CBSE Class 12 Question Paper 2025 Solution Air Conditioning and Refrigeration

Marking Scheme
Strictly Confidential
(For Internal and Restricted use only)
Senior School Certificate Examination, 2025
SUBJECT NAME AIR CONDITIONING AND REFRIGERATION (Q.P. CODE 351)

General Instructions: -

1 You are aware that evaluation is the most important process in the actual and correct
assessment of the candidates. A small mistake in evaluation may lead to serious problems
which may affect the future of the candidates, education system and teaching profession.
To avoid mistakes, it is requested that before starting evaluation, you must read and
understand the spot evaluation guidelines carefully.
2 “Evaluation policy is a confidential policy as it is related to the confidentiality of the
examinations conducted, Evaluation done and several other aspects. Its’ leakage to
public in any manner could lead to derailment of the examination system and affect
the life and future of millions of candidates. Sharing this policy/document to
anyone, publishing in any magazine and printing in News Paper/Website etc may
invite action under various rules of the Board and IPC.”
3 Evaluation is to be done as per instructions provided in the Marking Scheme. It should not
be done according to one’s own interpretation or any other consideration. Marking
Scheme should be strictly adhered to and religiously followed. However, while
evaluating, answers which are based on latest information or knowledge and/or are
innovative, they may be assessed for their correctness otherwise and due marks be
awarded to them. In class-X, while evaluating two competency-based questions,
please try to understand given answer and even if reply is not from marking scheme
but correct competency is enumerated by the candidate, due marks should be
awarded.
4 The Marking scheme carries only suggested value points for the answers
These are in the nature of Guidelines only and do not constitute the complete answer. The
students can have their own expression and if the expression is correct, the due marks
should be awarded accordingly.
5 The Head-Examiner must go through the first five answer books evaluated by each
evaluator on the first day, to ensure that evaluation has been carried out as per the
instructions given in the Marking Scheme. If there is any variation, the same should be
zero after delibration and discussion. The remaining answer books meant for evaluation
shall be given only after ensuring that there is no significant variation in the marking of
individual evaluators.
6 Evaluators will mark( √ ) wherever answer is correct. For wrong answer CROSS ‘X” be
marked. Evaluators will not put right (✓)while evaluating which gives an impression that
answer is correct and no marks are awarded. This is most common mistake which
evaluators are committing.
7 If a question has parts, please award marks on the right-hand side for each part. Marks
awarded for different parts of the question should then be totaled up and written in the left-
hand margin and encircled. This may be followed strictly.
8 If a question does not have any parts, marks must be awarded in the left-hand margin and
encircled. This may also be followed strictly.
9 If a student has attempted an extra question, answer of the question deserving more
marks should be retained and the other answer scored out with a note “Extra Question”.

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10 No marks to be deducted for the cumulative effect of an error. It should be penalized only
once.
11 A full scale of marks __________(example 0 to 80/70/60/50/40/30 marks as given in
Question Paper) has to be used. Please do not hesitate to award full marks if the answer
deserves it.
12 Every examiner has to necessarily do evaluation work for full working hours i.e., 8 hours
every day and evaluate 20 answer books per day in main subjects and 25 answer books
per day in other subjects (Details are given in Spot Guidelines).This is in view of the
reduced syllabus and number of questions in question paper.
13 Ensure that you do not make the following common types of errors committed by the
Examiner in the past:-
● Leaving answer or part thereof unassessed in an answer book.
● Giving more marks for an answer than assigned to it.
● Wrong totaling of marks awarded on an answer.
● Wrong transfer of marks from the inside pages of the answer book to the title page.
● Wrong question wise totaling on the title page.
● Wrong totaling of marks of the two columns on the title page.
● Wrong grand total.
● Marks in words and figures not tallying/not same.
● Wrong transfer of marks from the answer book to online award list.
● Answers marked as correct, but marks not awarded. (Ensure that the right tick mark
is correctly and clearly indicated. It should merely be a line. Same is with the X for
incorrect answer.)
● Half or a part of answer marked correct and the rest as wrong, but no marks awarded.
14 While evaluating the answer books if the answer is found to be totally incorrect, it should
be marked as cross (X) and awarded zero (0)Marks.
15 Any unassessed portion, non-carrying over of marks to the title page, or totaling error
detected by the candidate shall damage the prestige of all the personnel engaged in the
evaluation work as also of the Board. Hence, in order to uphold the prestige of all
concerned, it is again reiterated that the instructions be followed meticulously and
judiciously.
16 The Examiners should acquaint themselves with the guidelines given in the “Guidelines
for Spot Evaluation” before starting the actual evaluation.
17 Every Examiner shall also ensure that all the answers are evaluated, marks carried over to
the title page, correctly totaled and written in figures and words.
18 The candidates are entitled to obtain photocopy of the Answer Book on request on
payment of the prescribed processing fee. All Examiners/Additional Head Examiners/Head
Examiners are once again reminded that they must ensure that evaluation is carried out
strictly as per value points for each answer as given in the Marking Scheme.

