How to Build the Business Case for a VSD Compressor Upgrade

A variable speed drive compressor can start reducing electricity consumption as soon as it enters service, but that does not make every VSD upgrade commercially sound. Approval depends on measured demand, credible cost modelling, correct model selection and a funding plan that does not rely on an unconfirmed grant.

Anglian Compressors, a Branch of Atlas Copco Compressors, builds these cases from operating data rather than nominal motor size. The result should tell your finance team what the upgrade will cost, what it will save and what happens if production demand or electricity prices change.

Begin With a Measured Compressed Air Baseline

A month of electricity bills cannot show how the compressor behaves across a shift. You need a load profile that records demand, pressure and operating state over representative production periods.

Gather at least the following information:

• Compressor model, motor rating, age and control type
• Annual operating hours and shift patterns
• Loaded, unloaded and stopped hours
• Working pressure at the compressor and critical points of use
• Free air delivery required during minimum, average and peak demand
• Current electricity tariff, including the unit rate used for the calculation
• Planned production changes that could alter future demand
• Maintenance, breakdown and compressor hire costs
• SMARTLINK records or other available controller data

Seasonal operations need more than a quiet-week snapshot. A food packing site near King’s Lynn, for example, may have a very different profile during harvest peaks. Data from an unrepresentative week could produce a technically correct calculation for the wrong operating condition.

As a Branch of Atlas Copco Compressors, we measure that profile with AIRScan, which logs flow, pressure and power across representative shifts. AIRchitect then models how alternative compressor arrangements would perform against the same measured demand. Those records give the baseline, and the free energy audit behind it, a documented measurement source rather than an estimate.

Assess the Demand Pattern

If the compressor is continually loaded near its rated capacity, a VSD may offer less benefit than expected. If demand moves substantially during shifts, weekends or cleaning cycles, speed control has more room to reduce power consumption.

The history of VSD compressors explains how the technology developed from fixed-speed control into systems that match motor speed more closely to air demand.

Separate Avoidable Waste From Genuine Demand

Do not size a new compressor around losses that should be removed first. Leaks, excess discharge pressure, inappropriate open blowing and poor compressor sequencing all inflate the apparent air requirement.

Pressure instability also needs diagnosis. An air compressor not building pressure may have a control fault, internal bypass, excessive downstream demand or significant leakage. Replacing it with a larger VSD compressor could hide the fault while increasing capital expenditure.

The baseline must show the air your process requires after practical waste reductions, not the maximum volume the current system happens to produce.

Convert the Load Profile Into Annual Cost

The financial case should compare measured annual system cost with the expected cost of the proposed VSD installation.

Start with the existing compressor’s power demand at each operating state. Loaded power alone is not enough because a fixed-speed machine can consume a meaningful amount of electricity while unloaded and producing no useful air.

A simplified annual calculation is:

Annual energy cost = average input power in kW × annual operating hours × electricity price per kWh

For a variable load, calculate each demand band separately. If the system spends 2,000 hours at high demand, 1,500 hours at medium demand and 1,000 hours at low demand, model the power requirement for each band rather than applying one assumed saving across all 4,500 hours.

Illustrative Payback Calculation

The example below shows how variable speed compressor ROI is built up from those three bands. The power figures, the 22p per kWh unit rate and the £30,000 installed cost are all assumed for illustration. Replace every one of them with measured and quoted figures for your site.

Demand band	Hours	Fixed-speed kW	Fixed-speed kWh	VSD kW	VSD kWh
High	2,000	40	80,000	36	72,000
Medium	1,500	32	48,000	24	36,000
Low	1,000	24	24,000	12	12,000
Total	4,500		152,000		120,000

Fixed speed: 152,000 kWh × £0.22 = £33,440 a year. VSD: 120,000 kWh × £0.22 = £26,400 a year. Annual saving: £7,040.

Simple payback: £30,000 installed cost ÷ £7,040 annual saving = 4.3 years.

The saving in this example is 21%, well below the manufacturer’s upper figure, because the high-demand band leaves little room for speed reduction. If the conservative case cuts the saving by a fifth to £5,632 a year, payback extends to 5.3 years.

Use Site-Specific Savings

Manufacturer data for the GA VSDˢ range (atlascopco.com) states that the technology can reduce energy consumption by up to 60% compared with a fixed-speed compressor. That figure is an upper limit, not a default input for every proposal. Your business case should use the saving calculated from the site’s measured demand profile and the selected machine’s performance data.

Include the Whole Installation

A compressor quotation is only one part of the capital requirement. The approval figure may also need to include:

Cost item	What to confirm
Compressor package	Model, pressure variant, dryer option and controller
Electrical work	Supply capacity, protection, isolation and cabling
Mechanical installation	Connections, valves, ventilation and condensate drainage
Distribution changes	Pipework alterations and pressure-drop corrections
Production planning	Installation window, temporary air and commissioning
Compliance	Air receiver arrangements and any required Written Scheme of Examination (WSE) review
Maintenance	Service package, parts and monitoring costs
Decommissioning	Removal or retention of the existing compressor

Leaving these items outside the proposal makes the payback appear stronger, but it also creates a funding gap after approval. Procurement will trust a slightly longer payback more than an incomplete project cost.

