Saturday, May 16, 2020

HP 42S/Free42/DM42: Pulley Sheave and Fan Laws

HP 42S/Free42/DM42:  Pulley Sheave and Fan Laws

Basic Pulley Law - Fans

The basic relationship between a fan and it's motor is simple:

rpm1 * dai1 = rpm2 * dia2

where:

rpm1:   how fast the fan turns, in revolutions per minute
dia1:  the diameter of the fan's pulley
rpm2:  the speed of the motor, in revolutions per minute
dia2:  the diameter of the motor's pulley

A diagram is shown below of a motor-pulley/fan-pulley system.  The actual length of the pulleys will vary per system. 




The program PULLAW sets up the solver for this law.  To call up the solver, press [(shift)] [ 7 ] (SOLVER), select PULLAW.  

HP 42S/DM42/Free42 Solver Program: PULLAW

00 { 59-Byte Prgm }
01▸LBL "PULLAW"
02 MVAR "RPM1"
03 MVAR "DIA1"
04 MVAR "RPM2"
05 MVAR "DIA2"
06 RCL "RPM2"
07 RCL× "DIA2"
08 RCL "RPM1"
09 RCL× "DIA1"
10 -
11 END

Basic Sheave Law

The basic sheave law listed below describes how adjusting a sheave's airflow affects the motor's break horsepower:

bhp2 = (cfm2 / cfm1) ^ 3 * bhp1

CFM:  cubic foot per minute (airflow)
BHP:  brake horsepower of the motor 

The program SHLAW sets up the solver for this law.  To call up the solver, press [(shift)] [ 7 ] (SOLVER), select SHLAW.  

HP 42S/DM42/Free42 Solver Program: SHLAW

00 { 60-Byte Prgm }
01▸LBL "SHLAW"
02 MVAR "BHP1"
03 MVAR "CFM1"
04 MVAR "BHP2"
05 MVAR "CFM2"
06 RCL "CFM2"
07 RCL÷ "CFM1"
08 3
09 Y↑X
10 RCL× "BHP1"
11 RCL- "BHP2"
12 END

Basic Fan Laws

Three basic ratios are used in dealing in fan engineering, which are:

cfm ratio = cfm2 / cfm1 = rpm2 / rpm1

sp ratio = sp2 / sp1 = (rpm2 / rpm1) ^ 2

hp ratio = hp2/ hp1 = (rpm2 / rpm1) ^ 3

where:

rpm:  a fan's revolution per minute.   rmp1 and rpm2 represents different conditions of the fan

cfm:  airflow of a fan in cubic feet per minute.  You can use other units, just be consistent.

sp:  static pressure of a fan

hp:  break horsepower of a fan

HP 42S/Free42/DM42 Program: FANLAW

00 { 74-Byte Prgm }
01▸LBL "FANLAW"
02 "RPM2?"
03 PROMPT
04 "RPM1?"
05 PROMPT
06 ÷
07 STO 01
08 CLA
09 "CPM RATIO ="
10 AVIEW
11 STOP
12 ENTER
13 ENTER
14 ENTER
15 ×
16 STO 02
17 CLA
18 "SP RATIO ="
19 AVIEW
20 STOP
21 ×
22 CLA
23 "HP RATIO ="
24 AVIEW
25 .END.

Source:

Gladstone, John  Air Conditioning Testing and Balancing: A Field Practice Manual  Van Nostrand Reinhold Company.  New York.  1974  ISBN 0-442-22703-5

Barring any changes, this would be post 1111.   I know some people are still into the 11-11 thing... 

Eddie


All original content copyright, © 2011-2020.  Edward Shore.   Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited.  This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.