Impact of Cooling Tower Blade Modification on Energy Consumption- Issue Oct-Dec 2001

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Issue : October-December 2001

Impact of Cooling Tower Blade Modification on Energy Consumption

By P. Gupta
Mechanical Engineer – E
Nuclear Science Centre, New Delhi

Piyush Gupta is an M.Tech from IIT Delhi in Maintenance Engineering and Machine Dynamics. He has 21 years experience, of which 8 were with BHEL and 13 in his present position.

Atypical central air conditioning plant is a heat pump. It picks up heat in the air handling units or in the heat exchangers through a closed circulation system of chilled water. The heat so picked up is dispensed off through the medium of condenser water in the cooling towers. An efficient working of an air conditioning plant is therefore substantially dependent on efficient disposal of heat in the cooling towers. The Co-efficient of Performance (COP) of the plant is strongly dependent on the extent to which the condenser water temperature is pulled down in the cooling tower. The inputs for doing such a pull down in temperature is the energy consumed by the cooling tower fan motor for a given mass throughput of the ambient air corresponding to a given mass flow of condenser water. This causes the evaporative cooling of the condenser water. Any effort to reduce the power consumption of the cooling tower fan motor without affecting the total flow of air quantity will result in better, energy efficient operations.

An attempt is made to describe one such modification that was carried out in the cooling tower fans. The report also presents quantitative data on the savings so made and the pay back period of the investment.

System Description

The central air conditioning plant of 400 ton capacity at Nuclear Science Centre was commissioned in the year 1989. The plant is principally designed to remove the process heat loads of innumerous components of the particle accelerator system. The heat so picked up in the components is rejected by the air conditioning system through the use of two cooling towers of 200 ton capacity. The cooling towers are of FRP construction and are induced draft type. The cooling tower sucks in ambient air from the cooling tower base, by the use of 1500-mm diameter fan blades mounted directly on the top of the tower. The water is sprinkled vertically down through the use of distribution headers at the top, at the outlet of which nylon spray nozzles are installed at multiple exit points. This means that the cooling tower is a counter-flow type.

The cooling tower fan blades that suck in the ambient air are rotated at 960 rpm through the use of a 10 hp induction motor. The fan blades are made of aluminium casting.

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Basis of the Modification Proposed

Since the blades are made of aluminium sand-casting, the leading and the trailing edges of the blades are inherently defective as regards to the smoothness in the blade profile. The flow surface over the blades is also rough. This implies that electrical energy is being wasted due to the following two reasons:

There is a flow friction when air is flowing over the blade surface.
There is a loss of head due to the shock in the flow at entry and at exit, because of the rough profile of the blade edges.

Another major factor responsible for the wastage of electrical energy is, due to the wastage in motor amperage for rotation of heavy mass (inertia of the rotating mass) of the cooling tower fan blade and the hub.

Modification

Modification The aluminium casting blade and impeller set was replaced with a blade and impeller set made out of fibrereinforced plastic (FRP). The blade angle set on the new set of blades was 18° as against the 14°, set for the aluminium blades. This enabled greater mass throughput of air through the tower, as can be seen from Table 1 and 2. However, the motor amperage was still within the prescribed motor capacity. It is also evident from the tables that the efficiency of the towers also increased due to increased evaporation rate of water molecules due to increase in the air quantity.

Table 1 Cooling tower readings with aluminium blade and hub. The measured velocity is an average of sixteen readings.
Measured Velocity	Evaluated Velocity	Flow area	Air flow	Motor amps	Power Factor	Motor Power	DBT	WBT	CT in	CT out	η
meter/s	fpm	sq.ft.	cfm	amps		kW	°F	°F	°F	°F	%
7.06	1390	18.84	26,187	14.5	0.61	6.36	94	83	92	87	55

Table 2 Cooling Tower readings with FRP blade and hub. The measured velocity is an average of sixteen readings.
Measured Velocity	Evaluated Velocity	Flow area	Air flow	Motor amps	Power Factor	Motor Power	DBT	WBT	CT in	CT out	η
meter/s	fpm	sq.ft.	cfm	amps		kW	°F	°F	°F	°F	%
8.47	1667	18.84	31,406	10.9	0.62	4.85	94	83.5	92	86.5	65

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Comments

• It is noted that due to installation of FRP blades, the current drawn by the motor reduced. The savings in the energy consumed is 1.51 kW for each cooling tower.

The volume throughput of the air increased from 26,187 to 31,406 cfm. This increase is about 19.9%.
The efficiency of the tower increased by 9%.
The monthly savings just on motor current is: 1.51 × 30 × 24 × 4 = Rs. 4,349. This is assuming that the duty demand on the cooling tower is continuous for 24 hours a day for each day of the month as is the case at Nuclear Science Centre. The energy cost is assumed as Rs 4.00 per kWh.
With an investment of Rs 23,000 the pay back period is 23,000/4,349 = 6 months approximately. The pay back period will of course depend on the application concerned.
The annual savings for both the towers put together is approximately Rupees one lakh.
Installation of FRP fan blade hub has led to one additional benefit. The maintenance work of overhauling the motor has become easier. This is because the FRP hub does not seize on the motor shaft as the aluminum hub does and which had to be invariably broken for removal, every time the motor had to be serviced.
The cooling tower fan blades were replaced in August 2000. They have been in continuous round the clock operation for more than a year and their structural capability has been proven beyond doubt. It may be worth mentioning here, that the tip velocity at the blade periphery principally determines the design of the blade for its material properties. This is because the tear away stress is maximum at this section in the complete length of the blade, i.e. from where it is anchored onto the fan hub right up to the blade tip. The blade section thickness of course is determined by the aerodynamics of the airflow and the change in momentum of air throughput, that the blade section has to encounter.
There are innumerous manufacturers of FRP fan blades. They have to be given the data on the blade revolutions per minute, the diameter of the fan and the mass throughput of air besides the static pressure that has to be developed across the blade section to come up with a good design. Subsequently, they can manufacture the product for replacement in existing cooling towers.

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