Technical Planning of Ventilation System to Support Development W Undercut in 2021 at PT. Freeport Indonesia Underground Mining

Ventilation is an attempt to drain clean air into the mine and remove dirty air out of the mine. The main components of a mine ventilation system are intake, working, and exhaust. Intake is a tunnel and wells system where air flows from the surface into the mine. The purpose of the ventilation system in an underground mine is to provide and drain clean air into the mine for breathing and comfort of mine workers. Based on the Ventilation Design Criteria used by PTFI, the minimum airflow level required for every mine worker is 0.033 m3 / s / worker. Based on PTFI Ventilation Design Criteria, the minimum level in diluting smoke of heavy equipment diesel engine is 5 m3 / min or 0.08 m3 / s / kW. PT Freeport Indonesia is currently developing new underground mines namely Grasberg Block Caving (GBC) and Deep Mill Level Zone (DMLZ) which will be mined using the block caving method.


Introduction
Mine ventilation is the application of the principle of fluid mechanics which enters airflow into the mine pit, as the main tool responsible for air circulation both in number and direction throughout the mine [1]. Meanwhile, according to Malcolm McPherson, ventilation in underground mines will provide sufficient airflow both in quantity and quality to reduce bad air contamination to better conditions in all mining areas [2]. In the planning of development W Undercut, ventilation planning is also needed. This is because the development engineer's plans cannot be worked out if the air is insufficient. Therefore, for development planning, both short-term or long-term ventilation planning, will be followed for the same period. To provide a sufficient quantity of fresh air then drain and divide the fresh air to the desired levels through Drift or Raise using a fan and ventilation control system. So, it will create safe and comfortable working conditions for mine workers.
DMLZ ventilation system is currently operated and evolved under the continuous mine development process. The current supply of intake air is for intake access from Intake # 1 raise from DOZ, Intake # 2 raise from DOZ, Intake # 4 raise from DOZ, DMLZ spur, and conveyor access. Whereas for exhaust air use the Exhaust main fan on NED # 3 as DMLZ main fan # 1 and the fan on NED # 4 as DMLZ main fan # 2 and NED # 5 as DMLZ main fan # 3. Besides, there are also some auxiliary fans to help in draining clean air towards active headings and regulating the direction of dirty airflow to the surface.
The purpose of this research is to analyze the number of air requirements for the development of W Undercut, analyze Resistance and Head Loss to model the ventilation system in W Undercut using Ventilation software.

The air need for human
Every work activity carried out by humans requires different air requirements. The respiratory rate will be different for each human activity. The harder the work done, the greater of respiratory rate. [3]. This respiratory rate is related to the amount of oxygen in the work environment. Lack of oxygen will disrupt the respiratory rate. The physiological effects of this oxygen deprivation vary, among others, if the oxygen content is 17%, the respiratory rate will increase, 15% will feel dizzy, 13% will lose consciousness, and the most severe is convulsions to death if the oxygen content is 6% [4].

Resistance series of mine ventilation
There are two types of resistance calculations in Airflow Systems used in underground mining, namely:

Series ventilation
Req = Equivalent resistance of a series network is the sum of all resistances of each airway. So the general equation: Eq (1) Note: Rser = series circuit R1, R2, R3, = series 1 and so on

Parallel ventilation.
In mine ventilation, this practice is called partition and branching. There are two forms of separation, including natural separation which occurs when air quantity is divided between parallel branches by itself without regulation. And controlled separation occurs when the specified quantity of air is drained through each of the parallel branches by regulation. According to Kirchhoff's first law, a general equation can be written as follows: Eq (2) Note: Qtot = total air debit Q1, Q2, Q3 = air debit at the branching So when the airways are arranged in parallel type, the total amount of airflow is the sum of air flowing through individual airways. According to Kirchhoff's second law, it shows that the head losses for parallel airways are the same. Eq (3) Note: HL = Total Head Loss H1, H2, H3, …. = Head loss in branching When the airways and others are arranged in parallel type, the resistance equation can be calculated by Eq (4) Note : Rpar = parallel air resistance R1, R2, Rn = resistance in branching

