2026 NABTEB REFRIGERATION & AIR CONDITIONING WORKS SCIENCE ANSWERS


AIR CONDITIONING OBJ (TYPE C)
01-10: BBBBABBADC
11-20: BCCDAAABAA
21-30: CAABCCADCD
31-40: CABCBCADBC


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NOTE: In this theory, Type A, B, C & D are the same but reshuffled. Use the answers provided to trace the correct questions in your booklet before answering. WE USED TYPE-C IN SOLVING, SO TRACE FROM YOUR QUESTIONS BEFORE COPYING THE ANSWERS.

INSTRUCTION: ANSWER FIVE(5) QUESTIONS ONLY


(1a)
(i) Solid.
(ii) Liquid.
(iii) Gas.

(1bi)
(i) Conduction.
(ii) Convection.
(iii) Radiation.

(1bii)
(i) Conduction: This is the transfer of heat through a solid material from a hotter region to a colder region without the actual movement of the material. For example, heat passes through the walls of the evaporator or condenser tubes.
(ii) Convection: This is the transfer of heat through the movement of liquids or gases. In refrigeration systems, air or refrigerant circulates and carries heat from one place to another.
(iii) Radiation: This is the transfer of heat in the form of electromagnetic waves without the need for a material medium. Heat from the surroundings can be radiated to or from refrigeration equipment.

(1c)
°F = (9/5 × °C) + 32

Substituting ?40°C:
°F = (9/5 × ?40) + 32
°F = ?72 + 32
°F = ?40°F
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(2a)
(i) Reduces friction: It reduces friction between moving machine parts, thereby ensuring smooth operation.
(ii) Reduces wear and tear: It minimizes the wear of moving parts and prolongs the life of the equipment.
(iii) Cooling: It carries away heat generated by friction and helps to cool the moving parts.
(iv) Prevents corrosion: It forms a protective film on metal surfaces, thereby preventing rust and corrosion.
(v) Sealing: It helps to seal the clearance between moving parts, reducing leakage of gases or refrigerants.


(2b)
(i) Good viscosity: The oil should have sufficient thickness to maintain a lubricating film between moving parts.
(ii) High flash point: It should withstand high temperatures without catching fire easily.
(iii) Low pour point: The oil should remain fluid and flow easily at low temperatures.
(iv) Good chemical and thermal stability: It should resist oxidation and decomposition during operation.

(2c)
(i) Splash lubrication.
(ii) Pressure (force-feed) lubrication.
(iii) Wick or gravity-feed lubrication.
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(3a)
Heat load (Q) = U × A × ?T

Where:
U (Overall heat transfer coefficient) = 0.37W/m²K
A (Area) = 10 m²
?T (Temperature difference) = 36°C - 14°C = 22°C

Q = 0.37 × 10 × 22
Q = 81.4 W

(3bi)
Sensible heat is the heat added to or removed from a substance that causes a change in temperature without changing its physical state.
Example: Heating water from 20°C to 60°C without boiling it.

(3bii)
Latent heat is the heat added to or removed from a substance that causes a change of state without any change in temperature.
Example: Water changing to steam at 100°C or ice melting at 0°C.
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(4ai)
Soap solution method: A soap solution is applied to the suspected joints, fittings, or pipe connections. If there is a refrigerant leak, bubbles will form at the leaking point, indicating the exact location of the leak.

(4aii)
Electronic leak detector method: An electronic leak detector is moved around the refrigeration system components. The instrument senses the presence of escaping refrigerant gas and gives an audible or visual signal to indicate the location of the leak.

(4b)
A vacuum is created in a refrigeration system to remove air, moisture, and other non-condensable gases from the system before charging it with refrigerant. This helps to prevent corrosion, ice formation, high operating pressure, and poor system performance.
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(6a)
(i) To remove heat from the refrigerant vapour: The condenser rejects the heat absorbed by the refrigerant in the evaporator, together with the heat added during compression.
(ii) To convert refrigerant vapour into liquid: It changes the high-pressure, high-temperature refrigerant gas from the compressor into a high-pressure liquid by cooling and condensing it.

(6b)
(i) Air-cooled condenser
(ii) Water-cooled condenser
(iii) Evaporative condenser

(6c)
DIAGRAM
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(7ai)
(i) Conducting materials.
(ii) Insulating materials.

(7aii)
=CONDUCTING MATERIALS=
(i) Copper.

(ii) Aluminium.

=INSULATING MATERIALS=
(i) Rubber.
(ii) PVC (Polyvinyl Chloride).

(7b)
Insulation is the use of materials that resist the flow of heat, electricity, or sound in order to prevent energy loss and protect equipment and personnel.

(7c)
(i) It reduces heat gain or heat loss in refrigeration pipes and equipment.
(ii) It prevents condensation or sweating on cold surfaces.
(iii) It improves the efficiency of the refrigeration system by reducing energy losses.