Hi My Mechanic Cooling Tower Revisited:
System Calculations
To properly operate and maintain a cooling tower, there needs to be a basic understanding of the system water’s use. Water use of the cooling tower is the relationship between make-up, evapora- tion, and blowdown rates. There are a couple simple mathematical relationships between the blowdown rate, evaporation rate, make-up rate, and cycles of concentration of a cooling tower that are very useful to understand the principal flow rates.
The first relationship illustrates the overall mass balance consideration around a given cooling tower:
(1) Make-up = Blowdown Evaporation
In this case, the blowdown accounts for all system losses including leaks and drift, except for evaporation.
The second principal relationship defines cycles of concentration in terms of make- up flow and blowdown flow:
(2) Cycles of Concentration = Make-up ÷ Blowdown
This equation can be rearranged to either of the following to solve for the make-up rate or blowdown rate:
(3) Blowdown = Make-up ÷ Cycles of Concentration
(4) Make-up = Cycles of Concentration × Blowdown
If the evaporation rate and cycles of concentration are known, the blowdown rate can then be determined by substitut- ing equation 4 into equation 1:
(5) Cycles of Concentration ×
Blowdown = Blowdown +
Evaporation
Solving for blowdown:
(6) Blowdown = Evaporation ÷
(Cycles of Concentration -1)
Also, if the blowdown rate and cycles of concentration are known, the make-up rate can be determined by solving equation 4, and then the evaporation rate can be determined by solving equation 6 for evaporation:
(7) Evaporation = Blowdown × (Cycles of Concentration – 1)
Solving for blowdown:
(6) Blowdown = Evaporation ÷
(Cycles of Concentration -1)
Also, if the blowdown rate and cycles of concentration are known, the make-up rate can be determined by solving equation 4, and then the evaporation rate can be determined by solving equation 6 for evaporation:
(7) Evaporation = Blowdown × (Cycles of Concentration – 1)
System Concerns
Cooling towers are dynamic systems because of the nature of their operation and the environment they function within. Tower systems sit outside, open to the elements, which makes them susceptible to dirt and debris carried by the wind. Their structure is also popular for birds and bugs to live in or around, because of the warm, wet environment. These factors present a wide range of operational concerns that must be understood and managed to ensure optimal thermal performance and asset reliability. Below is a brief discussion on the four primary cooling system treatment concerns encountered in most open-recirculating cooling systems.
Cooling towers are dynamic systems because of the nature of their operation and the environment they function within. Tower systems sit outside, open to the elements, which makes them susceptible to dirt and debris carried by the wind. Their structure is also popular for birds and bugs to live in or around, because of the warm, wet environment. These factors present a wide range of operational concerns that must be understood and managed to ensure optimal thermal performance and asset reliability. Below is a brief discussion on the four primary cooling system treatment concerns encountered in most open-recirculating cooling systems.
Corrosion:
Corrosion is an electrochemical or chemical process that leads
to the destruction of the system metal-
lurgy. Figure 7 illustrates the nature of a
corrosion cell that may be encountered
throughout the cooling system metal-
lurgy. Metal is lost at the anode(3) and
deposited at the cathode.(4) The process is
enhanced by elevated dissolved mineral
content in the water and the presence of
oxygen, both of which are typical of most
cooling tower systems.
Figure 7. Example of a Corrosion Cell.
There are different types of corrosion encountered in cooling tower systems including pitting, galvanic, microbiologically influenced (Figure 8), and erosion corrosion, among others (expanded
discussion is available at www.gewater. com/handbook/cooling_water_systems/ ch_24_corrosion.jsp). Loss of system metallurgy, if pervasive enough, can result in failed heat exchangers, piping, or portions of the cooling tower itself.
Figure 8. Microbiologically Influenced Corrosion (Source: Taprogge GmbH).
Scaling;
Scaling is the precipitation of dissolved minerals components that have become saturated in solution. Figure 9 illustrates calcium carbonate scale collecting on a faucet head. Factors that contribute to scaling tendencies include water quality, pH, and temperature. Scale formation reduces the heat exchange ability of the system because of the insulating properties of scale, making the entire system work harder to meet the cooling demand. An expanded discussion for scaling is available at the following link; click here.
Figure 9. Calcium Carbonate Scale (Source: Hustvedt).
