Question

Prior to the advent of refrigeration, many different methods were used to keep food cool and prevent it from spoiling. One common method was to use clay pots. In this approach, two clay pots, an inner pot and an outer pot, were separated from each other with a thick layer of wet sand in between. The inner clay pot would cool if the atmosphere was dryallowing the food placed inside to stay cool.

a. What was happening to allow the inner clay pot to cool? Explain.

b. Why did this method of cooling require the atmosphere to be unsaturated (RH < 100 %)?

Answer 1

Sol: a.

The inner clay pot cooling is based on the principle of evaporation cooling. The evaporation of water needs energy which is taken from the ambient air leading to a fall in air temperature ant the clay pot cooler uses this principle with. It consist of two differently sized clay pots, wet sand and a cloth. Smaller clay pot is placed inside the larger clay pot and the space between them is filled with sand. This sand is watered and subsequently the opening of the small clay pot should be covered with a wet cloth. Due to the porosity of fired clay, a part of the water can diffuse through the clay end evaporate on the outside. This evaporation process consumes thermal energy from inside of the clay pot cooler as a result their is a decreasing in air temperature in the inner clay pot.

Sol: b.

Cooling of different foods require different temperatures. This method of cooling required the atmosphere to be unsaturated because if the relative humidity(R_H) is consistently high, evaporative cooling will not be a viable option, and therefore another system needs be considered. When the relative humidity is low, only a small portion of the total possible quantity of water vapour that the air is capable of holding is being held, and also the air is capable of taking additional moisture. Low relative humidity increase transpiration rates. On the other hand, when the relative humidity is high, the rate of water evaporation is low, and therefore cooling is also low.