If a pound of anhydrous ammonia is evaporated from —28.5°F in the flask, it absorbs 573 B.t.u., its latent heat of vaporization. Assuming no losses, this can cool three pounds of water at 79°F to freezing point and freeze it, requiring 47 B.t.u. per pound for cooling through 47 degrees and 144 B.t.u. per pound to overcome the latent heat of fusion. Three pounds of water X drop in temperature from 79 to 32 degrees X specific heat of water (unity) + three pounds of water X latent heat of fusion of water (144 B.t.u.) = latent heat of vaporization of ammonia required to cool and freeze the water.
After evaporation, ammonia must be liquefied again. This is achieved by applying pressure and providing a cooling medium like air or water; water is preferred due to its lower temperature and higher specific heat capacity compared to air. In an ice plant setup, an iron flask replaces the glass flask, with liquid ammonia escaping through a valve into an expansion coil where it vaporizes before being diverted either for absorption in water or compression back into liquid form.
Commercial systems consist of pipes inside coolers absorbing heat from products and outside condensers releasing heat to cooling media like air or water. A compressor is essential for raising the temperature and pressure of ammonia gas so that it can release heat to cooling water at ordinary temperatures, which must be a few degrees higher than the cooler's temperature.
Anhydrous ammonia boils under 16 pounds of pressure at 0°F and requires its boiling point raised by increasing pressure and temperature in order for heat transfer to occur with cooling media like water. In practice, almost any gas can serve as a working medium but anhydrous ammonia is most commonly used due to its practicality.
The direct-expansion compression system uses coils where ammonia evaporates directly from the atmosphere of cold-storage rooms, while the brine system circulates cooled sodium chloride or calcium chloride brine through air-cooling coils in cold-storage compartments. The congealing-tank system splits up the main brine tank into smaller tanks installed within each compartment to reduce costs and insulation losses.
<Callout type="important" title="Critical Efficiency Factor">Maintaining a sufficiently low ammonia evaporation pressure is crucial for both direct-expansion and brine systems, as it affects heat transfer efficiency.</Callout>
In the congealing-tank system, weak brine solutions are used to store more refrigeration in smaller volumes. This method avoids the costs associated with circulating pumps and insulation but requires careful management of ammonia pressure levels.
Key Takeaways
- Ammonia's latent heat of vaporization is crucial for refrigeration.
- Water and air are the most common cooling media, with water preferred due to its lower temperature and higher specific heat capacity.
- Commercial refrigeration systems require a compressor or generator to raise ammonia pressure and temperature for efficient heat transfer.
Practical Tips
- Use anhydrous ammonia as it is highly effective in commercial refrigeration systems.
- Maintain proper insulation around brine tanks and pipes to minimize heat loss.
- Ensure the cooling medium's temperature is a few degrees higher than the cooler's temperature for efficient operation.
Warnings & Risks
- Improper handling of anhydrous ammonia can be dangerous due to its toxicity and corrosiveness.
- Failure to maintain proper pressure levels in the system can lead to inefficiencies or equipment damage.
Modern Application
While this chapter focuses on early commercial refrigeration systems using ammonia, many principles remain relevant today. Understanding latent heat absorption and efficient heat transfer is crucial for modern refrigeration design. However, safety standards have improved significantly since 1912, making contemporary systems safer to operate.
Frequently Asked Questions
Q: What is the primary reason water is preferred over air as a cooling medium in commercial refrigeration?
Water has a higher specific heat capacity and can be maintained at a lower temperature compared to air, making it more effective for absorbing and transferring heat.
Q: How does the congealing-tank system differ from the brine-circulating system in terms of cost?
The congealing-tank system avoids the costs associated with circulating pumps and insulation by splitting up the main brine tank into smaller tanks installed within each compartment.
Q: What is the significance of maintaining a sufficiently low ammonia evaporation pressure in refrigeration systems?
Maintaining a sufficiently low ammonia evaporation pressure ensures efficient heat transfer from products to be refrigerated, which is critical for both direct-expansion and brine systems.