Fortunately, consultants and engineers operating in the conditioning industry have become aware of the importance of choosing each piece of equipment and system correctly for the total savings of an installation.
by Alfredo Sotolongo*
I feel great satisfaction to see that more and more consulting engineers are concerned about making the right selection of water pumps for the air conditioning system.
They make sure they take into consideration the following:
- If the pumping system is closed, as in the case of cold water from a chiller.
- If the system is open, as in the case of condensation water in cooling towers.
- When the pumps operate more than one at a time, either in parallel or in series.
- When the system is opened they take into consideration the NPSHR (Net Positive Suction Head Required) especially if the pump is separated from the tower and there is a considerable pressure drop between the outlet of the tower and the inlet of the pump.
When taking into consideration these basic concepts, it should be avoided that once the system begins to operate there are problems of cavitation, air in the system or lack of thermal capacity that at that time could be late and / or expensive to repair. Many times no problem is detected with the naked eye and yet the system does not operate at the efficiency for which it was designed, creating an additional expense in electricity consumption.
Speaking of efficiencies, each time a pump is selected, the possibility of justifying variable frequency drives, either external or as an integral part of the pump, must be investigated. This applies not only to variable flow pumps, but also to constant flow pumps. The selection program of armstong pumps allows to analyze the behavior of the pump and its consumption at different rotational speeds.
As I have explained in many previous articles, the relationship between water flow and electricity consumption is exponential to the cube, in other words, if a pump at 100% of its capacity consumes 100% of the energy, when the capacity drops 50%, the energy is reduced to 12.5%.
Example:
Maximum flow rate 1000 GPM and horsepower 100 HP. If we apply the equation of affinity laws, then:
New HP = Original HP (New GPM / Original GPM)³, where
New HP = 100 (500 / 1000)³ = 100 (0.125) = 12.5 HP
Because it is very difficult to determine during the design the exact point of operation of the pump, since it is subject to installation; the application of variable frequency drives in constant flow systems is becoming more and more popular every day. Instead of balancing the system by artificially creating pressure drops, the rotational speed is reduced and energy consumption is minimized. The energy saved helps to recover the investment of the variable frequency drive in a very short time.
Among the companies that manufacture water pumps for air conditioning systems, the one that is at the forefront in the research and development of equipment to reduce energy consumption is Armstrong, which, with the IVS concept, integrates the frequency inverter to the pump without the need for external sensors which offers the following advantages:
In the frequency inverter, the operating conditions of the pump corresponding to 10 different rotation speeds are programmed for the same impeller diameter. The data that are integrated are: power, pressure and flow.
For each value of electrical consumption corresponds a flow rate and a pressure differential, therefore, when there is a change in electricity consumption the controller compares it with the speed of rotation and brings the rotation speed to the coincident value in the operating curve of the system.
The most important achievement is that the electricity consumption will be, at all times, only the minimum necessary to meet the need of the pumping system.
Due to the ever-increasing cost of electrical energy, consulting engineers have managed to educate building owners to rely more on systems that minimize consumption, resulting in considerable savings of money for their customers and ecological benefits for the country.
If you need more information on any of the topics covered in this column, please contact me at the email: [email protected].
* President of Protec, Inc., is certified as a professional engineer in Puerto Rico and the state of Florida; has more than 40 years of experience in the application and sale of systems and equipment for energy conservation. He is a member of ASME (American Society of Mechanical Engineers), AEE (Association of Energy Engineers), ASHRAE and was president of the Miami chapter of that association.
