54) For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference (tm) for parallel flow is __________ that for counter flow.

55) A vapour absorption refrigerator uses __________ as a refrigerant.

56) Thermal conductivity of air increases with rise in temperature.

57) The material of pipe lines for a system using ammonia as a refrigerant should be copper.

58) The wave length corresponding to the maximum energy is inversely proportional to the absolute temperature. This statement is called Planck's law.