Flow control choices for air valves include unidirectional (one-way), bi-directional (two-way) or three-way.  In a one-way configuration the valve permits metered (restricted) flow in one direction and free flow in the reverse direction.  In a two-way configuration the valve provides for metered (restricted) flow in both directions.  In a three-way configuration the control provides flow control in three directions.  Flow compensation is an important consideration.  Choices include noncompensated, pressure compensated, and temperature compensated.  In a noncompensated flow control the amount of flow that passes through the orifice and the pressure drop across it are directly related. Flow through the valve varies with the fluid viscosity and pressure across the valve.  A pressure compensated flow control maintains accurate output flow regardless of the input pressure, up to rated limits. The orifice is adjusted to compensate for pressure variations and maintain a set flow rate.  A temperature compensated flow control maintains accurate output flow regardless of the input temperature, up to rated limits. Sharp-edged orifices or expansion rates of dissimilar metals are used to maintain a constant flow rate.  Air valves may have adjustability.  Choices include adjustable flow or fixed flow.  Important performance specifications to consider include metered flow for gas or air, and maximum operating pressure.Balance Valves

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