Test the VSD Compressor Payback Against Change

Calculate simple payback by dividing the net capital cost by the forecast annual saving. Then test the result against at least three cases:

• The measured case uses the audited demand profile and current electricity tariff.
• The conservative case reduces the expected energy saving and assumes no grant support.
• The growth case includes approved production changes and any additional air demand.

Also state the annual cash saving after the payback period. That figure often matters more to finance than the percentage reduction because it shows how the project affects operating expenditure in later years.

Do not count avoided breakdown costs unless you can support them with maintenance records, incident reports or documented hire expenditure. Reliability may strengthen the decision, but an invented downtime figure weakens the whole submission.

Select the VSD Model From the Duty Point

Motor rating is not a sufficient model-selection method. The proposed compressor must deliver the required FAD at the site’s working pressure while operating effectively across the measured demand range.

Compare candidate models using:

• FAD at the required discharge pressure
• Minimum and maximum controllable flow
• Specific energy consumption at relevant demand points
• Motor and drive operating range
• Cooling and ventilation requirements
• Air quality requirements
• Integrated or separate dryer arrangement
• Controller compatibility with the remaining compressor fleet
• Monitoring, maintenance and electrical requirements

Check Performance Across the Operating Range

A machine selected around peak demand alone may spend much of its life near the bottom of its control range. At that point, storage volume, control logic and how the compressor responds at minimum speed become important.

Where several compressors share a header, assess the system rather than replacing one machine in isolation. A correctly sized VSD unit may manage trim demand while fixed-speed machines provide base load. Poor sequencing can make an efficient compressor operate inefficiently.

There is also a case where the recommendation flips. A stable process running close to full load for most of the year may suit a properly sized fixed-speed machine, or another control arrangement, better than a VSD. The measured profile has to settle that question.

Use Grants to Reduce Capital Cost, Not to Rescue the Proposal

Regional energy-efficiency and manufacturing grants can reduce the amount funded from capital budgets. Availability, qualifying technologies, match-funding rules and application windows change, so confirm the live position with the relevant local authority or Growth Hub before including support in the approved project value.

Many schemes require approval before equipment is ordered or work begins. Treating a grant as retrospective funding can make an otherwise eligible project fail the application process.

Build the Case Without Grant Support

Build the financial case in two layers:

• Show that the project works without grant support using measured energy savings.
• Add confirmed grant support as a reduction in net capital expenditure and recalculate payback.

This approach prevents the project from collapsing if the application is unsuccessful. It also gives decision-makers a clear view of the underlying investment and the improved funded position.

Tax treatment can change the after-tax figure too. Full expensing applies to companies buying new, unused main-rate plant and machinery, and it is not an energy-efficiency-specific relief, so ask your finance team to confirm how the purchase qualifies before it enters the business case.

The remaining expenditure may come from an existing capital allocation, asset finance or another approved funding route. Compare each option on total cost, cash-flow timing, ownership conditions and any effect on grant eligibility. A low monthly payment does not automatically mean the lowest project cost.

Give Finance and Operations the Same Approval Pack

Finance needs a traceable return. Operations needs confidence that the proposed machine will supply the plant without creating a new reliability problem. One approval pack should address both.

Include:

• A short description of the existing system and the reason for review
• The measured demand and pressure profile
• Identified leaks, pressure losses or control issues
• The existing annual energy cost
• The selected model and its duty-point rationale
• The proposed annual energy consumption and saving
• The complete installed project cost
• Payback under measured and conservative assumptions
• Grant status, eligibility conditions and application deadline
• Installation, commissioning and temporary-air plans
• Maintenance, monitoring and warranty arrangements
• Key assumptions, exclusions and calculation sources

Evidence from comparable projects can help stakeholders judge delivery risk. The Masteroast case study describes a customer that has returned to us for further work as its requirements changed.

Record every assumption beside the calculation it affects. If production hours, electricity tariffs or required pressure change, the reviewer should be able to update the model without rebuilding it.

Bring Us the Data Before You Request the Compressor

A useful quotation starts with the current compressor details, operating hours, electricity tariff, pressure requirement, air-quality specification and any available SMARTLINK data. Tell us about planned production changes, known pressure problems, installation constraints and the site postcode so regional funding can be checked at the correct geographic level.

Our free energy audit measures how the system behaves and gives you a technical basis for comparing fixed-speed and VSD options. We can then specify the appropriate model, calculate the installed cost and provide the figures required for internal approval.

Our case studies set out what was installed on other sites and what changed afterwards, which gives your reviewers a reference point for delivery.

Use the form below to arrange your free energy audit and start the business case with measured data.