Difference between revisions of "Ventilation air heating"

Revision as of 17:51, 4 February 2016

Introduction

Mining is a global industry, and mines exist all over the world in a variety of different climates. As such, mines experience different temperature regulation needs. Some mines need little or no temperature regulation. These mines are typically shallow mines in mild or warm climates, as well as open pit mines. Very deep mines and mines in hot climates generally require cooling systems in order to maintain a reasonable working temperature for underground workers. Mines in cold climates often require heating systems to keep the air temperature above freezing in order to prevent ice buildup in the mine, especially in the wintertime. These heating systems can make up over 50% of all ventilation costs at an arctic mine^[1], which in turn can contribute up to 40% of a mine’s total energy costs^[2]. It is therefore important to design a mine air heating system that is both effective and efficient.

Ice Formation in Mines

In mines where air temperatures extend below freezing, there is the potential for ice and frost formation on the surfaces of the mine excavation. As hot, humid air from inside the mine travels upwards in elevation, often through a shaft or a ramp, the process of autocompression causes the air to expand in volume and decrease in temperature^[3]. The dry bulb temperature drops until it reaches its saturation level, at which point the water in the humid air precipitates out as fog or ice crystals^[3]. This precipitation forms a layer of ice on the surface of the excavation, and over time there can be significant ice buildup in a mine, which creates a major safety hazard.

Safety Hazards and Operational Challenges of Ice in Mines

An ice buildup in a mine can have a variety of safety hazards and operational difficulties associated with it. The ice can build up in a raise, shaft, or ramp to a point where it has significant mass^[3]. This ice mass has the potential to fall, especially if conditions warm up, which can pose a serious safety risk to anyone who could be in the path of the falling ice chunks.

The buildup of ice in airways over time can decrease the cross-sectional area of the airway, thereby increasing the amount of fan pressure required to ventilate the mine^[4]. This can lead to a substantial increase in fan operating costs and in extreme cases could even limit the amount of ventilation that the mine receives.

Visibility can be greatly reduced in areas where heavy precipitation occurs in the form of fog. This poses a safety hazard as well as an operational challenge because workers may not be able to see mobile equipment and other workers, which increases the risk of worker injury and can necessitate that mobile equipment slow down^[3].

In mines with a production shaft, ice formation can be a significant problem. Ice can damage shaft support members, cables, and pipes, which can disrupt hoisting operations and is also a serious safety concern^[1].

Ice formation on the floors of a mine can be a safety hazard, as workers could slip and fall on the ice, and mobile equipment could slide on the ice and cause injury^[1].

Finally, cold temperatures underground often necessitate the use of heavy gloves and protective clothing for workers^[1]. This can make tasks more difficult and can cause them to be less efficient.

Because of all the safety concerns and operational challenges that ice formation creates, it is important for mines operating in cold climates to heat their ventilation air above freezing so that ice does not form.

Recommended Air Temperatures

The air in the mine must be above 0°C in order to prevent ice formation. In hoisting shafts, where ice formation can be detrimental to operations and safety, it is recommended that air be heated to at least 5°C^[4]. In ventilation raises, where no operational activities are taking place, it is recommended that a temperature of at least 1.5°C is maintained^[4].

Because mine air heating is very energy intensive and can make up a large portion of a mine’s energy costs, it is important to ensure that a mine is not being heated to a point where energy is being wasted. Therefore, it is usually best to not greatly exceed these recommended temperatures.

Determining Required Heater Capacity

When selecting a mine air heater, the most important parameter to consider is the amount of heat that it can deliver to the mine. In order to select an appropriate air heater, the required heater capacity must first be determined.

The required capacity of an air heater depends on two main parameters:

• The volumetric flow rate of the air
• The required temperature change of the air

The volumetric flow rate of the air will often already be known, since it is generally determined based on the need to dilute diesel fumes, harmful gases, and dust in the underground mine workings. The required temperature change of the air depends on the initial temperature of the intake air, and the type of excavation through which the air is being transferred. The required temperature can be calculated by taking the absolute value of the initial air temperature minus the desired final air temperature.

English System

Many air heating systems are categorized using the English system units of Btu per hour. The required capacity of an air heater in Btu/h can be found using the following equation^[1]:

Q = (Air density, Lb/ft3) x (0.24 Btu/Lb·°F) x (60 min/h) x V x ΔT

Given an air density of 0.075 Lb/ft3, this equation can be simplified to:

Q = 1.08 x V x ΔT

Where:

• Q is the required heater capacity in Btu/h
• V is the volumetric flow rate in cubic feet per second
• ΔT is the required air temperature change in degrees Fahrenheit

This simplified equation is for air at standard air density (0.075 Lb/ft3). If the mine is in an area where the air density is different from standard, the equation can be adjusted by multiplying by the local air density divided by 0.075^[4].

Metric System

To find the required heater capacity in kilowatts, the following equation is used^[5]:

Q = 1.3 x V x ΔT

Where:

• Q is the required heater capacity in kilowatts
• V is the volumetric air flow rate in m3/s
• T is the required air temperature change in degrees Celsius

This equation is for air at standard air density (1.225 kg/m3). If the mine is in an area where the air density is different from standard, the equation can be adjusted by multiplying by the local air density divided by 1.225.

