ducts

This report is from a study of the performance of eight exhaust systems of a large medical center in Tennessee. The mechanical contractor of the project initiated this study when confronted with a claim of unacceptable deficiencies in all building exhaust systems. These systems were reportedly exhausting 32% to 43% less than design airflow as a consequence of excessive leakage in the duct system.

A simple duct system was installed in an attic test module for a large-scale climate simulator at a U. S. national laboratory. The goal of the tests and subsequent modeling was to develop an accurate method of assessing duct system performance in the laboratory, enabling limiting conditions to be imposed at will and results to be applied to residential attics with attic duct systems. Steady-state tests were done at a severe summer condition and a mild winter condition. In all tests the roof surface was heated above ambient air temperatures by infrared lights.

Traditionally, ventilation and air conditioning ducts have been manufactured with rectangular cross sections. The rectangular duct can easily be adapted, e.g. to restricted ceiling voids and plant rooms, however, often at the cost of efficient airflow design and possible cost savings. A round duct system normally performs better and offers a more economical solution, which is the reason why it always pays to study and compare the ductwork design before selecting which type is to be used for a specific application.

Describes the development of the Swedish duct tightness guidelines, the "AMA system". The latest version, due in 1998, aims to increase tightness requirements once again by introducing a tightness class D as the standard requirement for larger spiroduct systems. The concern about an increasing part of the Swedish population becoming allergic and asthmatic led to the Swedish Parliament introducing compulsory inspections of ventilation systems in 1990.