THERMAL CALCULATIONS


Tool : Airflow and pressure loss calculations, EBM PAPST PLC.


POWER SUPPLY SELECTION MATRIX

No. of Slots : 06 x 6U Compact PCI

Wattage per slot : 50 watts Max.

Total Power required : (50 x 6) + 50 W for accessories 350 W

Power supply wattage : 600 W

Max. Load with 75% eff. : 450 W

Please refer to the above matrix the power supply requirement has been selected with enough back up as far as the wattage needs are considered.

The air flow needs of the enclosure are calculated considering the delta in temperature between the ambient and internal to the enclosure. Please find below various matrices for different delta T’s. Delta T is the difference in temperature between the ambient air outside the system and the maximum permissible operating temperature within the system. For example some applications have a permissible temperature rise of 10 degrees within the system. The maximum temperature could be governed by a particular component rating. For various iterations we are considering the Delta T from 3 degree C onwards.
After entering the values in the two fields press CALCULATE. Two figures (M3/sec and CFM) will then appear that represent the airflow required to eradicate the level of waste heat.
Having calculated the airflow needed it is then necessary to establish what back pressure the fan needs to overcome to deliver the required flow rate.
In a large number of applications air is either blown into a cabinet or enclosure or extracted from it. This may occur through a large singular square slot or round hole, or alternatively, a series of slots and holes in varying arrangements. For example if a large square aperture has a series of slots punched out in the metal work in a grid like arrangement. Unless the openings be they square or round are singular, the design reduces the open area available for air to move through and increases the back pressure the fan has to work against.
The second stage of the tool therefore calculates the overall smallest available "open" area for air to pass through and culminates in a figure for back pressure in PASCAL’s. By clicking on either the SQUARE SLOT tab you will automatically select the input fields for the relevant design.
Using the square slot as an example, first enter the number of slots; this may be one entire slot or more usually a number of slots. Then enter the length and breadth (mm) of the slots. By pressing CALCULATE the result will be shown in meters2 and is the total available open area for air to travel through. The very last data field will now also show the Pressure loss the fan needs to overcome to deliver the required airflow.
This will now provide you with two measurements
1) Airflow in CFM needed to remove the waste heat form the system. This has been calculated based on the inputted data of Power (watts) and the permissible temperature rise (DELTA-T) within the system.
2) Pressure in PASCAL’s that the fan needs to overcome to deliver the calculated required airflow.
These values can now be used to help determine the correct fan or fans needed to cool your application by matching up your pressure and airflow results to the fan performance curves available under the data sheet section of our website. You may also use the Airflow result as one of the specifying criteria for our interactive fan selector
Even if you have a large singular slot or hole creating minimal back pressure there will most likely be a requirement for you to fit a protective finger guard (s) to the system that will have an effect on flow rate. Depending on the type of guard metal or plastic, the open area for air to flow through will be typically decreased by 7-10%. In this design the air entry opening is 120mm x 60 mm. So typically for pressure loss calculations we need to consider only 10% less for practical calculations. So an effective honeycomb opening of 110mm x 55mm will result in a pressure loss of 14.848 pa.
The design tool will give good approximations for the required airflow and the back pressure the fan(s) have to overcome to deliver it. However due to the wide variation in system design, particularly component layout, architecture and densities, the tool has to assume an unpopulated system.
Therefore once the pressure drop figure in PASCAL’s is obtained, it is possible to increase the accuracy of the results by factoring in the effects of the system being full of electronics.
For example: if it is estimated that the system will be 80% full of components, simply take the pressure loss result and add 80% of it again. This will not account for the particular configuration of the system in terms of how and where components are located and the airs' flow path, but will help to account for increased overall density.
In Project Code #0900412001 the above example holds good, so for the fan selection the select a pressure drop of 14.848 plus 80%. i.e. 26.73 pa.

For detailed calculator please log on to http://www.papstplc.com/tools/calculators/calculators


The general data fed into the calculator for the air flow and the pressure drop is as follows:

Power(watts) : 450
No. of slots : 06
Length(mm) : 60
Breadth(mm) : 120

i) When Delta T is 3 deg C
1. The air flow requirement in CFM is 277.513
2. The pressure drop in pa is . 26.73

ii) When Delta T is 4 deg C
1. The air flow requirement in CFM is 208.135
2. The pressure drop in pa is . 26.73

iii) When Delta T is 5 deg C
1. The air flow requirement in CFM is 166.508
2. The pressure drop in pa is . 26.73

iv) When Delta T is 6 deg C
1. The air flow requirement in CFM is 138.757
2. The pressure drop in pa is . 26.73

v) When Delta T is 7 deg C
1. The air flow requirement in CFM is 118.934
2. The pressure drop in pa is . 26.73

vi) When Delta T is 9 deg C
1. The air flow requirement in CFM is 92.504
2. The pressure drop in pa is . 26.73
vii) When Delta T is 8 deg C
1. The air flow requirement in CFM is 104.067
2. The pressure drop in pa is . 26.73

viii) When Delta T is 10 deg C
1. The air flow requirement in CFM is 83.254
2. The pressure drop in pa is . 26.73

Figure 1: fan selection Matrix

Figure 2: Flow rate vs. Pressure loss



Considering the above chart one fan P/N 5112N each for suction and exhaust is considered enough to keep the Delta T within the admissible limits. The fan selected is of 147 CFM and the pressure drop is about 26.73 Pascal. Both criteria are met as per the chart above. More over we proposed to use one fan for forced air ventilation at the sides and two similar fans for suction one each for the electronics and the power supply unit.