Why use VSD in existing HVAC installations

Transcription

1 Introduction... 2 Saving energy Controlling capacity to actual need Examples Variable speed duty Constant speed duty Reducing maximum kw demand Removing unneccessary energy loss caused by vanes and valves Table of contents Reduction in mechanical wear and tear Increasing comfort level Controlling comfort & reducing fan and pump noise Calculation of pay back time Conclusion MC.60.D VLT is a registered Danfoss trademark 1

2 Introduction Introduction Saving energy: Energy savings can be obtained by reducing the speed of some of the most energy consuming installations in buildings: pumps and fans, used in HVAC applications. Pumps and fans in old, existing HVAC installations, normally run at full speed all the time. Adjustment to actual flow or pressure demands is usually performed by valves, dampers or guide vanes. This is not energy efficient. When using a VSD to control the speed of a pump or fan, considerable energy savings can be gained. Reduction of mechanical wear and tear: Besides the energy savings, fitting VSDs on fans and pumps will reduce the wear of belts, bearings and valves, reducing the overall maintenance costs. Increasing comfort level: By reducing the speed of a centrifugal fan or pump, the acoustic noise level as well as the draught from fan systems are reduced. Water hammer from pump systems can be totally avoided. Controlling comfort: When using a VSD, accurate temperature and pressure control can be maintained. Calculation of pay back time: The energy savings will ensure a short pay back time of the investment in VSDs. Typically 1-2 years. Regarding Singapore (March 1996): The Singaporean government has introduced a writing off scheme for equipment used for energy saving purposes, via the taxes. This will make it even more attractive to invest in energy saving equipment. Typical advantageous applications: Experience shows that the payback time for the following applications is very short: VAV AHUs (fans and pumps) Secondary chilled water pumps Constant pressure ventilation fans Condenser fans Saving energy Save energy and reduce costs by: 1. Controlling the HVAC system s capacity to match the actual need. 2. Reducing maximum kw demand of the building. 3. Removing unnecessary energy loss caused by vanes and valves. 2 MC.60.D VLT is a registered Danfoss trademark

3 Controlling capacity to actual need Centrifugal pumps and fans run according to a Variable Torque characteristic. Changing the speed of a pump or fan will change the torque demand by the square of the speed (n): T = f (n 2 ). By changing the speed to 80% of the nominal value, the torque demand will thus only be about 64% of the nominal value. The power consumption, however, of a pump or fan motor is according to a cube law, P = f (n 3 ). This means that by reducing the speed to 80% of the maximum value, the power consumption will be reduced to about 50% of nominal value. Energy savings of up to 50%, or even more, is often possible in a typical installation, but only by using a VSD. Fig. 1, on this page, shows the typical power consumption of valve control and VSD control. Energy savings according to the actual need, will of course depend on the variations in need. On very warm days, aircon and ventilation systems will have to run at full speed, but this will only happen a few days over the year or a few hours a day. The rest of the year or during off-peak periods every day, energy will be saved. Energy saving E.g.: At 80% speed the power consumption is = = = 51.2% Fig % % Power 52 % Valve control Valve control Area of saved energy % Power 80% 100% Variable air systems using constant speed fans with dampers or guide vanes controlling the airflow, consume a lot of energy by constantly running the motor at full speed. Pumping circuits, e.g. chillers, running at constant speed waste chilled water by using by-pass loops, returning the excess chilled water not used by the installation. Energy is wasted both in pump motors and in unnecessarily cooled water. In comparison to power consumption in constant speed systems, power consumption in variable speed systems using VSD, can be calculated according to the actual need (P = n 3 ): MC.60.D VLT is a registered Danfoss trademark 3