351 2 P.T.O.

Set 4 >

MARKING SCHEME
AIR CONDITIONING AND REFRIGERATION

SECTION A
(Objective Type Questions)

1. Answer any 4 out of the given 6 questions on Employability Skills. 41=4

(i) It includes activities for which there is no apparent reward
but one derives enjoyment and satisfaction in doing them 1
(ii) (a) Microsoft Excel 1/2+1/2=1
(b) LibreOffice Calc
(c) Google sheets
Any two
(iii) A listener is considered to be an active listener if he pays
attention, asks question and looks at who is talking. 1
(iv) A purposeful activity to initiate, maintain and aggrandise
profit-oriented business. 1
(v) (a) Reusing scrap material 1/2+1/2=1
(b) Use of ecofriendly material etc.
(vi) File > Save as > Type file name > Save 1

2. Answer any 5 out of the given 7 questions. 51=5

(i) (A) Evaporative condenser 1

(ii) (D) Hot wire 1

(iii) (B) Ice plant 1

(iv) (D) H.P. cut out 1

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(v) (A) Double tube evaporator 1

(vi) (B) Transmission 1

(vii) (C) Sensible heating 1

3. Answer any 6 out of the given 7 questions. 61=6

(i) (B) Dehumidification 1

(ii) (C) Rectangular 1

(iii) (A) overload protector 1

(iv) (B) Refrigeration of milk and butter 1

(v) (C) Constant pressure expansion valve 1

(vi) (A) Year round air conditioning 1

(vii) (B) Degree of superheat 1

4. Answer any 5 out of the given 6 questions. 51=5

(i) (B) Pasteurization 1

(ii) (C) Air cooled condenser 1

(iii) (A) Dry type 1

(iv) (C) L.P. cutout 1

(v) (B) Conduction 1

(vi) (C) Dehumidification 1

5. Answer any 5 out of the given 6 questions. 51=5

(i) (D) Heating and humidification 1
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(ii) (B) Duct 1

(iii) (C) Finned evaporator 1

(iv) (A) Ice plant 1

(v) (D) Automatic expansion system 1

(vi) (A) Exhauster 1

6. Answer any 5 out of the given 6 questions. 51=5

(i) (A) Filter 1

(ii) (A) R-717 1

(iii) (C) Current starting relay 1

(iv) (A) Monochloro trifluoro methane 1

(v) (A) PUF 1

(vi) (B) water flows through the tubes 1

SECTION B
(Subjective Type Questions)

Answer any 3 out of the given 5 questions on Employability Skills. Answer each question
in 20 – 30 words. 32=6

7. (i) Noisy environment 1/24=2

(ii) Not maintaining an eye contact with the speaker

(iii) Not being attentive

(iv) Not clarifying doubts

8. (i) Going to work because you want to earn money 1+1=2
(ii) Paying taxes because you want to avoid a fine etc.

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9. (i) Row 1/24=2
(ii) Column
(iii) Cell
(iv) Name box
(v) Worksheet
(vi) Workbook
Any four

10. (i) Initiative 1/24=2
(ii) Willingness to take risks
(iii) Ability to learn from experience
(iv) Motivation
(v) Self-confidence
(vi) Hard work
(vii) Decision making
Any four

11. (i) Toxin free homes and cities 1/24=2
(ii) Reducing wastes and pollution
(iii) Preventing hazardous climate changes
(iv) Limiting greenhouse gas emissions

Answer any 3 out of the given 5 questions in 20 – 30 words each. 32=6

12. In this process, the air is passed over chemicals which have an
affinity for moisture. As the air comes in contact with these
chemicals, the moisture gets condensed out of the air. 2

13. Refrigeration systems commonly for preservation of meat product
is carcass chilling which hold rooms and operated with ammonia at

