Thermal insulation optimum thickness

Typical dimensions for the /5-alumina electrolyte tube are 380 mm long, with an outer diameter of 28 mm, and a wall thickness of 1.5 mm. A typical battery for automotive power might contain 980 of such cells (20 modules each of 49 cells) and have an open-circuit voltage of lOOV. Capacity exceeds. 50 kWh. The cells operate at an optimum temperature of 300-350°C (to ensure that the sodium polysulfides remain molten and that the /5-alumina solid electrolyte has an adequate Na" " ion conductivity). This means that the cells must be thermally insulated to reduce wasteful loss of heat atjd to maintain the electrodes molten even when not in operation. Such a system is about one-fifth of the weight of an equivalent lead-acid traction battery and has a similar life ( 1000 cycles). 

SOLUTION A cylindrical oven is to be insulated to reduce heat losses. The optimum thickness of insulation and the potential earnings are to be determined. Assumptions 1 Steady operating conditions exist. 2 Heal transfer through the insulation is one-dimensional. 3 Thermal conductivities are constant. 4 The thermal contact resistance at the interface is negligible. 5 The surfaces of the cylindrical oven can be treated as plain surfaces since its diameter is large. 

Thermal insulation AU hot insulation shall have optimum thickness (excessive thickness can cause more heat loss due to extra surface area exposed), and the cladding by aluminium sheets must be leak-proof—no ingress of moisture or rainwater shall be possible. Likewise, no hot gases or heat from any source shall be allowed to enter a refrigerated system. 

Answer by Author We have checked the results obtained on the thermal conductivity apparatus using NRC-type and Linde-type multilayer insulation. We have found that results reported by the companies producing these materials agree with the values we have measured. However, we have found that the optimum values of thermal conductivity are reached with different numbers of radiation shields. This may be due to the ability to measure the thickness accurately and control the load application with a flat-plate thermal conductivity apparatus. 

In designing a storage vessel for a cryogenic liquid, there are several considerations that should be made Is the system intended for long- or shortterm usage What type of liquid will be stored, and what rate of liquid loss can be tolerated Will the system be mobile or stationary What acceleration effects are anticipated for the system How much time and cost will be allowed for cooldown What will be the stratification and pressure effects on the system What type and thickness of material will be required for the inner and outer vessels and what will be the thermal conductivity of this material Other important aspects include the optimum type of insulation and the design of inner vessel supports, fill and drain lines, vapor vents, vapor diffusers, antislosh baffles, and safety devices. 

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