** Advanced Mine Ventilation **

** Major Design Project **

The objective of this design exercise is to design a suitable cooling system to cool air for air conditioning purposes. The system to be designed is a chilled water system, employing a single stage spray cooler to achieve the required air-cooling duty at the delivery end of the system. Additionally students are to design a suitable vapour compression refrigeration system to achieve this duty, a suitable chilled water piping system to achieve the required duty and suitable heat rejection facilities for the refrigeration plant using a cooling tower.

Details of the system are as follows:

**Spray cooler: **

- Air quantity to be cooled 300 m3/s.
- Inlet air condition; dry bulb temperature 32°C, wet bulb temperature 25°C.
- Outlet air temperature 7°C saturated.
- Single stage spray cooler.

**Reticulation (piping system): **

Steel pipes, insulation can be used, pipe diameter to be selected by the student. It can be assumed that the spray cooler is located 1000m below surface a total distance of 1800m from the proposed refrigeration plant site which is to be located on the surface. Ignore heat transfer from the pipe.

**Refrigeration unit: **

Refrigerant to be R12, vapour compression refrigeration system.

**Heat rejection: **

Cooling towers, inlet air temperature 32°C dry bulb, 25°C wet bulb.

Students may assume that the barometric pressure is 100kPa in all cases.

Students are to undertake a design of all elements of the system, as a hint work from the spray cooler backwards through the system. All assumptions should be detailed.

The expectation is that students will present a report, detailing their design including all calculations and relevant diagrams, engineering drawings etc. The report should be referenced as appropriate.

**Marking Schedule **

The report should be of a suitable standard with presentation being worth 10% of the total marks for this project.

The breakdown of the remaining marks is as follows:

Spray cooler design 20%

Reticulation system 20%

Refrigeration System 20%

Heat rejection system 20%

Other 10%

In addition the psychrometric (hygrometric) properties of moist air can be calculated from the attached formulae.

There is no size limit on the report in terms of page numbers. However students should note that the report should have the following structure:

- Executive summary of findings (no more than 2 pages)
- Main report
- Appendices to support material within the main report as appropriate.

SUMMARY OF PSYCHROMETRIC EQUATIONS (From McPherson M.J (1993) Subsurface Ventilation and Environmental Engineering, Chapman and Hall, London)

This is a convenient point to make a reference list of the more important equations that have been derived in the previous sections, and to re-order them in the sequence that they are required for most psychrometric calculations where P, tw, td are the measured variables. All temperatures are expressed in degrees Centigrade and pressures in Pascals.

(Equation (14.44) is used for esd with tw replaced by td). Note that in this list of important relationships, the need for the old troublesome psychrometric “constant” (Equation (14.35)) has been entirely eliminated.

Example

At entry to a level continuous underground airway of constant cross-section, psychrometer readings give P1 = 110.130 kPa, tw1 = 23 °C and td1 =28 °C. The corresponding readings at exit are P2 = 109.850 kPa, tw2 = 26 °C and td2 = 32 °C. If the volume flow of air at inlet is 25 m3 /s, calculate the heat and moisture added to the airstream during its passage through the airway

Questions:

**1:**

Students are to write a report to answer the following:

- What is your understanding of ventilation on demand and what relevance does it have for the mining industry?
- For a mine that you work at or can gain information from what would be the impact of ventilation on demand to the operation. Students should consider the effect on both production and development airflows. However a full network analysis determining the overall system impact is not required.
- Students should report on the infrastructure and any other operational changes that would be required to implement ventilation on demand at the site, eg control and monitoring requirements.

The expectation is of a report in a standard engineering format. All assumptions must be detailed and justified and must comply with any mine legislation. Students are also encouraged to detail any further research they believe is required to enable such a system to be developed.

**2:**

A steel pipe has an internal diameter of 150mm and an outside diameter of 160mm. The pipe carries water at a temperature of 10°C flowing at a velocity of 0.5 m/s. The air surrounding the pipe has a dry bulb temperature of 27°C. The air flow’s over the pipe with a velocity of 3 m/s. If the pipe is covered by insulation of 30mm thickness with a thermal conductivity of 0.035 W/m°, determine:

(a) The UA factor for 1m length of insulated pipe,

(b) The rate of heat gain by the water per m of pipe

(c) The temperature of the inner and outer surfaces of the pipe and the temperature of the outer surface of the insulation

(Note ksteel = 45 W/m°C, hc inside pipe = 1400 W/m2°C, hc outside insulation = 18 W/m2°C and radiative heat transfer coefficient outside insulation hr = 6.1 W/m2°C)

3:

A basic vapour compression refrigeration system is using refrigerant R12 .The evaporator temperature is 0°C and the condenser

temperature is 55°C .The isentropic efficiency of the compressor is 80%. Observations made on the chilled water circuit to the evaporator indicate that the cooling duty is 1800 kW. Assuming the refrigerant enters the compressor as a saturated vapour and leaves the condenser as a saturated liquid detail the necessary calculations and then plot the cycle on the R12 pressure/specific enthalpy diagram provided. Also calculate:

(a) The power input to the compressor;

(b) The rate of heat removal in the condenser;

(c) The actual coefficient of performance;

(d) The Carnot COP;

(e) The cycle efficiency.

Discuss briefly how the COP of this plant might be improved and illustrate the benefits by providing a freehand sketch of the modified refrigeration cycle on a appropriate property diagram.

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