Identifying and Resolving HVAC Stratification Issues

Air stratification is the tendency of two or more airstreams to remain separated in layers. Stratification is a common issue in air handling units when return air and outside air are introduced into the mixing box. Warm air naturally rises, but airstream velocity plays a large part in causing the air layers to remain separate. Numerous issues can arise where stratification exists, including freezestat trips, frozen coils, poor temperature control accuracy, insufficient fresh air distribution, poor economizer operation, and uneven velocity profile. Stratification can occur in both summer and winter.

The first step to correcting stratification is to perform a detailed temperature and velocity profile in the air handling unit to gain a better understanding of the issue, or a thermal imaging camera can be used. Once the cause of the stratification is identified, there are various options that can be used to correct the problem. These include baffles, dampers, and other blending devices. In some cases, there is not enough room to sufficiently blend the layers, and freezing can be avoided by adding glycol or other anti-freeze solutions to chilled water, or chilled water can be drained altogether. Averaging bulb thermostats can also be used to improve sensor accuracy.

About PJM

PJM Mechanical Contractors, Inc. is a multi-faceted mechanical contractor providing construction and installation, service and maintenance, and design/build services in a comprehensive range of mechanical disciplines that include HVAC, plumbing, controls, refrigeration, custom metal fabrication, piping, fire protection, and backflow preventer testing. PJM primarily serves clients in pharmaceuticals, manufacturing, healthcare, institutional, and other various commercial and industrial sectors.

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