate set of defrosting headers.
<Callout type="warning" title="Warning:">Using hot brine for defrosting can lead to uneven heating and potential damage if not managed carefully.</Callout>
Direct-expansion Coils<br>In the case of direct-expansion coils, the defrosting method probably most satisfactory where the cold-storage temperatures are above 32° Fahrenheit is to install sufficient coil surface to allow a part of the coils to be shut off at any time, so that the frost will melt without artificial heat and at the same time produce a certain amount of useful refrigeration. If it is necessary to force the defrosting process by the use of outside heat, a hot gas line from the condenser may be connected to the liquid-line connections to the separate coils just inside the expansion valves. The hot gas, after melting the ice as it passes through the coils, returns to the compressor together with the return gas from the remaining coils.
<Callout type="important" title="Important:">Forcing defrosting with outside heat can be energy-intensive and should only be used when necessary.</Callout>
Oil in the Refrigerating System<br>Next to unintelligent design, which sometimes provides for the operating engineer a plant that cannot be made to develop nearly as high efficiency as operating conditions should warrant, and unintelligent operation, which fails to get nearly as high efficiency as operating conditions and mechanical design should warrant, the worst foe to economy is foreign matter in the refrigerating system. In order that the oils used in the system shall not stiffen prohibitively at the low temperatures encountered and not be saponified by the ammonia, only very light mineral oils can* be employed. Such oils range from 22° to 30° Baume, corresponding to a specific gravity of from 0.924 to 0.88. These oils should have a cold test of about 0° Fahrenheit, to obtain which they will have a flash point of between 310° and 400° Fahrenheit.
<Callout type="risk" title="Risk:">Improper oil management can lead to reduced system efficiency and increased energy costs.</Callout>
Oil in Coils<br>The question of removing oil from the expansion coils, whether they are used for chilling cold-storage rooms, for chilling brine to be circulated through cold-storage rooms or for chilling the brine of an ice tank, is one which has received much attention, but, although much experimenting has been done and a few patents have been issued, no cheap and effective method has yet been devised. Where coils will drain so that there will be no danger of entrapping condensed moisture, the most effective method of removing the oil is to pump out the coils, disconnect them and blow them out with the highest pressure live steam available, letting the steam blow through each coil as long as any oil appears at the exhaust end. The brine may be left in the tank and heated up by steam, so as to prevent, as far as possible, the condensation of steam inside the pipes, or if the brine has been removed the same result may be obtained by leaving the covers on the tank and blowing in live steam.
<Callout type="gear" title="Gear:">Using high-pressure live steam for cleaning is effective but requires proper equipment to avoid injury.</Callout>
Permanent Gases<br>The action of so-called permanent gases in the condenser is less detrimental than that of oil in the expansion coils which forms an insulating lining of fairly high efficiency on the inside of the heat-absorbing surfaces. The principal effect of these gases, if their operation can be limited to the condensers, is to occupy space that should be available for the ammonia gas. This reduces the effective area of the condenser cooling surface, causing an increased head pressure with the usual amount of cooling water or an increased amount of cooling water to maintain the same head pressure.
<Callout type="warning" title="Warning:">Permanent gases can reduce efficiency and increase energy costs if not properly managed.</Callout>
Condensers should be purged as often as the accumulation of gases indicate that they need it. To do this most advantageously, the liquid outlet and hot-gas inlet valves of the coils to be purged should be closed and a liberal supply of cooling water allowed to flow over them for some time. The permanent gases can then be purged out through a small rubber tube, one end of which is connected to the purge valve; the other end should be immersed in a pail of water. If permanent gases escape when the purge valve is slowly opened, bubbles will rise to the surface of the water. If only ammonia escapes, the bubbles of ammonia gas will be dissolved in the water before reaching the surface, giving rise to a sharp crackling sound such as that caused by the condensation of steam in the process of heating water by the direct admission of steam through an open pipe.
<Callout type="important" title="Important:">Regularly purging condensers is crucial for maintaining system efficiency.</Callout>
Incrustation on Condenser Coils<br>While the comparatively high working temperature of condenser coils, together with the usually ample provisions for draining each separate coil, prevents the accumulation of such large quantities of oil as are often lodged in expansion coils, condenser coils are exposed to another source of loss of efficiency from without. Where the available cooling water is abnormally hard, or carries a large amount of suspended matter, ammonia condensers, and especially steam condensers, soon become coated with a deposit of scale or mud, which, if not properly removed, becomes a more or less effective insulator according to the composition of the deposit. The heat conductivity of metallic surfaces is not the same per degree difference in temperature at medium and low as it is for high temperatures, and it does not therefore follow that the resistance offered by the scale accumulating on the outside of atmospheric and submerged ammonia and steam condensers is the same as that of scale on the inside of a boiler. However, some slight idea of the extent of the loss may be gained from the fact that in steam-boiler practice, the insulating effect of scale results in a thermal loss corresponding to about 2 per cent of the fuel for each 1-64 inch in thickness of scale.
<Callout type="tip" title="Tip:">Regularly cleaning condenser coils can prevent significant energy losses and improve system efficiency.</Callout>
It seems superfluous to state that the heat-absorbing surfaces of brine-cooling coils and the heat-radiating surfaces of condenser coils should always be kept covered with brine and condenser water respectively. Nevertheless, it is not uncommon to see refrigerating plants operating with the brine so low in the tanks that the top expansion coils are exposed and the distribution of water over the condensers so irregular that a large portion of the surface is dry.
Key Takeaways
- Regularly defrost direct-expansion and brine coils to maintain system efficiency.
- Use appropriate oils with low flash points for refrigerating systems.
- Purge condensers regularly to remove permanent gases.
- Clean condenser coils to prevent scale buildup.
Practical Tips
- Regularly check and defrost your refrigerator coils, even if they are not ice-coated, to maintain efficiency.
- Use high-pressure steam for cleaning expansion coils when necessary, but ensure proper equipment is used to avoid injury.
- Monitor the quality of cooling water to prevent scale buildup on condenser surfaces.
Warnings & Risks
- Improper defrosting methods can lead to uneven heating and potential damage to the system.
- Using hot brine for defrosting requires careful management to avoid overheating or damage.
- Neglecting regular cleaning of condenser coils can result in significant energy losses.
Modern Application
While the specific techniques described in this chapter are from a bygone era, the principles of maintaining system efficiency through proper cleaning and maintenance still apply. Modern refrigeration systems face similar challenges with oil management, scale buildup, and gas accumulation. Understanding these historical practices can provide valuable insights into modern survival preparedness, especially for those setting up or maintaining small-scale refrigeration units in remote locations.
Frequently Asked Questions
Q: How often should I defrost my refrigerator coils?
Defrosting frequency depends on the amount of ice buildup. Regularly check your coils and defrost them when they start to accumulate significant ice, as this can reduce system efficiency.
Q: What type of oil is best for refrigeration systems?
Very light mineral oils with a specific gravity between 0.924 and 0.88 and a flash point between 310° and 400°F are recommended to prevent stiffening at low temperatures and saponification by ammonia.
Q: How can I effectively remove oil from the expansion coils?
Pump out the coils, disconnect them, and blow them with high-pressure live steam. Ensure no moisture is trapped during this process to avoid condensation inside the pipes.