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a system pressure corresponding to – 25C as the primary agent
and are of three general types : 2

(i) Chilled brine spray

(ii) Sprayed coil

(iii) Dry coils

14. The refrigerant is a heat carrying medium which during its cycle
(i.e. compression, condensation, expansion and evaporation) in the
refrigeration system absorbs heat from a low temperature system
and discards the heat so absorbed to the higher temperature system.
Examples are R-22, Ammonia etc. 1 1 + 1 =2
2 2

15. The transfer of heat from one region to another region normally
occurs due to the temperature difference between the two regions.
Modes of heat transfer: 1+1=2

(i) Conduction

(ii) Convection

(iii) Radiation

16. Blower Exhauster 1+1=2

When the fan is used to When the fan removes air or
discharge air against pressure gases from a space by suction it
at its outlet it is known as a is called as a exhauster
blower

Answer any 2 out of the given 3 questions in 30 – 50 words each. 23=6

17. The psychrometric process used in summer air conditioning is
‘cooling and dehumidification’. 1 1 +1 1 =3
2 2

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The air is passed over a cooling coil through or a cold water spray
whose effective temperature is less than the dew point temperature
of the entering air. In this process, the dry bulb temperature as well
as the specific humidity of air decreases. The final relative
humidity of the air is generally higher than that of entering air.

18. (i) The occupied zone of conditioned space for human occupancy is considered
upto height of about 2 metre from the floor. 1+1+1=3
(ii) The temperature variation of more than 2C should not be
allowed as a temperature difference of more than 2C
causes discomfort to the human body.
(iii) The movement of air around the human body should be
kept at 7·5 m/s approximately and it should not be more
than 15 m/s if the persons are sitting.
(iv) The air velocity of 20 m/s is not objectionable when the
occupants are moving.
Any three

19. (i) It is Dichloro difluoro methane 1+1+1=3
(ii) It is a colourless almost odourless liquid
(iii) Its boiling point is – 29C at atmospheric pressure
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(iv) Water is slightly soluble in it at – 18C
(v) The latent heat of R-12 at – 15C is 159 kJ/kg
(vi) The cylinder colour code is white
Any three

Answer any 3 out of the given 5 questions in 50 – 80 words each. 34=12

20. A well labelled appropriate diagram the conditioned air is supplied
from the air conditioning plant through ducts to the outlets which
distribute it to the occupied zone of the space or room. In this
system, the conditioned air is delivered from a central planum, in
separate ducts to each of the outlets. 2+2=4

21. A unitary air conditioning system is a factory-made system that
combines heating and cooling in one unit or a few sections.

They are often used in commercial buildings like hotels and
apartment complexes.

It is commonly preferred for 15 TR capacity or above or around
200 cubic meter/win of air flow. 2

The unitary air conditioning systems are classified as follows:

(Any two)=2

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Single packaged system: All components are in one cabinet.
 Split system: Components are in separate assemblies but are designed to
 work together.
 Ductless systems: All primary components are in one AC unit, like a
 rooftop package unit .
 Ducted split systems: Indoor and outdoor units are connected to form one
 System.

22. A well labelled appropriate diagram. 4

23. A well labelled appropriate diagram.

In an electrical circuit, some kind of protection against excessive
current is provided. If this excessive current is maintained for a
long period, the motor winding would become overheated and burn
out might result. A bi-metal disc type overload protector with a
small heater which supplements external heat is generally used.

If the current increases beyond a safe point, the temperature of the
heater coil in the overload protector increases. This causes the
thermally sensitive bi-metal contact to spring away as to open the
circuit and stop the motor. If the switch is of the automatic reset
type, the circuit remains open until the temperature drops and
contacts are again closed. 2+2=4

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24. R-134 a is an eco-friendly refrigerant its properties are as follows:
1+3 (Any three) =4

(i) It is the CFC free, environmental friendly refrigerant to
replace R-12, R-22.
(ii) This refrigerant is a hydrofluorocarbon and has a zero
ozone depletion potential and a low greenhouse effect.
(iii) It is non-flammable, non-explosive and non-toxic.
(iv) It has high chemical stability though it has a relatively high
affinity for moisture.
(v) It has a low boiling temperature of – 26·2C at atmospheric
pressure.
(vi) Heat transfer coefficients are significantly higher for R-134
a than R-12.

351 11 P.T.O.