4 Energy saving How many hours at a speed (i.e. flow requirement) of: 100% = Power consumed 100% 90% = 72.9% 80% = 51.2% 70% = 34.3% 60% = 21.6% 50% = 12.5% Multiplying the number of hours (h) running at the various speeds by the associated amount of power (kw), and adding the values, will show the total energy consumption (kwh) per year for the actual motor driven by a VSD. Fig. 2, below, shows a typical capacity characteristic of a pump as well as the associated motor shaft output in kw. Practical figures may vary with different pump designs. Fig. 2 Pump characteristics: Fig. 2, below, shows a typical capacity characteristic of a pump as well as the associated motor shaft output in kw. Practical figures may vary with different pump designs. Valve regulation follows A 1 to B 1 curve. By speed regulation it is possible to utilize the pumps characteristic. Thereby power consumption follows the dotted curve from A 1 to C 1. Compared to constant speed operation, the difference from the above mentioned variable speed calculations, represents the savings in energy. By repeating this calculation for other pump and fan systems, within a building, the total energy saving of the whole building can be calculated. Two theoretical examples based on general experience as well as one practical example, all showing the necessary calculations, are given on the following pages. 4 MC.60.D VLT is a registered Danfoss trademark

5 (Calculations on pay back time for all three examples will be shown at the end of part 1). Example 1 A 15 kw AHU operating for 12 hours during week days and 6 hours during week-end. In total 66 hours per week. Energy saving a) Energy cost at constant speed: Energy consumption per week: 15 kw x 66h = 990 kwh Electricity rate is US$ 0.10 / kwh. Energy cost per year: 990 kwh x US$ 0.10 x 52 = US$ 5,148 b) Energy cost at variable speed: Assume average speed is 75% corresponding to 42% power consumption. Energy consumption per week: 42% of 15 kw x 66h = 416 kwh Energy cost per year: 416 kwh x US$ 0.10 x 52 = US$ 2,163 c) Value of energy saved by using variable speed: US$ 5,148 - US$ 2,163 = US$ 2,985 e) Investment for running variable speed (Approximate figures): VSD price : US$ 3,700 Transmitter for closed loop operation (e.g. 1 pressure transmitter) : US$ 300 Labour hours for mounting of drive and transmitter ( 4 h) : US$ 200 Installation of drive and transmitter (20 m of ctrl.cable) and test run of application : US$ 100 Total installation cost : US$ 4,300 Note: All calculations are shown in US$, as a neutral currency. When you convert the figures to your currency, you will see that the energy savings and pay-back time are just as favourable. MC.60.D VLT is a registered Danfoss trademark 5

6 Energy saving Example 2 A 15 kw AHU operating for 24 hours during week days and week-ends. In total 24h x 365 days = 8760 hours per year. a) Energy cost at constant speed: Energy consumption per year: 15 kw x 8760h = 131,400 kwh Electricity rate is US$ 0.10 / kwh. Energy cost per year: 131,400 kwh x US$ 0.10 = US$ 13,140 b) Energy cost at variable speed: Assume average speed is 80% corresponding to 51.2% power consumption. Energy consumption per week: 51.2% of 15 kw x 8760h = 67,276 kwh Energy cost per year: 67,276 kwh x US$ 0.10 = US$ 6,728 c) Energy cost saved by using variable speed: US$ 13,140 - US$ 6,728 = US$ 6,412 e) Investment for running variable speed (Approximate figures): VSD price : US$ 3,700 Transmitter for closed loop operation (e.g. 1 pressure transmitter) : US$ 300 Labour hours for mounting of drive and transmitter ( 4 h) : US$ 200 Installation of drive and transmitter (20 m of ctrl.cable) and test run of application : US$ 100 Total installation cost : US$ 4,300 6 MC.60.D VLT is a registered Danfoss trademark

7 Example 3 Shows the design calculations for the Asian Museum of History, Hong Kong. Given is the building s cooling load curve for a period of 24 hours: Fig. 3 Energy saving Based on fig. 3, a block cooling load profile was made, giving some expected average fan speed requirement values in %, using VSD s, to meet the cooling load: Fig. 4 % is expected fan speed to meet cooling load MC.60.D VLT is a registered Danfoss trademark 7

8 Energy saving From Good Practice Guide book no. 2 issued by the UK Department of Energy, graphs were used to convert the fan speeds [%], found in fig. 4, to power consumption [%]. Fig VSD 2. Guide vane 3. Damper 4. Hydraulic/Eddy current coupling Variable Speed Duty Based on avg. fan speeds as noted in fig. 4 and the power consumption (kw) relating from fig. 5, the energy consumption in the different periods, as well as the total energy consumption (kwh), for Variable Speed Duty can be calculated: Fig. 6 Period Av. Fan Speed % Power Actual Hours kwh s In period % Power kw Run Used Per day MC.60.D VLT is a registered Danfoss trademark

