erd efficiency - fedco

Fluid Equipment Development Company, LLC www.fedco-usa.comOffices in USA, Singapore and Dubai [email protected]

ERD EfficiencyWhat Systems Builders and Consultants

Need to Know

White Paper

MK-WP-11


Executive Summary

There is no standard definition of efficiency for energy recovery devices (ERDs). Each supplier is free to create their own definition that (not surprisingly) often favor their particular technology.

This White Paper addresses the impact of ERDs on the energy consumption of the RO process using an objec-tive and fully transparent criteria. The analysis shows that published ERD efficiency data has virtually no correlation to the actual energy consumption of the RO system.

Two new factors are presented, transfer efficiency and field efficiency that allow a more accurate characteriza-tion of ERDs in realistic field operation. The impact of various ERDs on energy consumption of a hypothetical 20,000 m3/day SWRO train is presented.

In large scale systems, the analysis clearly indicates tur-bochargers provide the same or better energy consump-tion than all other ERDs including Isobaric Chambers.

In particular, isobaric chambers, that purport to have a 97% efficiency have, in fact, an efficiency in range of 83-85% in most field applications. An unfortunate consequence is that efficiency requirements of 95+% are often included in project specifications precluding other ERDs that can offer a significantly lower cost of permeate through comparable energy consumption and reduced CAPEX and OPEX.

It is time to open project specification to all field-prov-en ERDs and let the market decide the winner.

ERDs in RO

Reverse osmosis (RO) systems reject a portion of the high pressure (HP) feed as HP brine. Energy Recovery Devices (ERDs) recover brine hydraulic energy thereby reducing net energy consumption. This paper will focus on two ERD types that appear to dominate the ERD market:

• Turbocharger or Turbo - an integrated pump/turbine that uses brine energy to produce a pressure boost the feed. Refer to Figure 1;

Motorized Turbocharger or HEMI – A type of turbo that uses a motor to adjust rotor speed. Refer to Figure 2;

• Isobaric Chamber/work exchanger - direct acting positive displacement pump in parallel with the HP pump. Refer to Figure 3.

Please keep in mind that the ultimate criterion for ERD selection is the impact on the cost of permeate. This criterion includes the CAPEX and OPEX of the ERD system hence encompasses many factors beyond “ERD efficiency”. Other papers address these broader issues.

What is ERD Efficiency?

There is no industry accepted definition of ERD effi-ciency. ERD suppliers have offered various definitions which, not surprisingly, favor their particular equipment resulting in confusing and contradictory claims.

A definition that should be universally acceptable is an efficiency that characterizes the reduction in energy consumption of the RO system.

This paper will reach that goal by using two efficiency definitions. Transfer efficiency (ETRN) is the ratio of hydraulic energy transferred to the feed stream divided by the hydraulic energy available in the brine stream:

ERD Efficiency — Whats Systems Builders and Consultants Need to Know

Figure 3

Figure 2

Figure 1


ETRN = (∆PF x QF) / (∆PR x QR) [1]

Where

∆PF = pressure boost in the feed stream

QF = feed flow passing through the ERD

∆PR = brine pressure drop through the ERD

QR = brine flow passing through the ERD

Field efficiency, EFIELD, captures factors related to ERD auxiliary equipment and system performance impacted by the ERD not otherwise captured by transfer efficien-cy. A field efficiency of 100% indicates that the ERD has no adverse impact on system performance. Net efficiency, ENET, is defined as:

ENET = ETRN x EFIELD [2]

Net efficiency is the key parameter in ERD efficiency evaluation. Let’s see how various ERDs compare in transfer efficiency, field efficiency and net efficiency.

Turbochargers

The turbo is a booster pump driven by an integral brine turbine. Figure 4 displays a turbo rated to 3,000 m3/h. The highest reported turbo transfer efficiency is 84%1.

