REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - mixed air temperature, outdoor, return

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - mixed air temperature 

PROBLEM:

An air-conditioning system draws 10% outdoor air at a dry bulb temperature of 42 C / 108 F. If the return air is at 28 C / 82 F, determine the dry bulb temperature of the mixed air.

Given

10% outdoor air

42C/108F outdoor air temperature 

28C/82F return air temperature 

Unknown 

dry bulb temperature of mixed air

SOLUTION:

(% Outdoor Air  x  Temperature)  +  

(% Indoor Return Air  x Temperature)  = 100%  x  Air Mixture Temperature

using Celcius temperatures and substituting given values, we have

( 10% x 42 )  +  

( 90% x 28 ) = 100% x Air Mixture Temp

Mixed air temp 

= 4.2 + 25.2 

= 29.4 C / 85 F

*  Cheers :)  *

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - compressor discharge, suction pressures

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - compressor  discharge, suction pressures

PROBLEM 

(from sample questions on): https://www.red-seal.ca/

The compression ratio of a refrigeration system is 7:1. If the discharge pressure is 300 psig, what is the suction pressure?

Given 

7:1 compression ratio

300 psig discharge pressure (gage)

Unknown 

suction pressure 

SOLUTION 

Cr = Pd abs/Ps abs

Ps abs = Pd abs/Cr

where: 

Cr = compression ratio

       

Pd abs = absolute discharge pressure 

Ps abs = absolute suction pressure 

Absolute Pressure 

= gage pressure 

+  14.7 

Pd abs

= 300 psig + 14.7

= 314.7 psia

substituting the values, we get, 

Ps abs = 314.7/7

Ps abs = 45 psia

Ps gage = 45 - 14.7

Ps gage = 30.3 psig

Checking

Cr = 314.7/45

Cr = 7

*

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - fan speed, static pressure, power, amperage

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - fan speed, static pressure, power, amperage 

PROBLEM 

The rpm of a 3 HP ducted fan moving 1,200 CFM is increased from 900 rpm to 1800 rpm. If the initial static pressure is 0.08 in. w.c., what is the resulting CFM, static pressure, and HP?

Given

cfm1 = 1200 cfm

sp1 = 0.08 in. w.c.

HP1 = 3 HP

rpm1 = 900 rpm

rpm2 = 1800 rpm

Unknown 

cfm2

sp2

HP2

SOLUTION 

Using the Fan Laws,

cfm2 =

cfm1 x (rpm2/rpm1)

cfm2 

= 1200 x 1800/900

= 1200 x 2

= 2400 cfm

when the fan speed rpm is doubled, the cfm is also doubled 

sp2 = sp1 x 

         (rpm2/rpm1)^2

sp2 = 0.08 x 

          (1800/900)^2

sp2 = 0.08 x (2)^2

sp2 = 0.08 x 4

sp2 = 0.32 in. w.c.

when the fan speed rpm is doubled, the static pressure is increased 4 times

HP2 = HP1 x 

         (rpm2/rpm1)^3

HP2 = 3 x 

          (1800/900)^3

HP2 = 3 x (2)^3

HP2 = 3 x 8

HP2 = 24 HP

when the fan speed rpm is doubled, the Power needed to drive the fan is increased 8 times

if the supply voltage is constant, 

then the current draw (amperage) 

is also increased 8 times

*

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - wattage of duct heater

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - wattage of duct heater 

PROBLEM 

(from sample questions on): 

https://www.red-seal.ca/

A duct heater is supplied with 238 VAC and drawing 19.9 A. What is the heater's wattage? 

Given

E = 238 VAC

I = 19.9 A

Unknown 

duct heater wattage 

SOLUTION 

P = I x E

P = 19.9 x 238

P = 4,736 watts

*

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - resistance of compressor motor windings

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - resistance of compressor motor windings

PROBLEM 

(from sample questions on): 

https://www.red-seal.ca/

When diagnosing the motor windings of a hermetic compressor, the following values are noted: 

Common-to-start: 

9 ohms 

Start-to-run: 

12 ohms 

What is the common-to-run value? 

Given

resistance of 

hermetic compressor windings

Common-to-start: 

9 ohms 

Start-to-run: 

12 ohms 

Unknown 

common-to-run value

SOLUTION 

common-to-run value

= Start-to-run value - Common-to-start value

= 12 - 9

=  3 ohms

*

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - minimum sleeve size for insulated pipes

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - minimum sleeve size for pipes with insulation 

PROBLEM:

The suction line of a walk-in cooler is 5/8 inch and has an insulation of 1/2 inch. The discharge line is 3/8 inch and has 1/4 inch of  insulation. Determine the minimum required sleeve size of pipe through a concrete block exterior wall.

Given

suction line = 5/8 in.

suction line insulation = 1/2 inch

discharge line = 3/8

discharge line insulation = 1/4 inch

Unknown 

Size of sleeve

SOLUTION 

Sleeve size of pipe

= suction line 

   + ( 2 * insulation )

   + discharge line

   + ( 2 * insulation)

=  5/8 + (2 * 1/2) 

   + 3/8 + (2 * 1/4)

= 5/8 + 1 + 3/8 + 1/2

= 2  1/2  inch

***

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - capacitance of series capacitors

Watch this video in my YouTube channel 

Eternalson17

YouTube link

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - capacitance of series capacitors 

PROBLEM:

Two capacitors are wired in series. The capacitance of the first capacitor is 50 micro Farads (uF). If the second capacitor has 150 uF, solve for the total capacitance of the circuit.

Given 

wiring = series 

capacitance1 = 50 uF

capacitance2 = 150 uF

Unknown 

Total capacitance of circuit

SOLUTION:

Series Capacitors:

Total Capacitance Ct =  1/ [ (1/C1) + (1/C2) ]

Ct = 1/ [ (1/50) + (1/150) ]

Ct = 1/(0.02 + 0.0067)

Ct = 1/0.0267

Ct = 37.5 uF

Alternate formula:

Ct = C1 x C2 / ( C1 + C2 )

Ct = 50 x 150 / (50 + 150)

Ct = 7500/200

Ct = 37.5 uF

*