9 For the alternative, Constant Speed Duty, calculations of the energy consumption were also made: Constant Speed Duty (Average values): Assume 50% of fan load (16 kw) for 24h = 384 kwh (One group of fans constantly running) Assume 50% of fan load (16 kw) for 12h = 192 kwh (One group of fans running day time only) Energy consumed per day: 576 kwh Energy saving Now the energy savings from running Variable Speed Duty, as opposed to Constant fan Speed Duty, can be calculated: Based on running 365 days / year: Constant Speed Duty 365 x 576 kwh = 210,240 kwh Variable Speed Duty 365 x 199 kwh = 72,625 kwh Saving = 137,615 kwh Savings in energy costs per year: 137,615 kwh at US$ 0.1 / kwh = US$ 13,762 This saving represents a 65% reduction of the energy bill! Total cost of Danfoss VLT 6000 HVAC drives in this case were = US$ 17,406 Reducing maximum kw demand Fig. 7 When using dedicated types of Variable Speed Drives, the high starting current of the motors will be avoided, as these drives include soft starting feature whereby zero starting current is produced, apart from the magnetising current. Thereby the maximum demand current is reduced. This adds to the energy savings already obtained with the pumps and fans. Fig. 7, on this page, shows the differences in starting current for various types of drive systems. In Singapore Maximum Demand is today charged on the highest continuous power consumption, lasting at least half an hour, over a period of one month. In a retrofit situation existing starters (f.ex Y/ -starters) and power factor correction equipment (PFC capacitors) can be removed reducing energy-loss and maintenance relating to these components. 1 = VLT 6000 HVAC 2 = Star-Delta 3 = Soft Starter 4 = Direct-on-line MC.60.D VLT is a registered Danfoss trademark 9

10 Energy saving Removing unnecessary energy loss caused by vanes and valves Dampers, guide vanes and valves in fan or pump systems cause a pressure drop across them dependent on the speed of the air- or water flow passing through. When retrofitting with VSD s dampers, guide vanes or valves can just be left fully open, or simply removed. Using a VSD, reduces or even eliminates the unnecessary pressure drops and thereby the unnecessary power consumption caused. This improves overall efficiency and capacity. In constant speed pump systems bypass loops are commonly used as part of the regulating function, keeping constant pressure at varying flow. Bypass systems for pumps can be equally eliminated by using a VSD to control the flow of the pump according to the actual need. Reduction in mechanical wear and tear When a HVAC installation no longer runs at full speed, additional benefits are provided: Besides the energy savings, fitting VSD on fans: Increases lifetime of belt drives on fans Less wear on bearings, as the shock load on belt and bearings is removed through soft start/stop. Modern VSD s include belt monitoring, meaning longer intervals between maintenance. and because vanes, dampers and actuators are no longer needed: No more jammed guide vanes / dampers No more loss of performance No guide vane maintenance costs Lower noise levels / better air quality control Longer intervals between air filter changing (lower speed = less dirt) On pumps, fitting VSD will: Remove water hammer ( at start/stop situations) Prevent blown valves Reduce leakages as pressure is kept constant As it appears from the above, several nuisance problems can be avoided and considerable savings can be gained. Increasing comfort level Noise from fans: As a by-product of the energy saving from reducing fan speeds, acoustic air movement noise from the fan is reduced. Noise from fans in a quiet environment can be very annoying. If a fan, sized for full occupancy, for example, is supplying a partly occupied room, the acoustic noise level can be as high as e.g. 70 dba. The noise level will vary dependent on the throttling with dampers or guide vanes. The more throttling the more noise. Keeping dampers or guide vanes fully open and reducing the speed from 100% to f.ex. 50%, reduces the noise by 16.5 dba. (A reduction of 10 dba corresponds to a 50% reduction of the noise level), according to standard fan catalogues. The following formula shows how to calculate the above. Calculating noise reduction: 55 x Log (dba) New speed Old speed 10 MC.60.D VLT is a registered Danfoss trademark