Turbochargers convert brine hydraulic energy to me-chanical energy and then back to hydraulic energy in the feed stream within a single casing. The separation of energy transfers permits the turbo to handle brine and feed flows that differ in pressures and flow rates. The turbo boosts feed pressure between the HPP and membranes (per Figure 1) or boosts pressure between membrane stages among various options.

The turbo field efficiency, EFIELD, is 100% as it has no adverse impact on RO system performance. Therefore, field efficiency equals transfer efficiency.

Motorized turbos will be discussed below.

Isobaric Chambers

The most common isobaric chambers today uses a rotating ceramic cylinder with multiple passage that place brine into direct contact with the feed. This pa-per will refer to it as the “CIC”.

Figure 5 shows a typical CIC array with manifolds (each yellow cylinder is a CIC). The mandatory HP booster pump, flow meters, control valves, etc. are not shown. Multiple unit installations are typical due to a maximum unit capacity of up to about 70 m3/h.

CIC efficiency is difficult to determine for several rea-sons. The first problem - what is a CIC? Does it include


Figure 4

Figure 5



the mandatory high-pressure booster pump and exten-sive piping, flow meters and valves? The supplier implies no, it’s not part of the ERD nor are the associated losses chargeable to the CIC. It is easy to understand why – the supplier can claim a great efficiency.

Let’s bring clarity to CIC performance by determining ETRN and EFIELD using the following factors:

• Corrected efficiency equation• Impact of feed inlet pressure• Impact on HPP performance• Brine intrusion in the feed stream• HP Booster pump losses

• Manifold losses

To keep the analysis neutral, published performance data from the CIC supplier will be used including a 96.5% efficiency2.

Each item above will be assigned a field adjustment factor. These factors determine EFIELD.

Incorrect Efficiency Equation

While preparing data for this paper, it was discovered that a widely used CIC efficiency formula does not prop-erly calculate efficiency. Please refer to Addendum 1 for an explanation on this matter. For SWRO applications, the true CIC efficiency is about 0.4 percentage points lower than indicated in various publication. The adjustment factor is 99.6%

Impact of Feed Inlet Pressure

Per Addendum 1, CIC efficiency is strongly dependent on feed inlet pressure. The CIC supplier did not state the inlet pressure used in reporting efficiency.

CIC feed pressure must be at least 1 bar higher than brine pressure for brine channel scavenging and brine pressure must be at least 1.0 bar to suppress cavitation. A 3.0 bar feed inlet provides minimal margin to ensure proper CIC operation. The adjustment factor is 98.5%.

Impact on High Pressure Pump Performance

As a reminder, the turbo saves HPP energy by reducing the ∆P of the entire feed flow HPP while the CIC saves HPP energy by reducing the flow of the HPP to approxi-mately the permeate flow.

Which HPP (full or partial flow) has a higher efficien-cy? Figure 6 plots efficiency of centrifugal pumps as a function of Specific Speed (NS) and best efficiency point capacities3. NS is a design parameter that characterizes pump geometry. A given pump may be better (or worse) than the plotted value but pumps of equal design, qual-ity and NS would have efficiencies in proportion to the values in Figure 6.

NS values for HPPs generally range from 1,200 to 1,800. HPP best efficiency point capacities from 70 to 2,300 m3/h at an NS of 1,500, derived from Figure 6, is given by:

HPPEFF = 0.03483 * ln(Q) + 0.623742 [3]

where Q is in m3/h. For example, a RO system with a feed flow of 1,000 m3/h would have a nominal HPP efficiency of 86.4%. The HPP flow rate at 42% recov-ery with a CIC would be 420 m3/h with an efficiency of 83.4% as shown in Figure 6. This is a 3.5% loss or a 96.5% adjustment factor of HPP efficiency from CIC usage. This ratio is essentially constant over a broad range of NS values.Brine Intrusion in the Feed Stream

The CIC allows high pressure brine to mix with the mem-brane feed. One source of leakage in from direct contact

Figure 6



between feed and brine streams. Another source is dependent on process conditions such as “imbalanced” operation and stoppage of rotors due to biofouling or debris. Let’s accept the manufacturer’s claim of a 3% increase of TDS at the membrane inlet and ignore the potential for process related mixing4. Assuming 38,000 ppm feed water, to keep permeate flux at the design value, feed pressure must increase from 59.6 to 61.2; about 1.6 bar5. This increases HPP pumping energy by 2.7% resulting in a 97.3% adjustment factor.