Fouling;
Fouling occurs when suspended particles fall out of solution forming deposits. Common foulants include organic matter, process oils, and silt (fine dirt particles that blow into the tower system, or enter in the make-up water supply). Factors that lead to fouling are low water velocities(5), corrosion, and process leaks. Fouling deposits, similar to scale deposits, impede the heat exchange capabilities of the system by providing an insulating barrier to the system metallurgy. Fouling in the tower fill can plug film fill reducing the evaporative surface area, leading to lower thermal efficiency of the system.
Microbiological Activity;
Microbiological activity is micro-organisms that live and grow in the cooling tower and cooling system. Cooling towers present the perfect environment for biological activity due to the warm, moist environment. There are two distinct categories of biological activity in the tower system. The first being planktonic, which is bioactivity suspended, or floating in solution. The other is sessile biogrowth, which is the category given to all biological activity, biofilms, or biofouling that stick to a surface in the cooling system. Biofilms are problematic for multiple reasons. They have strong insulating properties, they contribute to fouling and corrosion, and the bi-products they create that contribute to further micro-biological activity. They can be found in and around the tower structure, or they can be found in chiller bundles, on heat exchangers surfaces, (see Figure 10), and in the system piping. Additionally, biofilms and algae mats are problematic because they are difficult to kill. Careful monitoring of biocide treatments, along with routine measurements of biological activity are important to ensure bio-activity is controlled and limited throughout the cooling system.(6)
Figure 10. Biofouled Heat Exchanger (Source: Taprogge GmbH).
(3) The anode in a corrosion cell is defined as the site where metal is lost from the system structure and goes into solution.
(4) The cathode in a corrosion cell is defined as the site where the metal lost at the anode is deposited.
(5) Low water velocities may occur in poorly designed or improperly operated heat exchangers, in the cooling system piping, or in locations across the tower fill where uniform distribution is not maintained.
(6) Beyond the operational and mechanical problems bioactivity causes in cooling tower systems, there is a human health issue if the system develops a specific bacterium known as Legionella. For more information regarding Legionella and Legionnaires’ disease go to www.cdc.gov/legionella/patient_facts.htm.
DECSA INSTALLATIONS REVIEW:
Closed circuit
coolers
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Hi MiMechanic Fans Revisted:
Table 1: Axial Fans Typical Peak Efficiency;
Fan Efficiency, An Increasingly Important Selection Criteria:
The Importance of Fan Efficiency:
Why is fan efficiency so important? As a general rule, successive generations of electronic enclosures such as personal computers, telecommunications cabinets, as well as system routers, pack increasing functionality into smaller and smaller spaces. Accompanying this trend is the need to remove ever higher levels of heat energy from within those enclosures. Thermal engineers will often force air through a system using fans to regulate the internal temperatures; however as the aerodynamic performance increases so will input power.
In modern day equipment racks it's not uncommon for the total fan load to be a significant factor in the system's power budget. Coupled with the advent of equipment efficiency legislation and a growing awareness of cost of ownership, fan efficiency is becoming a critical selection parameter. Engineers now need to gain an understanding of fan efficiency, balancing it against more familiar metrics such as airflow and noise.
Understanding Fan Static Efficiency:
Fan manufacturers typically provide static efficiency as the value of efficiency, while total efficiency includes the outlet velocity term. Fan total efficiency is calculated using total pressure. Static efficiency is calculated using only static pressure.
Positive static pressure is created as a fan moves air through a system. Negative static pressure is what all other components in the airflow path create as they resist air movement. Different fan types will generate different airflow values while creating a positive static pressure to balance the negative static pressure caused by system obstructions. The fan performance curve (see Fig 1) is a representation of the airflow (X axis) that a particular fan type produces to overcome given static pressure values (Y axis).
Total pressure is the summation of static pressure and outlet velocity pressure. Outlet velocity pressure does not contribute to a fans ability to remove system heat energy; therefore it's not normally included in fan efficiency calculations.
