Mechanical & Marine Systems Engineering

Saturday, October 29, 2022

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

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Eternalson17

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

PROBLEM:

Two capacitors having 50 uF and 150 uF respectively are wired in parallel. What is the value of the total capacitance?

Given

capacitance1 = 50 uF

capacitance2 = 150 uF

Unknown

Total capacitance of circuit

SOLUTION:

Parallel Capacitors: additive

Total Capacitance, Ct = C1 + C2

Ct = 50 + 150

Ct = 200 uF

Note:

If these same two capacitors were wired in series, the total capacitance would be 37.5 uF, reduced to almost 20%.

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - cooler/reefer tonnage capacity

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Eternalson17

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REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - cooler/reefer tonnage capacity

PROBLEM:

An 8 x 20 ft sea container reefer walkin cooler has an hourly load of 22,500 Btu/hr and operates 24 hours a day. The defrost system runs for 15 minutes every hour. Calculate the tonnage capacity in Tons of Refrigeration and in Kilowatts.

Given

22,500 Btu/hr hourly load

24 hours operation

15 minutes defrost per hour

Unknown

Reefer capacity in tons and KW

SOLUTION:

Total defrost time

= 15 minutes x 24

= 360 minutes

= 6 hours

Total Run-Time

= 24 - 6

= 18 hours

Q = hourly load

x 24/run time

Walk in Cooler Condensing Unit capacity

= ( Hourly Load x 24 hr ) / 18 hr Run-Time

= ( 22,500 x 24 ) / 18

= 30,000 Btu/hr

Tonnage capacity = 30,000/12,000 = 2.5 tons

Capacity in Kilowatts = 2.5 x 3.5 = 8.8 KW

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - water flow rate of chiller

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Eternalson17

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REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - water flow rate of chiller

PROBLEM:

A chiller has a capacity of 75 tons with a heat rejection ratio (HRR) of 1.3. If the change in temperature is 10F, solve for the water flow rate in US gallons per minute and imperial gallons per minute.

Given

chiller capacity = 75 tons

heat rejection ratio = 1.3

delta T = 10F

Unknown

US gpm

imperial gpm

SOLUTION:

Converting tons to Btu/hr

Q = 75 tons x 12,000 Btu/hr per ton = 900,000 Btu/hr

Incorporating the HRR and rearranging the main equation,

US gpm = Q x HRR/(500 x delta T)

US gpm = 900,000 x 1.3/(500 x 10)

US gpm = 234 US gallons

To solve for imperial gallons, change the constant to 600

imperial gpm = Q x HRR/(600 x delta T)

imperial gpm = 900,000 x 1.3/(600 x 10)

imperial gpm = 195 imperial gpm

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - water temperature entering chiller

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Eternalson17

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REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - water temperature entering chiller

PROBLEM:

A 60 ton chiller circulates 160 US gallons of water. What is the temperature rise? If the water supply temperature is 45F, determine the temperature of the water entering the chiller.

Given

Cooling capacity = 60 tons

gpm = 160 US gallons

suppy temperature = 45F

Unknown

Temperature rise = ? or change in temperature (delta T)

Temperature of water entering chiller = ? or return temperature

SOLUTION:

Converting tons to Btu/hr

Q = 60 tons x 12,000 Btu/hr per ton = 720,000 Btu/hr

Rearranging the main equation Q = 500 x gpm x delta T

delta T = Q/(500 x gpm)

delta T = 720,000/(500 x 160)

delta T = 9 F

return temperature = delta T + supply temperature

return temperature = 9 + 45

return temperature = 54 F

REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - chiller cooling capacity (tonnage)

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Eternalson17

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REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - chiller cooling capacity (tonnage) PROBLEM: Calculate the tonnage of a water chiller having a water flow rate of 240 US gallons with a supply temperature of 45F and a return temperature of 55F.

Given gpm = 240 US gallons supply temperature = 45F return temperature = 55F Unknown Q = ? cooling capacity in tons of refrigeration

SOLUTION: Q = 500 × gpm x delta T Q = 500 × 240 x (55 - 45) Q = 500 × 240 x 10 Q = 1,200,000 Btu/hr

The conversion is 1 Ton = 12,000 Btu/hr, so the final answer is, Tonnage = 1,200,000/12,000 Tonnage = 100 tons *