Exam 2014, questions PDF

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Matric No: _________________ EDINBURGH NAPIER UNIVERSITY SCHOOL OF ENGINEERING & THE BUILT ENVIRONMENT

BUILDING SERVICES ENGINEERING A

MEC10113

Academic session:

2014/15

3 Hours Exam duration: (excluding reading time)

Diet:

December 2014

Reading time:

15 Minutes

Total exam time: 3 hours 15 minutes

Please read full instructions before commencing writing Exam paper information •

Total number of pages:

7

•

Number of questions:

4

•

Question number 1 is compulsory. Additionally, you have to answer any TWO of the remaining THREE questions.

Special instructions •

Exam paper to be returned

Special items •

NONE

Examiner(s):

Mr N Taylor and Professor T Muneer

1

1a- With respect to air-conditioning systems and using appropriate sketches critically discuss the following: (i) By-pass factor (2) (ii) Apparatus dew-point (2) (iii) Sensible heat ratio (2) (iv) Difference between relative humidity and percentage saturation. Identify the engineering societies that promulgate the use of the above psychrometric properties. (2) [8 marks] 1b- Define the following terms: (i)

Luminous Efficacy;

(3)

(ii)

Utilization Factor;

(2)

(iii) Maintained Illuminance.

(3)

Discuss the effects that each may have on the running costs of a lighting installation. [8 marks] 1c(i) Critically discuss two methods of spectral frequency analysis of noise by comparing these methods and briefly discussing applications where each might be used. (6) (ii) Briefly define the term ‘speed of sound’ and quantify the speed of sound in dry air at C, at sea level. (2) [8 marks] Total [24 marks]

2

2 The octave frequency analysis of noise from a machine, measured at a distance of 3.6 m from the machine in anechoic conditions, produced the following sound pressure levels: Frequency (Hz)

Sound Pressure Level (dB)

63

61

125

68

250

59

500

73

1k

79

2k

63

4k

42

8k

18

The A-weighting adjustments for the octave centre frequencies are: 63Hz , -26dB ; 125Hz , - 16dB ; 250Hz , - 9dB ;500Hz ,- 3 dB ;1 kHz , 0dB ;2kHz, +1 dB ; 4 kHz , + 1dB ; 8 kHz , (i)

-1dB.

Calculate the A-weighted sound power level (LW(A)) of the machine given that the radiation of sound for the measurements is hemispherical and given that: Lp(A) = Lw(A) – 10*Log10*(2πr²)

(ii)

(8)

A machine is to be placed in a workshop in a position such that the directivity factor (D) is 6. The unadjusted sound power level of the machine (Lw) is 81.4dB. The workshop is rectangular in plan with dimensions of 28m long x 9m wide x 5m high. The average sound absorption coefficient of the workshop internal surfaces is 0.19 at 500 Hz. Calculate the sound pressure level (Lp) at a worker’s position at 500 Hz; given that the worker’s position is 10.5 m from the machine and given that: 3

R = ΣSά / (1-ά), and

4  D + L p = L W + 10 log10  dB 2 R   4π r

(10)

[Total 18 marks] 3 The plan and section through an office in a multi-storey office building are given together with the specifications of the current and new lighting scheme. The current lighting in the office is by means of 42 Standard Dispersive Industrial Reflectors each using a 200W bulb of luminous efficacy 12.6 lumens/W. The fittings are arranged in a manner which satisfies the appropriate spacing: mounting height ratios. This lighting scheme is based on a Utilisation Factor of 0.514 and a Maintenance Factor of 0.9 for an air-conditioned office. A new lighting scheme is envisaged using Thorlux luminaires, Jubilee-XL standard range. These linear fittings each 1.2m in length are louvered with high reflectivity aluminum. Each fitting contains two T5 tubes of power 28W and luminous efficacy of 65 lumens/W. The fittings will be ceiling mounted. The DLOR of the fittings is 74%. The illuminance on a working plane 0.85m above the floor is to be 500 lux. The ceiling and wall reflectances are to be 70% and 50% respectively. The office will still be air-conditioned. (i)

Using the manufacturers data calculate the number of fittings required and design a dimensioned sketch of the suggested ceiling layout complying with the appropriate spacing: mounting height ratios. (9)

(ii)

Calculate and compare the original and new working plane illuminance. Comment on your findings. (3) Calculate the annual savings in lighting energy costs delivered by the new system given that the annual lighting period is 860 hours and the tariff is 12.6p/kWh. (3)

(iii)

(iv)

Critically appraise the suitability of this lighting system and discuss what steps and considerations you might take in improving the energy efficiency of the installation. (3) [Total 18 marks]

4

PLAN A

3m

office 5m 2.4m

corridor

20m

A

0.5m office

2m

3.8m 2m

SECTION A - A

5

4. a- In winter, a meeting room with a large window is to be maintained at comfort conditions. The inside glass temperature on the design day is 4.4oC. Condensation on the window is highly undesirable. The room is to accommodate 18 adult males (264kJ/hour, sensible-; 211kJ/hour, latent load; per person). The heat loss through the walls, ceiling, and floor is 35,448kJ/hour. There are 640 watts of lights in the room. a. Determine the sensible-heat loss or gain. (3) b. Determine the maximum permissible relative humidity within the room that will ensure that condensation does not occur on the inner pane of the window. The room temperature is 180C. (5)

c. If the heating system provides air at 35oC, determine the required air flow (m3/s) and the maximum relative humidity permissible in the incoming air. (10) [Total 18 marks]

6

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