Hot Gas Bypass - Trane Odyssey TTA0724 A/D B Series Application Manual

Hot Gas Bypass

Many years ago, hot gas bypass (HGBP) was added to
HVAC systems to correct a number of operational
problems. Hoping to avoid such problems altogether, it
eventually became common practice for designers to
specify hot gas bypass in systems. Unfortunately, the
practice often degraded rather than improved reliability.
Hot gas bypass increases the minimum refrigerant charge;
it also inflates the first cost of the system. Besides adding
more paths for potential refrigerant leaks, hot gas bypass
increases the likelihood of refrigerant distribution and oil
return problems. Finally, hot gas bypass uses excessive
amounts of energy by preventing the compressors from
cycling with fluctuating loads.
Trane now has more than 15 years experience in the
successful use of packaged rooftop equipment without hot
gas bypass in commercial comfort-cooling applications. To
prevent evaporator freeze-up, our equipment typically
includes Trane® Frostat™ coil frost protection.
Like hot gas bypass, the Frostat system protects the coil
from freezing, but it does so by turning off compressors at
14
the Frostat sensor detects the conditions suitable for
evaporator coil frosting. The compressor is released to
operate when the coil temperature rises a few degrees
above the frost threshold. The Frostat control strategy
reduces the overall energy consumption of the system
while maintaining system control.
Systems should be designed to avoid HGBP whenever
possible. However, HGBP is necessary for some
appreciations such as 100% OA.
When using HGBP or a Rawal APR valve, the total "linear"
line length should be limited to 75 feet.
Additional Resources
• Refer to the Trane Application Guide Hot Gas Bypass
Installation Guideline for Direct Expansion (DX)
Equipment (APP-APG017*- EN) for more information
such as design, pipe size, and maximum hot gas
bypass length.
• Refer to the Engineers Newsletter, "Hot Gas Bypass –
Blessing or a Curse?" (ADM-APM007-EN).
SS-APG008H-EN