HP Booster Pump

A booster pump (See Figure 4) is needed to generate about a 3-4 bar boost in the feed stream exiting the CIC unit to compensate for membrane and CIC flow channel losses. This type of pump generally has reduced efficien-cy due to high bearing and seal drag from high inlet pressure. The reduced efficiency is captured in a 99.4% adjustment factor6.

Manifold Losses

For a large RO train, up to 25 CIC units may be required. Each unit has four connections (feed in and out, brine in and out) thus four manifolds are required. The man-ifold losses result in about 1% reduction of efficiency yielding a 99% adjustment factor7.

Eliminate the VFD on the HPP

Adding a motor to the turbocharger (motorized turbo-charger or HEMI per Figure 2) permits it to control of feed and brine pressure independent of the HPP. A rela-tively small VFD is required for the HEMI motor, howev-er, associated losses are negligible. The HEMI motor and VFD are typically low voltage.Since the HEMI eliminates the HPP VFD, it would be fair to charge all other ERD types with HPP VFD losses.The VFD on the HPP would be replaced by either a DOL contactor or a solid-state motor starter. If DOL, a feed control valve is needed only during system startup and shutdown for gradual pressure ramp up and ramp down respectively. The valve is fully open during system operation.

Alternatively, a soft starter may be used. It eliminates high inrush current, has zero (0) electrical losses during system operation, costs from 25% to 33% of that of a similar rated VFD, has no power harmonics and is programmable to provide ramp speeds as required for system startup and shutdown7. A soft starter or DOL reduces energy consumption by about 4.1% via elimi-nation of VFD losses (3.5%) and improved HPP motor efficiency from pure sine wave power (0.6%)8. The adjustment factor is 95.9%.

Net Efficiency

Figure 7 compiles field factors and the resulting net efficiencies. The lesson is that the HPB, HEMI are CIC are all relatively close in SEC. The ERD selection process should include CAPEX, reliability, ease of operation, etc.The CIC manufacturer offers an incomplete self-assess-ment of CIC efficiency. Why?

Figure 7


Black Box Performance

Imagine a black box enveloping the entire high-pressure skid per Figure 8. This box has:

• two (2) inputs; low pressure feed from the pretreat-ment system and electricity;

• two (2) outputs; permeate and low-pressure brine.

A third type of input is money in the form of CAPEX and OPEX to build the box and money for maintenance and downtime. This third input will be ignored in this paper.

Specific Energy Consumption (SEC) best describes black box performance and is usually expressed as the electrical energy needed to produce one (1) cubic meters (metric ton, 1,000 liters) of permeate measured in kWh/m3.

Let’s look at SEC calculations for a 20,000 m3/day sea-water RO train with no ERD, PIT, turbocharger, CIC and HEMI. Detailed performance information for each ERD (plus no ERD for reference) is shown in Figure 11.

The analysis was performed using HPP efficiency per equation [4], ERD efficiencies per Figure 9, VFD efficien-cy of 95.9% and HPP motor efficiency of 97% except the HEMI motor which was assigned a lower efficiency which varied in accordance with loading.

SEC values are summarized in Figure 9. Note that the HEMI has the lowest SEC. What is particularly fasci-nating is that the HEMI also has the lowest published efficiency claim.

Several membrane projection programs include estimates of SECs based on the specified type of ERD. Unfortunately, these projections have understated turbo efficiency by over 15% in some cases and ignore the various losses associated with the CIC discussed above. Until these projection programs are updated to reflect current ERD transfer and field efficiency values, they are of little value in determining SEC.

