Water is often available but at a prohibitively high temperature, with specific heat of .2377. Its weight is only .0706 per cubic foot, requiring 69.6 cubic feet to absorb one B.t.u., or 16,136,000 cubic feet for the negative heat of one ton of refrigeration. Water's specific heat being 1.0 and its weight 62.5 pounds per cubic foot means only .016 cubic feet is needed to absorb one B.t.u. Assuming a rise in temperature of one degree, 4,608 cubic feet will carry away 288,000 B.t.u., or an amount equivalent to the negative heat of a ton of refrigeration. Water's comparatively small specific heat allows us to employ it only because of its cheapness. When water is too expensive, air is used instead due to its extremely low specific heat.
The temperature of boiling-point and latent heat of evaporation of water is unfortunately too high for direct use in refrigeration systems at usual working pressures. Ammonia might be employed as a working medium in steam systems for power production, but commercially it's impractical. In refrigerating systems, ammonia or other mediums are used to splice out water which is the real arm of every form of heat dredge.
The striking similarities between a steam and a refrigerating system make such a system convenient for illustrating the principles of operation of a direct expansion refrigerating system. Except that the cycle of operations is reversed, a steam system consisting of boiler, engine, surface condenser, and means of returning condensation to the boiler, is mechanically and thermodynamically an almost exact counterpart of a refrigerating system.
<Callout type="important" title="Key Principle">The fundamental principle in both systems is that heat always flows from higher temperature to lower temperature.</Callout>
In these two similar systems, shell boilers have their counterparts in coolers of the shell type and water tube boilers in direct expansion coils. Each is a receptacle for evaporation of its respective working medium, accomplished by absorption of heat liberated in the combustion of coal or animal/solar heat.
Refrigerating furnace gases and cold storage air: Heat gravitates from furnace gases to water in boiler because it's at lower temperature; water is heated and furnace refrigerated. In refrigeration systems, heat gravitates from comparatively hot air of cold-storage compartment to ammonia in expansion coils because the ammonia is at a lower temperature.
<Callout type="warning" title="Safety Hazard">Condensation cannot take place if outside air or cooling water is far above vapor temperatures.</Callout>
The difference lies in that whereas in steam systems, vapors are hotter than needed to be condensed by warm room air; in refrigeration systems, ammonia is not hot enough to condense at atmospheric temperature. Ammonia vapors must be raised before they can absorb more heat in expansion coils and boiler.
<Callout type="tip" title="Practical Tip">Use steam-driven compressors to raise ammonia vapor temperatures for effective condensation.</Callout>
In absorption systems, a generator brings steam and ammonia into close proximity. The object of this chapter is to continue the analogy between flow of water and heat, making more practicable concrete application of involved principles.
Water flows from one place to another only when there's difference in pressure or static head; heat flows from higher temperature to lower temperature. Rate at which either water or heat will flow is directly proportional to difference in pressure/temperature tending to cause the flow and inversely proportional to resistance opposing that flow.
Key Takeaways
- Water's specific heat is significantly higher than air, making it more efficient for absorbing heat but less practical due to volume requirements.
- Ammonia and other refrigerants are used in modern systems because they can reach lower temperatures compared to water.
- The principles of steam engines and refrigeration systems are fundamentally similar, with the cycle reversed in refrigeration.
Practical Tips
- Use ammonia or other suitable refrigerants for efficient heat absorption at low temperatures.
- Understand that heat always flows from higher temperature to lower temperature, just like water flows downhill.
Warnings & Risks
- Condensation cannot occur if the cooling medium is too warm compared to vapor temperatures.
- Direct use of water as a refrigerant is impractical due to high boiling point and latent heat requirements.
Modern Application
While this chapter focuses on historical refrigeration systems, understanding these principles remains crucial for modern survival preparedness. The concepts of heat transfer and the mechanics of refrigeration are still relevant in designing efficient cooling solutions during emergencies or off-grid living scenarios.
Frequently Asked Questions
Q: Why is water less practical than ammonia as a refrigerant?
Water's boiling point at atmospheric pressure is too high, making it impractical for direct use in refrigeration systems. Ammonia can reach lower temperatures and thus is more suitable.
Q: What are the key similarities between steam engines and refrigeration systems?
Both systems involve cycles where a working medium (steam or ammonia) absorbs heat, expands to do work, and then releases heat as it condenses. The main difference lies in the direction of energy flow.
Q: How does the principle of heat transfer apply to refrigeration?
Heat always flows from higher temperature to lower temperature. In refrigeration systems, this means transferring heat from a warmer area (e.g., inside a cold storage room) to a cooler one (e.g., outside environment).