Feature . . . Export CA Conditions  

The calculated SA (CA) Tdb, RH and airflow can be fed (exported) to the main AHU module as precooled (treated) outdoor air. The current activity's state is retained when you recall back this module for input changes, etc.

Design Approach explained . . . 

Sensible vs Enthalpy Wheel

Two models are provided:

• Sensible-only wheel recovers sensible heat only. Only Sensible Effectiveness is required as input.
• Enthalpy wheel recovers both sensible + latent heat. Sensible Effectiveness + Latent or Total Effectiveness are required as inputs.

Some latent benefit is achieved if the hot humid outdoor air stream is cooled sufficiently below its dew point to condense moisture on the evaporator end of the HP. Condensation occurrence can enhance sensible effectiveness.

Airflow and Energy Recovery

If Standard (nominal) Airflow option is selected, all airflows calculated are standard airflow. Standard equations with fixed constant are used for energy recovery calculations as follows:

• Sensible heat = 1.20 x airflow x dT (SI units)
• Latent heat = 3.0 x airflow x dw (SI units)

If Standard (nominal) Airflow option is turned off, all airflows calculated are actual airflow, i.e. with air density variations. The airflow is calculated at the state point conditions. The governing equations used for energy recovery calculations are as follows:

• Sensible energy = m x Cp x dT.
• Latent energy = m x hfg x dw.

The calculations assume no heat transfer between heat exchanger and its surroundings, nor are there gains from cross leakage, fans or frost control, etc.

Effectiveness. . .  

For air-to-air heat exchangers, ASHRAE Standard defines effectiveness (E) as

E = Actual transfer of moisture or energy / Max possible transfer between airstreams

Assuming no water vapor condensation occurs, the leaving supply air condition is

SA = OA - {Es * [Cmin / ms*Cp] * (OA-EAin)}

and the leaving exhaust air condition is

EAout = EAin + {Es * [Cmin / me*Cp] * (OA-EAin)}

where

Es = Sensible Effectiveness.

SA, OA, EAin, EAout = dry bulb temperature (locations as shown in the schematic diagram above).

ms = mass flow rate of supply air

me = mass flow rate of exhaust air

Cp = specific heat.

Cmin = smaller of (ms*Cp) and (me*Cp).

Typically, Heat Recovery Wheels have a sensible heat transfer effectiveness of 50% - 80% and total effectiveness of 55% - 85%.

Sensible Eff > Total Eff > Latent Eff

Unbalanced airflow reduces heat transfer, even though it increases the HRW effectiveness. As flow ratio increases, effectiveness decreases.

Psychro Chart . . .  

For better understanding, a Psychro chart showing the OA to SA (CA) process is plotted for each calculation. The psychro chart can be saved as an image file.

Calculation Examples . . .  

The following design calculations are done with aPsychroAC program.

Worked Example 1 (IP units)

Given:

OA airflow = 8500 CFM

OA conditions: 95 ^oF DB, 81 ^oF WB

EA airflow = 10600 CFM

EA conditions: 75 ^oF DB, 63 ^oF WB

Effectivenes (sensible) = 70 %

Effectivenes (total) = 56.7 %