In part two of this article, we will discuss the effect of surface area and other variables on the efficiency of heat exchangers, why and how fins are used, the effect that a pressure drop has on their operation and examining the use of internal versus external heat exchangers in vacuum furnaces.
In part one we identified the variables most impactful of heat exchanger design and explained the significance of the convection heat transfer coefficient, the thermal conductivity of the tube material, fin material, and the importance of turbulent flow. Reviewing Newton’s law of cooling, Q = hA(Ta – Tb), another variable playing a critical role is the surface area (A) available for heat transfer. Put simply, the greater the area of heat exchange the more effective the heat exchanger will be. For this reason, one of the most important design goals for heat exchanger engineering is the maximizing of the area available for heat transfer while at the same time minimizing the physical size of the exchanger.
Heat Exchange Area
For many cases of heat exchanger design, including vacuum furnace quench systems, the heat energy must be transferred between a fluid with a high heat transfer coefficient and a fluid with low heat transfer coefficient. This is the case for finned heat exchangers used in vacuum furnace quench systems, where the high heat transfer fluid is water and the low heat transfer fluid is air. If the area available for heat exchange were the same on both sides of the heat exchanger (which would be the case with unfinned tubes), its performance would be limited to the less effective heat transfer occurring between the hot air and the tubes. In order to make the exchanger operate efficiently, the tube surface area would need to be made prohibitively large.
The use of fins can be used to increase the heat exchange area on the air side of the exchanger which will help achieve a relative balance between the heat transfer rates on both the air and the water sides, as well as the tubes. The addition of fins to a heat exchanger increases the effective heat transfer area on the air side by 15 times (or more) in comparison to a non-finned design. However, the amount of heat transfer does not increase by the same proportion because the effectiveness of heat transfer via the fin surface area is not as great as that of the tube surface area. This is because the heat that is absorbed at the end of the fins experiences more heat flow resistance than both the base of the fins and the tube wall, resulting in diminishing returns as the fin length increases. The use of fins allows for a very compact and efficient heat exchanger design.
In addition to increasing the fin length, another way the fin surface area can be increased is by increasing the number of fins. Much in the same way that increasing the fin length beyond an optimum length results in no further gains in heat transfer efficiency, a similar relationship exists between the heat transfer and the number of fins utilized (Fig. 3). There is an optimum number of fins per linear meter of tube length that will offer the greatest contribution to heat transfer. Beyond this optimum number, additional fins added to the design will result in decreasing the heat transfer efficiency due to an unwanted resistance to airflow within the heat exchanger. Having more fins than necessary will also be a waste of materials.
One of the factors to consider when designing and selecting a heat exchanger is the resistance to flow, on both the air side (Fig. 4) and the water side of the unit.
The methodologies used to optimize the heat transfer efficiency of a heat exchanger (increased fin area, turbulent fluid flow, and increased fin density) all increase the pressure drop across the exchanger and its resistance to the flow of air. This is important for several reasons. First, the fan required to push the quench air through the exchanger must be carefully sized to account for the pressure drop through the exchanger. If the pressure drop is not properly accounted for and the blower needs to be increased after the fact, it can be problematic. Due to the three laws of fan design (Fig. 5), the power required to move a given volume of air through a heat exchanger varies to the third power of the ratio of any change in airflow volume.
For example, if you undersize a quench system blower and later realize that you need to increase the airflow by a factor of 2, the power required to operate the fan will increase by a factor of (23), or 8 times! This would require a blower motor 8 times more powerful than the original one and quite likely a whole new blower unit. When comparing relative rates of heat transfer for variously sized quench blowers (Table 1), the fans laws are not the only impediment to increased cooling rates – the improvement in quench speed is proportionally much less than an increase in blower size. Everything else being equal, increasing the blower’s power will result in a disproportionally small improvement in cooling rate.
Another reason the pressure drop through the heat exchanger is so critical is due to the heat generated by the friction of the air moving through the unit. A greater pressure drop results in an increase in friction and therefore in heat generated. This additional heat generated from friction must be also removed by the heat exchanger in addition to the heat being removed from the furnace load during the quenching process. Keeping this in mind, a quench fan with a 150 kW (200 HP) motor, for example, adds nearly 150 kW (500,000 BTUH) to the heat load which represents a 5°C (9°F) increase in quench air temperature for a 5 bar, 300 m3/min (10600 ft3/min) quench system. Failure to take this into consideration can result in an undersized quench cooling system.
Internal and External Heat Exchangers
Vacuum furnaces can be designed with the blower, heat exchanger and quench piping either on the inside or the outside of the vacuum chamber, referred to as internal and external quench designs, respectively. Although these designs provide comparable performance, there are benefits and drawbacks to each.
The internal quench system offers the advantage of compactness since it occupies less floor space than the external design. This is somewhat offset out by the need to leave clearance around the furnace for maintenance access via personnel and forklift, minimizing the net gain in floor space. Another benefit of the internal design is that a separate housing for the quench blower and heat exchanger is not required. This comes at a price, however, in that the quench blower motor, drive shaft, and bearings are closer to heat generated in the furnace hot zone, and therefore more vulnerable to heat-related failure. Furthermore, these components are more difficult to repair or replace in the internal quench design and can sometimes require the removal of the entire hot zone. Also, since the heat exchanger is inside the furnace chamber, significant damage can occur to the furnace interior if it develops a water leak.
An often overlooked aspect of heat exchanger design is the supply of cooling water. Flow, pressure, pressure drop, and even the quality of the water play a pivotal role in ensuring that the heat exchanger achieves its full potential and its design parameters are not compromised.
We often take heat exchanger performance for granted and it is often one of the last places we look when problems arise. We now know to ask the right questions of our OEM suppliers and question our cooling water suppliers to ensure that these systems are working at peak efficiency.
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