PL-Asrcf? FIG. 5.— STANDARD WALL. T2-T3 the losses or transmission by conduction through the material, and T3-T4 convection and radiation from the cold air in the box to the inner surface. The convection and radiation drop is caused by a very thin layer or surface film surrounding each surface. Fig. 5 shows a standard wall as used in many cold storage buildings and ice storages. The unit transmission through the combination wall can very easily be determined from the following formula : B.t.u./ sq. ft./hr./°MD = 1 / X Ki C . Ci C2 Ca 110 HOUSEHOLD REFRIGERATION X being the thickness of the material, C the unit coefficient for each material as in Table XLII, K^ and K^ surface coefficients for the inner and outer surfaces respectively. Substituting— B.t.u./sq.ft./hr./°MD=: 1 / 13 4 1 1 1.10 5 .279 6.25 4.2 If this value is now used as the factor 'C' in the thickness fundamental formula (1) given on page 101, the total heat leakage through the walls can be obtained. It has been found that the resistance to heat flow at the surface due to radiation and convection is very small in comparison to internal thermal conductivity of the material itself, so these two factors can be omitted, particularly when good insulation of normal thickness is used. This can be demonstrated by the omission of the factors K^ and Kg in the above formula resulting in a final value of .0585 instead of .0548 or 6% greater. Referring to Fig. 6, a cross section of a standard ice box is given. Assuming this to be a 9 cu. ft. refrigerator, the outside surface would be 54 sq. ft. and the inside surface 34.5 sq. ft. or an average of 44.25 sq. ft. The unit heat transmission would be = .1225 B.t.u./sq. ft./°MD/hr. <Callout type="important" title="Critical Formula">The formula for calculating the total heat leakage through walls is crucial for designing efficient refrigerators.</Callout> Insulation.— The most important factors entering into the choice of an insulator are as follows : 1. Thermal conductivity. 2. Odorless and sanitary. 3. Compact. 4. Vermin and fire resistant. 5. Not easy to disintegrate or settle. 6. Durable in service. 7. Reasonable in cost. 8. Structurally strong and easy to ship, handle, and install. 9. Conform to variation on surface of lining.
Key Takeaways
- Understanding heat transfer methods is crucial for designing efficient refrigerators.
- The choice of insulation material depends on several factors including thermal conductivity and cost-effectiveness.
- Air spaces are not ideal insulators due to high radiation loss.
Practical Tips
- Use corkboard or mineral wool as they offer good thermal resistance and durability.
- Ensure the insulation is compact, structurally strong, and easy to install for practical use.
Warnings & Risks
- Radiation and convection losses at surfaces can significantly impact heat transfer efficiency if not properly managed.
- Improper installation of insulation materials may lead to moisture issues affecting performance.
Modern Application
While the chapter focuses on historical refrigeration techniques, understanding these principles remains crucial for modern survival preparedness. Knowledge of thermal conductivity and efficient insulation is vital in building emergency shelters or maintaining food preservation systems during power outages.
Frequently Asked Questions
Q: What are the key factors to consider when choosing an insulator?
The most important factors include thermal conductivity, odorlessness, compactness, resistance to vermin and fire, durability, cost-effectiveness, structural strength, ease of handling, and adaptability to surface variations.
Q: Why are air spaces not ideal for insulation in refrigerators?
Air spaces have high radiation loss due to the large temperature differential between walls, making them less effective insulators compared to materials like cork or mineral wool which reduce heat transfer more efficiently.
Q: What is the significance of surface coefficients K1 and K2 in the formula for calculating heat transmission?
Surface coefficients K1 and K2 represent the resistance to heat flow at the inner and outer surfaces respectively, accounting for radiation and convection losses. Their inclusion or omission can significantly affect the calculated value of heat transmission.