30 May 2005

UNITARY A/C: THE INFLUENCE OF PRODUCT DESIGN AND AIR FLOW RATES ON SENSIBLE HEAT RATIO


Concerns continue to persist that higher efficiency unitary A/C designs are developed at the
expense of higher Sensible Heat Ratios (SHRs) and degraded dehumidification. In this paper,
SHR is investigated from a fundamental psychrometric standpoint. The factors influencing SHR
are identified and related to the system design variables that influence seasonal energy efficiency
ratio (SEER). In the end, it is shown that SEER has very little impact on SHR, while indoor
airflow is the major variable, which was probably well understood by the earlier industry pioneers
who wisely set the limiting rating condition at 450 scfm per ton (37.5 scfm per 1000 Btuh) of
rated capacity. (Note: “scfm” stands for “standard cfm,” the air flow rate for the standard air
density of 0.075 lbm/ft3).