11 The relationship is explained as follows: Comfort increase Fig. 8 Sound level on air quantity regulation Noise is a determining factor in for example central Airconditioning for hotels, hospitals etc. Draught: By reducing the speed of a centrifugal fan or pump, the acoustic noise level as well as the draught from fan systems, can be very much reduced too. That increases comfort and may reduce sickness leave. Fig. 9 Leaks: Leaking water from pump systems, usually caused by water hammer, can be totally avoided by using a VSD with zero starting surge and a ramp to allow an extended starting period. This also extends quality and life time on pipework and fittings. Controlling comfort: When using a VSD, accurate temperature and pressure control can be maintained. See Fig. 9. MC.60.D VLT is a registered Danfoss trademark 11

12 Savings Calculation of pay back time When considering energy savings it is important to know the investments necessary to obtain the expected energy savings. And most interesting of all, maybe, to know about the pay back time of the investment. Calculation of the pay back time is initially done for each VSD installed and eventually the data for other frequency converters are added. First of all the investment necessary for obtaining energy savings has to be calculated: - The price of the frequency converter together with necessary transmitters. - The cost of installing the frequency converter incl. wiring of power and control signal as well as the necessary changes in pipe- and/or duct work. Secondly: - The energy savings, calculated as the difference between the energy consumption in a constant speed system compared to a frequency converter controlled system, based on the examples described on pages 7, 8 and 9, multiplied by the electricity rate per kwh, has to be established. Thirdly: - The savings on maintenance, has to be added to establish the total savings. Example 1: Pay back time based on theoretical example 1: Total installation cost of VSD = Energy saving per year US$ 4,300 = 1.44 years US$ 2,985 Note: Savings on maintenance has not been added in this example, as it depends on the actual situation, but the formula indicates that the pay back time will be even shorter, than estimated here. Example 2: Pay back time based on theoretical example 2: Total installation cost of VSD = Energy saving per year US$ 4,300 = 0.67 years US$ 6,412 Note: Savings on maintenance has not been added in this example, as it depends on the actual situation, but the formula indicates that the pay back time will be even shorter, than estimated here. Eventually the pay back time can be calculated as the total cost of having a VSD installed, divided by the total savings (energy and maintenance) for one year: Pay back time (simple): Total cost of installing VSD Total energy savings for one year Pay back time (real): Total cost of installing VSD (Total energy sav. pr. year + savings on maintenance/spare parts) Example 3: Pay back time based on practical example 3: Total cost of VSD = Energy saving per year US$ 17,406 = 1.26 years US$ 13,762 Note: Savings on maintenance has not been added in this example, as it depends on the actual situation, but the formula indicates that the pay back time will be even shorter, than estimated here. 12 MC.60.D VLT is a registered Danfoss trademark

13 Conclusion As described in the various sections of this paper, the majority of existing HVAC installations, which are running with constant speed pumps and fans, can be converted into profitable, cost efficient, energy saving (money saving) installations, by investing in Variable Speed Drives and a few transmitters. All together, a huge reduction of the energy bill, often 50% or more, a very short pay back time of the investment, typically 1 or 2 years and increased comfort level are obtained. Savings Furthermore, the cost reduction for maintenance and replaceable parts is obvious and the improved comfort level will enhance the indoor climate and eventually increase the human work output. Typical applications where substantial energy savings can be achieved by using VSD: Supply fans Simple retrofit with VSD is possible with modern dedicated HVAC drives including basic and vital HVAC components. Return fans Exhaust fans Smoke extract fans Condenser fans Induced draft boiler fans Cooling tower fans Condenser pumps Chilled water pumps Hot water/domestic water pumps District heating supply pumps District cooling pumps Pressure booster pumps Next step The booklet, HOW to use VSD in Existing HVAC Installations MA.35.J1.02, Retrofit Guideline Part II, will explain in details how a VSD is installed in the easiest possible way. The HOW booklet will also describe how unnecessary problems can be avoided. MC.60.D VLT is a registered Danfoss trademark 13

14 For your notes/calculations 14 MC.60.D VLT is a registered Danfoss trademark

15 For your notes/calculations MC.60.D VLT is a registered Danfoss trademark 15

16 For your notes/calculations 16 MC.60.D VLT is a registered Danfoss trademark