Calculating Fan Efficiency:
As with any energy converter, efficiency is the ratio of input and output power:-
Fan efficiency = Pout / Pin
Fan input power (Pin) is:-
Pin (Watts) = V <Volts> x I <Amps>
Fan output power (Pout) or airpower using Metric units is:-
Pout (Watts) = Air pressure <m3/sec> x Air flow <Pascal's>
Using standard units the formula becomes:-
Pout (Watts) = (Air pressure <inch H2O> x Air flow <cfm>) / 8.5
Example:-
A 48V fan drawing 1A working at an operating point of 200 cfm and 0.5 inch H2O
Pin = 48 x 1 = 48 W
Pout = (200 x 0.5) / 8.5 = 11.76 W
Fan efficiency = 11.76 / 48 = 0.245 or 24.5 %
Fan efficiency = Pout / Pin
Fan input power (Pin) is:-
Pin (Watts) = V <Volts> x I <Amps>
Fan output power (Pout) or airpower using Metric units is:-
Pout (Watts) = Air pressure <m3/sec> x Air flow <Pascal's>
Using standard units the formula becomes:-
Pout (Watts) = (Air pressure <inch H2O> x Air flow <cfm>) / 8.5
Example:-
A 48V fan drawing 1A working at an operating point of 200 cfm and 0.5 inch H2O
Pin = 48 x 1 = 48 W
Pout = (200 x 0.5) / 8.5 = 11.76 W
Fan efficiency = 11.76 / 48 = 0.245 or 24.5 %
The Fan Efficiency Curve:
Fan efficiency varies dramatically as a function of aerodynamic loading. Because airpower is the product of flow and pressure, a fan working in the free air condition (no backflow pressure) has zero pressure and thus is producing no airpower and by definition has zero efficiency. Similarly, a fan in the fully shut off condition (no flow) has zero flow and is also producing no airpower and zero efficiency. The peak efficiency of an axial fan typically occurs at a pressure point of 1/3rd the maximum pressure.
Figure 1 below represents a performance plot of a 120mm size axial fan with curves for both airflow and efficiency.
Figure 1: Pressure vs. Flow Curve - 120mm Axial Fan
As a general rule, fan efficiency increases with blade diameter and speed. Fan manufacturers are now focusing on higher efficiency fans, resulting in new designs with significantly increased peak efficiency compared to older designs. Table 1 provides an indication of peak efficiency values for different standard axial fan sizes and the comparative improvement with newer generation designs.
Table 1: Axial Fans Typical Peak Efficiency;
| Form Factor | Old | New |
|---|---|---|
| 40 x 40 | 10% | 25% |
| 60 x 60 | 14% | 30% |
| 80 x 80 | 16% | 33% |
| 92 x 92 | 18% | 35% |
| 120 x 120 | 24% | 40% |
| 172 round | 35% | 45% |
Fan Selection Taking Account of Efficiency:
Historically, fans were chosen by finding a standard form factor to occupy the available space and then matching airflow performance against system requirement; typically using free flow as a figure of merit. This approach has the potential for missing significant power savings which could be realized by carefully matching fan efficiency to the system operating point.
In the example shown below, (Fig. 2), selecting the fan based upon free air performance would favor the high flow fan option. Overlaying the system resistance line on the performance curve shows the high flow fan would achieve the required performance of 110cfm at 0.48 inch H2O.
However, comparing this fan efficiency at the operating point against an alternative lower free air flow fan design, it can be seen the second design would actually provide higher efficiency while still meeting the duty point.
Figure 2: Pressure Vs. Flow Curve with Fan Efficiency and System impedance
Benefits of Selecting High Efficiency Fans:
Higher levels of power are required to cool the large amount of heat generated by today's high end servers. As a result, more electrical power will be needed to be allocated to the system's cooling components. In some instances, 25% or more of the total power budget for a high end rack system is allocated to the cooling fans.
Using high efficiency fans has a cascade effect on system design. Power supplies can be down sized saving weight and space and the fans power distribution network can be minimized.
The long term benefit of specifying high efficiency fans is a reduction of ownership costs. Large data centers can contain tens of thousands of servers with anywhere between 10 and 50 fans in each.
A few percentage points improvement in the efficiency of every fan installed can quickly represent many thousands of dollars in annual energy savings.
High efficiency fans can be more costly than older fan types, and this can be seen as a deterrent. Engineers and purchasing managers should understand the wider implications of using these newer fan designs.
System level savings can result from the lower power requirements and substantial energy savings can be realized by the end user.
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