Thinking of Using Variable Frequency Drives (VFD'S) on Boiler Feedwater Pumps? Not so fast...

Transcription

1 Third Quarter, 2014 Newsletter HISTORIC QUOTES: "All things are difficult before they are easy. -Thomas Fuller Thinking of Using Variable Frequency Drives (VFD'S) on Boiler Feedwater Pumps? Not so fast... "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do." - Mark Twain "Don't let the fear of losing be greater than the excitement of winning." -Robert Kiyosaki Or So, you are thinking of using a variable frequency drive (VFD) on a boiler feedwater pump to save money? Why not, you say, if my boiler is only running at 50%, why run my feedwater pump at full speed? If I install a VFD and spin the pump at half speed, I can save half the energy cost. Unfortunately, the laws of physics just don't work that way. When it comes to pumps there are a special set of laws known as the Affinity Laws. These laws define the relationship between pump speed, flow, pressure and horsepower. Simply stated, the change in flow is

2 proportional to the change in speed, the change in pressure (head) is proportional to the change in speed squared, and the change in horsepower is proportional to the change in speed cubed. Expressed as equations these are: - Google "boiler coal to gas conversion company" Lipten is Number 1! - Lipten was awarded a contract for a boiler coal to natural gas conversion of third 210,000 lb/hr boiler after successful conversion a customer's other two similar boilers. - Lipten was awarded a contract for conversion of two 150,000 lb/hr coal boilers to natural gas firing. - Lipten and our partner were awarded a contract for the design-build installation of two 120,000 lbs/hr boilers. RPM1/RPM2 = FLOW1/FLOW2 (RPM1/RPM2)^2 = HEAD1/HEAD2 (RPM1/RPM2)^3 = HP1/HP2 So, how does this apply to a VFD on your feedwater pump? Let's see what happens when we apply the affinity laws to your pump running at 50%... Speed: True, if your 3600 RPM pump is now running at 1800 RPM, the ratio would be 1800/3600 = 0.5, and the flow would be cut in half, just what you want. But wait! No water is getting into your boiler! What's going on? Pressure: It's the second affinity law that got you! At 50% speed, the pressure is reduced by (0.5)^2 or down to 25%. Boiler feedwater pumps are sized to run at ASME code requirements, meaning that you furnish feedwater to your boiler at least 3% higher pressure than the relief valve setting on the boiler (plus pressure drops between the pump and the boiler). So, say your boiler operating at 120 PSI has a maximum working pressure of 150PSI. That means your feedwater pump was probably designed to operate at around 160 PSI. At 50% speed, the pump is only putting out 40 PSI, certainly not enough to overcome the 160 psi at the boiler! Working the equations backwards to maintain at least 150 PSI, the slowest the pump can spin is 150/160 x 3600 = 3486 RPM or just a 3% speed decrease! Horsepower and $Cost$: Energy usage = horsepower. So can you really save on your energy costs? With just a 3% speed reduction, the motor horsepower decreases (3486/3600)^3 which is 9% less horsepower. On a 10HP motor, that's 1HP or.745kw. Using a rate of $0.07 /KWH, a 24/7/365 operation would save no more than $450 a year. So, if the cost of installing a VFD was only $5,000, you've got a payback greater than 10 years. Even if you have a larger boiler with a 100HP

3 feedwater pump, you are saving ~$4,500 a year with a VFD. Assuming a very conservative $25,000 cost for the VFD installation means a payback greater than 5 years. But what if...? What if a boiler ran at 600 psi and is being reconfigured to run at 150 psi? The feedwater pumps would be oversized for the new operating conditions. Therefore, the pumps may be able to reduce speed significantly without dropping under the required pressure. This could be a significant cost savings. Also, what if your electrical rates are double those used in the examples? Your savings would double. So, is using a VFD on a boiler feedwater pump a good idea? Maybe. Just make sure you are using the right figures in your evaluation and don't break any (affinity) laws! DILBERT (2010) Scott Adams. Used by permission of UNIVERSAL UCLICK. All rights reserved. Did you know...

4 Radiant Heat vs Convective Heat Radiant heat is transferred through space to a surface by electromagnetic radiation (think infrared light). This is exactly how the sun warms the earth. Convective heat requires a fluid that can flow, like a liquid or a gas. Heat energy is transferred by the contact of the fluid with a surface. In the case of a boiler, water is heated by both forms of heat. In the combustion area, the heat released from burning fuel (coal, gas, oil, etc.) radiates energy from the flame to the walls of the boiler tubes (radiant heat). Beyond the combustion area, the flow of the hot flue gas passes over boiler tubes transferring additional heat (convective heat). Lipten Race Car Update The second stop of the season for the #21 Lipten Energy Solutions Acura RSX was the Historic Road Atlanta in Braselton, Georgia for the 37th annual HSR MITTY. During this two day event, driver Eric Filgueiras dominated his class, finishing 2nd on Saturday and 1st on Sunday. With a class of 10 cars and an overall field of 54 cars on the track, Filgueiras quickly found his footing with both car and circuit, proving his ability as a driver who can compete among some of the best.

5 The 20 year-old pilot saw crowded racing conditions Saturday afternoon for race 1 with substantial traffic to work through. With the races being 7 lap features, each driver had to make their moves quickly, with Eric doing so, battling to end the first race 2nd in class and 16th overall. On Sunday afternoon everything went Filgueiras' way, as he made quick work of the class leader and overall position, ending the race 1st in class and 11th overall! It was a great weekend all around for the up-and-coming driver. "I couldn't be more pleased with how this weekend went for us," explained Filgueiras, "I wasn't sure what to expect with this being my second weekend in this car and never having driven at Road Atlanta. I love the course and the event that James Redman and everyone at HSR puts on is absolutely top notch. I especially want to thank Lipten Energy Solutions, Kovi Consulting, and everyone who continues to get behind my efforts week in and week out." Lipten is Hiring! Lipten is looking for experienced power plant mechanical engineers, project managers, software developers and service technicians. If you're interested in a dynamic career with a healthy, growing firm, you've come to the right place. Lipten Company offers competitive salaries and a comprehensive benefits package including medical, dental and 401(k) as well as three weeks paid personal time off in addition to ten paid holidays. Lipten is an equal opportunity employer Click here to see if you are interested in potentially being part of the Lipten winning team. Lipten Mission Statement Lipten will make "power" a source of value for our clients by providing superior engineering, procurement and construction of steam, hot water, compressed air, chilled water and electrical systems. Lipten operates profitably throughout the world in an ethical, environmentally sensitive and socially responsible manner while focusing on the core principals of systems' safety, reliability and efficiency. Essential to achieving our goals is our dedication to maintaining a positive working environment.

6 Please Click Here to view newsletters and more on the LIPTEN website. Lipten is an Engineering, Procurement and Construction (EPC) firm that specializes in Central Energy Plant (CEP) design and construction. Lipten also has a controls group that provides standard and custom control solutions. Lipten provides steam generation, power generation, chilled water systems, compressed air systems, water treatment systems, controls and related Energy Center equipment and services. Lipten's level of support can vary from an advisory role to complete turn-key facility construction. Services include: design, engineering, drafting (CAD), equipment specifications, procurement, installation, construction management, site audits, start-up, operator training and maintenance.