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-only Heat Recovry

HP heat exchanger is useful for sensible-only heat transfer, i.e., no moisture is transferred. Only Sensible Effectiveness is required as input.

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)

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.

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 Pipes have a sensible heat transfer effectiveness of 45% - 65%. Effectiveness decreases as coil face velocity increases; Effectiveness increases as numbers of coil row increases (and pressure drop increases).

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 = 8250 CFM

OA conditions: 90 ^oF DB, 50% RH

EA airflow = 8250 CFM

EA conditions: 76 ^oF DB, 60% RH

Effectivenes (sensible) = 58 ％

SA = Supply Air

CA = Conditioned Air

OA = Outdoor Air

EA = Exhaust Air

DB = Dry bulb

WB = Wet bulb

Tdb = Dry bulb temp (^o F)

RH = Relative humidity (%)

w = Moisture content (lb/lb)

m = Mass flowrate (lb/min)

h = Enthalpy (Btu/lb)

Worked Example 2 - Precooling Air Reheater (SI units)

Wrap-around Heat pipe heat exchanger is used as precooling air reheater in a dedicated outdoor air system (DOAS). Precool heat pipe coil is to precool outdoor air, and reheat heat pipe coil is to reheat the leaving air after the DOAS cooling coil. The reheated air is to be around 24 ^oC (desired neutral temperature) for direct delivery into an air-conditioned space.

Given:

OA airflow = 1.6 CMS

OA conditions: 35 ^oC DB, 27 ^oC WB

EA airflow = 1.6 CMS

EA conditions: 11.2 ^oC DB, 11 ^oC WB

Effectivenes (sensible) = 58.4 %