Mine Air Heaters

In the past, mine heating systems consisted of steam coils that were heated by wood, coal, oil, or gas furnaces, as well as electrical coils. These systems are not used any more because of their inefficiency^[1].

Other methods of heating can include the use of waste heat from compressor stations, as well as the use of controlled recirculation of mine air. Additionally, glycol or heat pump systems can be used to recover heat from mine exhaust air^[1].

Types of Mine Air Heaters

There are two general types of mine air heaters: direct combustion heaters and indirect combustion heaters^[3]. Each type has its advantages and disadvantages, and a mine must decide what type of heater best suits its needs. In general, a direct combustion air heating system is preferred because it is more efficient and is cheaper to purchase and to operate.

The figure below shows the fundamental difference between a direct and indirect heating system^[6]. In a direct system, combustion occurs directly in the air stream. In an indirect system, combustion occurs in a separate chamber and heat is transferred to the air steam, with no direct interaction between the combustion chamber and the air stream.

Direct Combustion Air Heaters

Direct combustion air heaters involve the combustion of fuel, typically natural gas or propane, directly within the stream of air that is to be heated. They are quite simple systems, since they require no compartmentalization between the burners and the air stream, and thus do not need heat exchangers to transfer the heat of combustion from the combustion chamber to the air. Because of their simplicity, direct combustion air heaters require little maintenance and have very low maintenance costs^[3]. They also have very high efficiencies, since all of the heat produced by combustion enters the airstream, and no heat exchangers are used^[1]. The typical efficiency of a direct combustion heater using propane as fuel is 90%^[4].

The main disadvantage of direct combustion air heaters is that all of the products of combustion enter the air^[3]. The burning of natural gas or propane generally produces small amounts of carbon monoxide gas, which can be dangerous if regulated levels are exceeded. The carbon monoxide, and other products of combustion, enters the intake air of the mine and adds contaminants to the air that mine workers are breathing. The carbon monoxide production from direct combustion air heaters can be between 10 and 20 parts per million^[1]. The carbon monoxide levels in the air must be monitored, and the addition of carbon monoxide from direct combustion in the intake air must be offset by a decrease in the production of carbon monoxide by mining activities. This is to ensure that the carbon monoxide concentration in the mine air that workers are exposed to remains below the regulated limits^[3].

In mines that are very remote, particularly arctic mines, it may be difficult to obtain the natural gas or propane that a direct combustion air heating system needs. This is because remote arctic mines are often only accessible by ice road for a small portion of the year, so they get their fuel shipped in bulk^[3]. Because of this, it is often much more convenient to use diesel fuel, which has a higher energy density than natural gas and propane, and is safer to transport^[3]. The figure below shows the heating values of different fuels. Diesel fuel has the highest heating value at 143,000 Btu/gal^[1].

When mines use diesel fuel to heat their air, they must use an indirect combustion system^[3].

Indirect Combustion Air Heaters

Indirect combustion air heaters involve the combustion of fuel, typically diesel, inside a combustion chamber that is separate from the air stream that is to be heated. The heat from the combustion chamber is transferred to the air via a heat exchanger. The use of a heat exchange to transfer heat from the combustion chamber to the air involves a heat loss of 15 to 25%^[1]. Because of this, indirect combustion heating systems are less efficient than direct combustion systems. The typical efficiency of an indirect combustion heater using diesel fuel is 80%^[4]

The combustion must take place in a chamber that is separate from the mine air because of the fumes that are produced by the combustion. Because diesel fuel does not burn as cleanly as natural gas and propane, it cannot be used in direct combustion systems. However, since the products of combustion are not mixing with the air stream, there will be no contamination of the mine intake air^[3].

Indirect combustion heating systems involve a much more complicated process than direct systems, and are less reliable. Because of this, the maintenance costs for indirect heating systems are much higher than the costs for direct systems^[3]. Indirect combustion heating systems will generally be used when the mine has easy access to diesel fuel and it is inconvenient or unfeasible to use propane or natural gas.

Direct vs Indirect Air Heaters

The table below summarizes the advantages and disadvantages of direct and indirect air heating systems. As previously mentioned, it is generally preferable to use direct air heating when natural gas or propane is available as fuel, because this type of system is more efficient and easier to maintain.

Fuel Costs

Fuel consumption makes up a large portion of a mine air heating system's operating costs^[3]. The fuel costs of a heating system can be calculated based on the power requirements of the system, which are calculated using the equations previously given in this article. The calculated power requirements can then be used in combination with the energy density of the fuel to obtain the amount of fuel that is needed to heat the mine.

Example

Calculations

This example will demonstrate how to calculate a mine’s required heater capacity and its fuel consumption costs.

