Design-calculations: With the exception of the proprietary system mentioned which can be purchased as a package, the design calculations for these systems are largely empirical and little performance data are available for the user. In normal ventilation practice, air is considered to be incompressible since static pressures vary only slightly from atmospheric pressure. However, in LVHV system the extreme pressures required introduce problems of air density, compressibility, and viscosity which are not easily solved. Also, pressure drop data of small diameter pipe, especially flexible tubing, is not commonly available. For practical purposes, the turbine exhaust flow rate required. Resistance in the pipe should be kept as low as possible; flexible tubing of less than 1- to 1.5-inch diameter should be limited to 10 feet or less. In most applications, this is not a severe problem.