The OA inputs accept Conditioned Air (CA) conditions such as treated outdoor air from Precool Coil / Precool Air Unit (PAU), Heat Recovery Wheel (HRW), Run-Around Coil (RAC), Heat Pipe (HP), etc.

Precool Coil / PAU, HRW, RAC and HP calculations are included as separate modules with "Export CA" feature. The calculated SA (CA) conditions can be fed (exported) to the main AHU module as precooled (treated) outdoor air. You can recall back the PAU, HRW, RAC or HP module with its current activity's state retained. 

Highlights:

• Carrier ESHF (Effective Sensible Heat Factor) method or Supply Air Temperature method

• Recirculating or 100% OA (for SA Temperature method only) system

• Reheat option

• Fan heat gain option (draw-thru arrangement only)

• Plot & save psychrometric chart

• Built-in guides and explanations

• rhoAIR module

• Air mixing psychrometric module

• Precool Coil / Precool Air Unit (PAU) module

• Heat Recovery Wheel (HRW) module

• Heat Pipe (HP) module

• Run-Around Coil (RAC) module

• print results in pdf (for some modules only)

• in SI-IP Units

Design Approach explained . . . 

Two widely used design approaches to psychrometric air-conditioning calculations are modelled in aPsychroAC. They are

• Carrier ESHF (Effective Sensible Heat Factor) method

• SA (Supply Air) Temperature method.

In the Carrier ESHF method, you start with specifying the coil performance, i.e., coil BF (bypass factor) that is generally representative of the type of equipment you intend to select. The supply air temperature condition is then calculated.

In the SA Temperature method, you start with fixing the supply air temperature condition. The coil performance is then calculated. The norm range of design temperature difference between SA and RA temperatures is about 15 - 22 oF (8 - 12 oC) with a common design target of 18 - 20 oF (10 - 11 oC).

The two methods give comparable results but not exactly the same (in aPsychroAC) for the following reasons:

• in Carrier ESHF method, fixed constant is used in the governing equations as given in Carrier system design manual. In this case, Standard air flowrate is used.

• in SA Temperature method, mass flow rate and specific volume of the state-point air is calculated. In this case, Actual air flowrate is used. (Did you know ... Actual vs Standard air flowrate ? see Wiki here ... )

Re-circulating or 100% OA System . . .  

Both re-circulating and 100% OA systems can be performed. For 100% OA system, it is more friendly to adopt the SA Temperature approach. Therefore, the option of 100% OA system is limited to the SA Temperature method. For 100% re-circulating system, input OA Airflow = 0.

Psychrometric Chart . . .  

No more manual plotting of psychro chart. With aPsychroAC, psychro chart showing the air-conditioning process is automatically plotted for each calculation. The psychro chart tells you the results in a graphical manner to visualize the process as shown in the chart below. The psychro chart can be saved as an image file for your calculation report.

OA=Outdoor Air; MA=Mixed Air; RA=Room Air; SA=Supply Air; off=off-coil; on=on-coil; ADP=coil ADP; GSHF=Grand Sensible Heat Factor; RSHF=Room Sensible Heat Factor

For digital psychrometric chart plotting, see Psychro Visual (Windows OS program)

Input Data. . .  

Only two information is required from your cooling load calculation, i.e., RSH (room sensible heat) and RLH (room latent heat).

OA ventilation airflow is usually determined by nos. of people x recommended outdoor air requirements.  

Effect of Coil BF (Bypass Factor) . . .  

Coil BF represents that portion of the air which is considered to pass through the coil completely unaltered.

Smaller BF has the following effects:

• higher coil ADP; increases apparatus COP

• lower supply air temperature (larger differential temperature); less supply airflow, smaller fan

• more rows of coil (more coil surface); higher fan static

Cooling coil is not 100% efficient. Some of the air going through the coil is not cooled to the coil ADP temperature. In actual air-conditioning cooling coil application, the BF will be greater than zero. Carrier system design manual recommends the typical coil BF for various design applications as follows:

Relationship of Coil ADP, Supply Air & Chilled Water Temperature . . .  

Coil ADP (Apparatus Dew Point) or Effective Surface Temperature is the coil surface dew point temperature required to accomplish a cooling & dehumidifying process. Coil ADP is at the point where the GSHF line crosses the saturation line on the psychrometric chart. The relationship of coil ADP to SA & chilled water temperature is as follows:

Chilled Water Temp < Coil ADP < SA Temp 

In DX system, lower coil ADP means lower saturated suction (refrigerant) temperature at the compressor inlet. In chilled water system, lower coil ADP means lower chilled water supply temperature. This results in less efficient range (higher kW/Ton) of operation.

In aPsychroAC, you will be prompted when no coil ADP is found, i.e., the ESHF line or GSHF line does not intersect the saturation line as displayed below: