PE Mechanical common mistakes
PE Mechanical · Understand the traps and how to avoid them.
Applies the familiar 1.08 and 0.68 air constants to actual cfm at altitude or at nonstandard temperature without correcting for air density.
These constants are memorized from standard-air sensible and latent equations and are easy to use without checking the assumptions behind them.
What to do instead
Derive the constants from 60 times density times specific heat, or correct standard-air results for actual density using the site altitude and entering-air temperature; label each term with units.
Mixes two airstreams by averaging dry-bulb temperatures and humidity ratios weighted by volumetric flow (cfm) rather than mass flow.
cfm is the most visible flow value on drawings and schedules, and averaging temperatures feels intuitive.
What to do instead
Convert each stream to mass flow using cfm divided by specific volume, then mass-weight enthalpy and humidity ratio; verify the mixture state on the psychrometric chart.
Computes a cooling-coil load from dry-bulb temperature drop alone and omits the latent portion and bypass factor.
The sensible equation is simpler and is often the first one learned, while latent load requires humidity-ratio or enthalpy differences.
What to do instead
Use entering and leaving air enthalpies and humidity ratios to separate sensible and latent loads; determine apparatus dew point and bypass factor from the coil condition line.
Adds every room peak load together and sizes central equipment to that sum without applying diversity or coincidence factors.
Summing peak values is conservative and avoids judgment about schedules, but it overstates block load.
What to do instead
Apply documented ASHRAE diversity and coincidence factors, compare sum-of-peaks with block load, and state the basis for the selected factor.
Treats outdoor-air ventilation load as sensible-only when the outdoor dew point is above the room dew point.
Candidates focus on the temperature difference and forget that moisture removal can dominate the ventilation load.
What to do instead
Calculate both sensible and latent outdoor-air loads from enthalpy and humidity-ratio differences, and include them in the coil and equipment loads.
Claims energy-recovery savings from effectiveness alone without subtracting the added fan and duct pressure drop, defrost energy, and control penalty.
Effectiveness is a single attractive number and appears to represent free energy recovery.
What to do instead
Perform a net annual energy analysis that includes supply and exhaust fan power, added pressure drop, frost control or defrost, and part-load control effects.
Selects a fan or pump from a catalog curve at standard air or water and applies affinity laws without correcting for actual density, speed, or viscosity.
Catalog data are normally shown at standard conditions, and the correction steps are easy to skip under time pressure.
What to do instead
Correct fan power for actual density and speed, correct pump performance for viscosity and speed, and verify the operating point at the actual system curve.
Uses COP, EER, and kW/ton interchangeably or treats Carnot COP as the actual cycle COP.
The three efficiency metrics are common in refrigeration practice, and each has a different numerical scale.
What to do instead
Convert consistently using COP equals EER divided by 3.412 and kW/ton equals 12 divided by COP; compute actual cycle performance from real state points, not Carnot temperatures.
Sets a proportional-only controller and ignores offset, or interprets proportional band as a temperature range rather than a percentage of sensor span.
Proportional band, gain, and offset are related by formulas that are easy to confuse, and controller documentation uses different conventions.
What to do instead
Convert proportional band to controller gain using the sensor span, calculate the steady-state offset for the expected load, and add integral reset when offset is unacceptable.
Enters refrigerant property tables with gauge pressure instead of absolute pressure, or reads saturated vapor enthalpy for a superheated state.
Pressure gauges read psig, while refrigerant tables require psia, and the saturated and superheated columns are adjacent.
What to do instead
Add local atmospheric pressure to convert psig to psia before table lookup, and confirm the state region from temperature and pressure before selecting enthalpy or entropy.