Psychrometric, heat recovery software
Psychrometric Air-Conditioning calculation (Android): a PsychroAC
calculate and plot air-conditioning cooling & dehumidification pyschrometric process; with precool air unit, heat pipe, run-around coil, heat recovery unit . . .
To take a glance at all pocketEngineer software and OS requirements, click Software List.
Go to a PsychroAC module:
Run-around Coil module
aPsychroAC: Run-around Coil (RAC) module
This standalone module is provided as a quick design and analysis tool for Run-Around Coil (RAC) heat recovery.
In air-conditioning applications, RAC is used to recover energy from cool exhaust air to precool incoming hot outdoor air. With RAC system, the exhaust and supply airstreams can be located far away from each other.
The program is suitable for:
- calculation of Supply Air (SA) or Conditioned Air (CA) conditions.
- calculation of energy recovery.
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
RAC system operates for sensible heat recovery only. Only Sensible Effectiveness is required as input.
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, Run-Around Coils or Loops have a sensible heat transfer effectiveness of 55% - 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 oC DB, 50% RH
EA airflow = 8250 CFM
EA conditions: 76 oC 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 - PAU with Run-Around Coil (SI units)
Cool toilet exhaust air is used to precool incoming outdoor air before entering precool air unit (PAU).
Given:
PAU OA ventilation = 45325 CMH
OA conditions: 34 oC DB, 28 oC WB
Toilet EA airflow = 14400 CMH
EA conditions: 24 oC DB, 18.1 oC WB
Effectivenes (sensible) = 55 %
Run-Around Coil inputs
Run-Around Coil results
Run-Around Coil results
Run-Around Coil psychro chart plot
Psychro Chart plot for Worked Example 2
SA = Supply Air
CA = Conditioned Air
OA = Outdoor Air
EA = Exhaust Air
DB = Dry bulb
WB = Wet bulb
Tdb = Dry bulb temp (o C)
RH = Relative humidity (%)
w = Moisture content (g/kg)
m = Mass flowrate (kg/s)
h = Enthalpy (kJ/kg)
Remarks:
(1) Run-around coil heat exchanger is chosen for its zero cross contamination.
(2) Also, it offers great flexibility in locations of the exhaust and supply airstreams.