Given information:

• A remote arctic mine is using an indirect heating system that consumes diesel fuel
• The mine has a ventilation airflow of 100 m3/s
• The initial temperature of the air is -25°C and it must be heated to 5°C
• The energy density of diesel fuel is 39 MJ/L^[7]
• The efficiency of the heating system is 80%^[4]
• The cost of diesel fuel is $1.10/L

The required change in temperature is 30°C

The power consumption of the heater is: Q = 1.3 x V x ΔT Q = 1.3 x 100 x 30 Q = 3900 Kw

Assuming that the heaters will be working at a constant load for 24 hours each day, we can convert the heater power consumption to a daily energy requirement:

3900(Kw) x 3600(KJ/h/Kw) x 24(h/day) = 336,960 MJ/day

Because the heating system is 80% efficient, the daily energy requirement is:

336,960 MJ/day ÷ 0.8 = 421,200 MJ/day

Now that we know the daily energy requirement of the heaters, the fuel requirements can be calculated:

421,200(MJ/day) ÷ 39(MJ/L diesel) = 10,800 L/day

At a cost of $1.10 per litre, the total daily cost of fuel for the heating system would be:

10,800(L/day) x $1.10/L = $11,880 per day.

Discussion

The daily fuel cost was found in this example, but the yearly fuel cost was not. The reason for this is that the temperature at the mine site will change seasonally, thus changing the ΔT value for the heating system. This change means that the energy requirements for the system can change significantly over the year, so it would be inaccurate to assume the total yearly cost based on one day. However, an average yearly temperature can be used to estimate yearly fuel costs, although this value may not be sufficiently accurate.

If the mine experienced this calculated daily fuel cost of $11,880 for an entire year, the yearly fuel cost would be over $4 million. This is a substantial amount, and would likely make up a large portion of their ventilation operating costs. In Northern Canada, air heating costs can make up over 50% of a mine's total ventilation costs^[1]. As such, it is very important to design a system that is efficient as possible.

Cost Reduction

In order to minimize operating costs, a mine air heating system must be designed and operated efficiently. There are multiple ways with which energy can be saved and operating costs can be reduced.

Desired Heating Temperature Reduction

The simplest way to reduce the energy consumption of a mine air heating system is to reduce the temperature to which the system is heating the air. By reducing the target temperature, less energy has to be expended by the heating system, since the air is not being heated to as high a temperature. Reducing the temperature reduces the ΔT variable in the heater power consumption equation, and thus reduces the total amount of energy that the heating system requires. In an arctic mine in Canada, it has been demonstrated that adjusting the set temperature from 5°C to 4°C in a mine can lead to savings of approximately 5% per year^[3].

Feedback-Based Control

It is important to ensure that the air heating system is heating air to the desired temperature so that no ice or frost formation occurs. It is also important to ensure that the air is not being heated beyond the desired temperature, as this is unnecessary and inefficient. In order to monitor and control the heating system, and to make sure that no additional energy is being used unnecessarily, a feedback-based control system can be implemented. The feedback-based system monitors the output temperature of the air coming from the heater. It then adjusts the burner throttle to either increase or decrease the heat output so that the desired output temperature is achieved^[3]. By using this system, a mine can ensure that no additional energy is being consumed to heat air beyond the desired temperature.

Exhaust Air Heat Recovery

The exhaust air from a mine is generally discharged at a much higher temperature than the outside air^[8]. Some of the heat from this exhaust air can be recaptured and used to heat the intake air. When the heat from exhaust air is recovered and used to heat intake air, it reduces the heating requirements of the primary heaters, which saves energy and fuel.

The most common type of exhaust heat recovery system is a closed-loop glycol circuit. This system uses glycol to transfer heat from the exhaust air to the intake air. A schematic of a closed-loop glycol circuit is shown below^[8].

An analysis by H. Dello Sbarba et. al^[8]. showed that closed-loop glycol heat recovery circuits at cold-climate mines can provide energy cost savings of over $600,000 per year. The payback period for these systems can be less than four years.

Exhaust air heat recovery systems are best used when the exhaust air raise is close to the intake raise. A larger distance between the exhaust and intake raises means that the glycol must travel a further distance, losing more heat along the way and making the system less efficient^[8]. Capital costs are also lower when the exhaust and intake raises are near each other.

References

1. ↑ ^{1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11} American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (2015). 2015 ASHRAE Handbook - Heating, Ventilating, and Air-Conditioning Applications (SI Edition). pp. 29.10 - 29.11.
2. ↑ De Souza, E, (2014). Improving the Energy Efficiency of Mine Fan Assemblages. Air Finders Inc.
3. ↑ ^{3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15} De Souza, E., & Wilson, A. (2015). Arctic Mine Air Heating. Air Finders Inc.
4. ↑ ^{4.0 4.1 4.2 4.3 4.4 4.5 4.6} De la Vergne, J. HRMH - Ventilation and Air Conditioning. MineDesignWiki, Centre for Excellence in Mining Innovation
5. ↑ Spirax Sarco (2013). Heating Formulas.
6. ↑ GFS Booth Blog. Direct vs. Indirect Fired Air Heaters (Figure)
7. ↑ World Nuclear Association, (2010). Heat Values of Various Fuels
8. ↑ ^{8.0 8.1 8.2 8.3} H. Dello Sbarba et al., Economics of exhaust air heat recovery systems for mine ventilation, International Journal of Mining, Reclamation and Environment Vol.26 Issue 3 (2012) 185-198.