High Recovery RO

A multiple turbo arrangement called the “Multi-Stage Multi-Turbo” or MSMT uses a two-stage RO membrane arrangement that can increase recovery to 60% in sea-water applications. See Figure 10.

The MSMT reduces OPEX by significantly reducing the volume of feed water hence reducing intake and pre-treatment pumping energy as well as well reduced chemical consumption. Membrane efficiency is improved by minimizing excessive transmembrane pressure and reduced fouling potential via a more uniform permeate flux throughout the array.

Figure 8

Figure 9

Figure 10


Figure 11 presents OPEX data typical for a large SWRO facility for recovery at 45% and 60%9. Cost reductions are achieved in chemicals as well as intake and pretreat-ment pumping energy. RO process energy has increased by 6% from high recovery operation. The net savings in OPEX for the entire facility is 2.6% which is equiva-lent to a 9.5% reduction in RO energy cost.

The MSMT provides a significant CAPEX savings as well through reduced size of intake and outfall structures, reduced pretreatment capacity, reduced process pipe diameters, smaller pumps and smaller facilities. Please refer to Figure 12 for clarity on this matter.

Field Data

Efforts were made to find SEC data for large scale RO systems using CICs for comparisons with turbochargers. However, the available data was not presented with the necessary supporting data to confirm accuracy.

One major user that has experience with both CICs and turbochargers in large scale SWRO systems has con-firmed the general accuracy of the data in this analysis. The user wishes to remain anonymous but has provided the author with access to relevant performance data.

The CIC appears to suffer from unstable performance brought about by stuck rotors or biofouling resulting in sharply higher SEC. The frequency and severity are not well documented. Turbos do not suffer from such issues or other upset conditions common to the CIC.

Final Thoughts

Until the industry can agree on a comprehensive defini-tion of ERD efficiency, confusing and misleading claims will continue to complicate the ERD selection process.It is high time that the industry and most especially RO consultants scrutinize ERD efficiency claims. The first step to bring balance to the ERD market is to open the project specifiations to any ERD with a proven track record. This would bring a higher level of competitive-ness resulting in lower cost facilities able to provide lower cost permeate.

Please contact FEDCO with comments, objections or clarifications on any data or calculation presented in this paper.


Figure 11

Figure 12



Analysis of ERD Performance in a SWRO system

• 20,000 m3/day permeate output

• Pelton Impulse Turbine included with a 90% efficiency

• VFD used with HPP for all ERDs except the HEMI

• HPP efficiency derived from equation [3]

• HPP off-design point performance based on typical H-Q and E-Q curves

• ERD efficiencies assumed to equal supplier claims

• Four duty points represent typical SWRO operating conditions

• Effects of brine mixing on feed pressure for CIC estimated using membrane projection software

Figure 13



Addendum 1

Isobaric Efficiency Calculations

The CIC supplier published the following equation which has been widely used in various technical papers:

PXEFF = Σ (Pressure x Flow)OUT )/ Σ (Pressure x Flow)IN) X 100%1 [1]

PXEFF is the sum all hydraulic energies exiting the CIC divided by the sum of all hydraulic energies entering the CIC. Figure 1 shows four (4) flows relevant to the CIC. Labels are consistent with published terminology;

• A = low pressure feed IN • F = high pressure brine IN • C = high pressure feed OUT • G = low pressure brine OUT

Expanding equation [1]:

PXEFF = (C + G) / (F + A) [2]

= ((QF x PFOUT) + (QR x PROUT)) / ((QF x PFIN) + (QR x PRIN) [3]

Using the same terms as above, lets write the transfer efficiency equation as defined in this analysis:

ETRN = (∆Pf x QF) / (∆PR x QR) = (C - A) / (F - G) [4] = ((QF x PFOUT) - (QR x PFIN)) / ((QR x PRIN)- (QR x PFOUT)) [5]

Recalling that transfer efficiency is the ratio of feed energy added to the brine energy available, the fact that equation 3 mixes feed and brine energies in both the numerator and denominator rules out the potential to calculate transfer effi-ciency. But let’s see what it does calculate by comparing equations 3 and 5 for an CIC application using CIC performance data from a technical paper written by an CIC suppleir2.