Mining airways resistance
Air pressure and the quantity of airflow are related to find the resistance value. The resistance of ventilation is the value of resistance experienced by the flow of ventilation air inside the underground mine [5]. As long as the airways have not changed (has a fixed roughness, length, area, and perimeter), the resistance in the airways is constant. So, the correlation between air pressure and the quantity of airflow is formulated with the equation R: Resistance (Ns 2 / m 8 ) P = Pressure (Pa) and Q = Air Quantity (cfm or m 3 / s) Eq (5)

Calculation of air quantity
After getting the value of the air velocity and the surface area of the tunnel drift, it can be calculated the air quantity that passes through the tunnel. To measure the air quantity passing through the tunnel, it can be calculated by the equation Q = air quantity (m 3 / s), V = air velocity (m / s), and A = area (m 2 ). Eq (6)

Head loss calculation
Head loss is the loss of air debit that occurs due to velocity (Hv), friction (Hf), and bends or channel size changes (Hx). Head loss is the sum of friction loss and shock loss, then: Eq (7) HL = Hf + Hx Head Velocity Although it is not a head loss, technically it can be considered as a loss. Head velocity is a function of airflow velocity, which Hv = velocity head, V = velocity of flow (fps), g = acceleration of gravity (ft / Shock loss Shock loss occurs as a result of changes in flow direction in the channel or cross-sectional area of the air channel and addition to friction losses. Although the quantity is only about 10% -30% of the total head loss in mine ventilation, it must still be considered. Which Hx = Shock loss, X = shock loss factor [5] .Eq (11) Length equivalent (Leq) is a representation of shock loss due to bends, branches, widening or narrowing, and so on. The value of the equivalent length can be seen in table 1.

Methodology
This research was conducted in several stages, namely literature study, data collection, and data processing. Research data were processed using ventilation software. For more details, the research flowchart can be seen in the figure below (figure 1)

Auxiliary fan
There are several types of auxiliary fans that PT Freeport Indonesia has, but for W Undercut itself according to the concept of Preliminary Ventilation Requirements DMLZ W Undercut PTFI only will use Auxiliary Fan Spendrup 125-0800-1800, 150 HP to flow the air from the Intake Raise to Drift. One of the advantages of having an auxiliary fan is to help the main fan so that it generates additional total flow compared to only using the main fan [8]. Each fan flows air to two drifts. The auxiliary fan installed on this raise serves to appeal air because the air cannot flow according to the air quantity air that is expected. Table 2. Calculation of auxiliary fan

Airflow budget
Airflow Budget can be calculated based on the number of equipment used plus the number of workers and also based on segments calculated by decreasing PTFI's Multi-Criteria Flow Requirements. Calculation of airflow budget is very important in planning ventilation systems to analyze and provide airflow budget for equipment and people who work in development activities and others. To determine air requirements further based on equipment and workers as well as air requirements in development activities can be seen in table 3 and 4 below   Table 3. Air requirements by equipment and workers [9] Note: Air requirements for equipment based on PTFI Ventilation Design Criteria are converted from kW to HP (House Power)

Resistance and losses in airways
In planning a mine ventilation system in terms of quantity, it is necessary to analyze airflow resistance in the airways and ventilation control system that causes a decrease in air debit. An analysis is carried out on the vent bag on active faces. 6 faces are active in the development, and it needs to be analyzed to get a picture of the amount of resistance and head loss caused by friction factor and shockloss. Table 6 shows the resistance in ventilation control in PT Freeport Indonesia. While table 7 shows the analysis of resistance and head loss in a 52-inch vent bag.

Conclusion
Based on the analysis of total air needs, it can be calculated that for the development segment, the total air needs per segment is 15 m3 / s per face. It is derived from the decrease in Multi-Criteria Flow Requirements, PTFI. There are six active faces so that the total of air needs for active development in W Undercut is 180 m3 / s.