High pressure, high velocity duct systems have been gaining in popularity due to cost savings resulting from reduced space and material requirements. Compact duct systems hold particular advantage for use in multi-story buildings. The high construction cost on a per sq ft basis put a premium on reducing the space needed for air distribution systems. But use of compact, high velocity systems led to noise problems. This in turn generated interest in and use of duct liners to absorb the unwanted sound. Rather significant use of duct liners, based on noise considerations, had already developed when the "energy crisis" made the thermal insulation of duct systems highly desirable. Now the combined use of duct liners to control noise and to reduce energy loss has generated a booming duct 1iner market. The use of thermal and/or acoustical insulations as air duct liners is at an all-time high and growing rapidly. Design data, and in particular friction loss data, were nearly nonexistent. The designer of duct systems had very meager data on 1iner resistance. (See Fig. 1.) As the number of liner types and methods of manufacture grew, the design problem became more confusing. The 1972 ASHRAE Handbook of Fundamentals [1] states on page 469: Presently there is little information on friction loss in fibrous glass ducts or ducts lined with acoustical material. Refer to manufacturer's published data for friction loss in fibrous glass ducts."

Manufacturers I data on friction loss is rather spotty. Some manufacturers supply extensive data while others supply none, and the reliability and consistency of the available data is unknown. It appears that each manufacturer may use their own test method as no industrywide standard method is available.

This situation prompted ASHRAE' s Technical Committee TC5.2, Duct Design, to initiate a research study on duct 1iner flow losses. The study was designated ASHRAE RP-176, Energy Loss Due to Friction of Thermal and/or Acoustical Insulations When Used as Air Duct Liners. It was established to determine the pressu~e drop characteristics of straight rectangular ducts when 1ined with various types of fiber glass liner. This paper summarizes the results of that study.