PRIN = 58.2 bar PROUT = 1.7 bar PFIN = 2.6 bar PFOUT = 57.2 bar QR = 56.1 m3/h QF = 55.5 m3/h Figure 2 shows ETRN and PXEFF for equations 3 and 5 in tabular form as well as a data plot. For low feed inlet pressures, ETRN is only slightly lower than PXEFF but the deviation increases rapidly with increasing feed pressure. CIC efficiency sensitivity to feed inlet pressure should be noted as well.

Figure 2

Figure 1


ERD Efficiency — Whats Systems Builders and Consultants Need to KnowERD Efficiency — Whats Systems Builders and Consultants Need to Know

To illustrate the fallacy of PXEFF, at a feed inlet pressure, PFIN, of 57.2 bar, no feed energy is transferred from the brine as there is zero (0) feed pressure boost. As expected, ETRN is -1.1% reflecting internal CIC losses, however, PXEFF = 49.2% which is clearly absurd.

It is remarkable how an invalid equation provided by the CIC supplier has been accepted in the industry for so many years.

To understand why the CIC has such characteristics, keep in mind that the brine pushes directly against the feed Thus, the feed discharge pressure cannot exceed brine pressure regardless of the feed inlet pressure. Thus, feed inlet pressure represents a 100% loss of energy. In comparison, a turbo adds pressure boost to the inlet pressure thus the feed inlet energy is conserved.

In addition, losses in the CIC are related to flow rates and not pressure differentials. Therefore, at constant flow, the losses become proportionately larger as the pressure differential decreases as shown by Figure 3. Again, the CEFF equation fails to calculate a sensible efficiency.

Figure 3

Fluid Equipment Development Company, LLC www.fedco-usa.comOffi ces in USA, Singapore and Dubai [email protected]

ERD Effi ciency — Whats Systems Builders and Consultants Need to Know

About FEDCO

Founded in 1997 as a pump and ERD development partnership with a large RO system builder, Fluid Equipment Devel-opment Company, LLC, evolved to a vertically integrated design, manufacturing and global sales/marketing company. FEDCO holds numerous patents in pump and turbine design and reverse osmosis system design. FEDCO is a privately held company with a proven commitment to its customers, suppliers, end users, its community and the environment.

Manufacturing and Head Offi ce - Monroe Michigan

FOOTNOTES

1 – Private communications with Fluid Equipment Development Company, LLC. 2 - Richard Stover PhD The PX 3 - Pump Handbook. McGraw Hill4 - Miguel Angel Sanz, Richard L. Stover, PhD. Low Energy Consumption in the Perth Seawater Desalination Plant. IDA

World Congress-Maspalomas, Gran Canaria –Spain October 21-26, 2007 WaterReuse Research Foundation. Evaluation and Optimization of Emerging and Existing Energy Recovery Devices for

Desalination and Wastewater Membrane Treatment Plants. 2013.5 – Hydranautics membrane projection 2018. Membrane type SWC4 MAX, 14.5 fl ux, 0.86 fouling, 3.0 years age, 38,071

raw water TDS, 7.0 pH6 – Based on an HP Booster pump ∆P = 3.5 bar with an effi ciency 5% lower than the HPP, HPP ∆P of 61 bar, and re-

covery of 44%. For further explanation, if the HP booster pump effi ciency equals the HPP effi ciency, then this loss would be zero.

7 - Yin, Tang. What is a Reduced Voltage Soft Starter. VFDs.com, 7/11/2014. www.vfds.com/blog/what-is-a-soft-starter8 - Private communications with an industry expert on VFDs and AC motors.9 – Trends & Developments in Advanced Energy Recovery Technologies for Desalination. Energy Recovery, Inc. Presenta-

tion, May 30, 2013.

erd efficiency - fedco

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