There are numerous opportunities for energy savings in cleanrooms, including HVAC (heating, ventilation, and air conditioning), process cooling, compressed air, and other facility systems. The following are ten energy efficiency strategies for both new and existing facilities. These tips offer reliable technology, minimal risk, and low or no cost.

1. Low Face Velocity (LFV) Design

Face velocity refers to the speed at which air passes through filters or heating/cooling coils in air handling units. LFV design utilizes larger air handlers and smaller fans to reduce air velocity, thereby lowering energy consumption and equipment life cycle costs. Many engineers conventionally design air handlers with a face velocity of 500 feet per minute, which saves design time but increases operating costs.

According to the "square law," pressure drop is proportional to the square of the velocity. If the face velocity is reduced by 20%, the pressure drop decreases by 36%; a 50% reduction results in a 75% decrease. Based on the "cube law," fan energy consumption changes with the cube of the airflow. Reducing airflow by 50% results in an 88% reduction in fan energy use.

Therefore, larger air handlers, filters, and coil surface areas use less fan energy, allowing for smaller fans and motors. Smaller fans generate less heat, easing the cooling load. Thinner coils are easier to clean and more efficient, allowing for higher chilled water temperatures. Filters also perform better and last longer at lower face velocities.

LFV design reduces air and water pressure drops and minimizes coil moisture retention. Streamlined designs with fewer sharp angles can reduce pressure drop by 10–15%.

2. Air Change Rates

Cleanrooms maintain cleanliness and particle counts by circulating air at a specific rate, typically measured in air changes per hour (ACH). This rate impacts fan size, building design, and energy use. Reducing airflow, while maintaining cleanliness, can cut both construction and energy costs. A 20% reduction in ACH can reduce fan size by 50%.

Though cleanliness is the priority, newer research has found ways to reduce ACH without compromising it. There’s no consensus on the optimal ACH. Many outdated standards were based on inefficient filters. For ISO Class 5 cleanrooms, recommended ACH can vary from 250 to over 700.

A U.S. national lab found that ISO Class 5 cleanrooms typically operate at 90–250 ACH—much lower than traditional recommendations—with no impact on cleanliness or productivity. An ACH of 200 is generally sufficient, with 300 as a conservative upper limit.

3. Motor Efficiency

Motors consume the majority of electrical power in cleanrooms. Continuously running motors use significant electricity each month. Upgrading to efficient motors and right-sizing them during retrofits can yield excellent economic returns. Even small efficiency gains can significantly increase profit.

High-efficiency motors are not necessarily expensive. Prioritize load reduction before resizing motors. Variable speed drives (VSDs) can further improve performance when load varies.

4. Variable Speed Chillers

VSD chillers save considerable energy and cost. While some cleanroom designers believe chillers run at constant loads and don't need VSDs, the reality is most chillers operate below full load. VSD chillers run at 90–95% of full load, saving energy. A 1,000-ton chiller operating at 70% load with a VSD can save $20,000–$30,000 annually. At $0.05/kWh, the investment can be recouped in about a year.

Multiple-stage chillers rarely run at high load. Often, extra chillers are run to ensure backup capacity, leading to 60–80% operation. VSD chillers can improve efficiency and reliability significantly.

5. Dual-Temperature Chilled Water Loops

Cooling systems are typically designed for peak loads, which rarely occur. The chilled water temperature is dictated by rare high-heat demands, resulting in excess capacity and low efficiency. Chillers work less efficiently at lower water temperatures; each 1°F increase in chilled water supply temperature improves efficiency by over 1%. Dividing loads into dual-temperature loops improves performance.

Parallel loop systems can form two subsystems: a medium-temp loop (e.g., 55–65°F) for most loads, and a smaller high-efficiency loop (e.g., 39–43°F) for demanding equipment. This setup can increase chiller efficiency by 25% or more.

6. Optimized Cooling Towers

Efficient cooling towers improve chiller performance by lowering condenser water temperature. All towers should operate in parallel for maximum surface area and optimal evaporative cooling.

Traditional towers consume ~100W per ton of cooling. Efficiency can be boosted by reducing approach temperatures, improving airflow design, using VSD-driven fans, lowering tower height (to reduce pump lift), and increasing fill area.

Typical temperature differences between entering and leaving water should be 3–5°F. Parallel operation, VSD fans, and larger surface areas enhance efficiency, especially when demand varies. Using manifolded piping allows towers to share loads and operate more efficiently.

7. Free Cooling

Using outdoor air for cooling is highly cost-effective and widely used in commercial buildings. In cleanrooms, "free cooling" systems generate chilled water using low-temperature or low-humidity ambient conditions, reducing or eliminating chiller use. Energy use can drop from 0.5 to 0.05 kW/ton.

Direct heat exchange with the process load extends the usability of outdoor air compared to secondary/tertiary exchanges. Plate heat exchangers allow close temperature differentials (as little as 2°F). Many regions have extended periods annually where free cooling is viable.

8. Heat Recovery

Many facilities expend large amounts of energy for both heating and cooling, often without integrating the systems. Recovered heat can preheat outdoor air, reheat supply air, or serve other uses. Air handling unit (AHU) preheat coils can use waste heat from hot water, while reheat coils can capture heat from compressors or chiller condensers, reducing both chiller and boiler loads.

Heat exchangers enable indirect heat recovery between incompatible media.

9. Variable Speed Pumps

Older VSDs were unreliable and complex, making many engineers hesitant. Today’s VSDs are more reliable and affordable. Many mission-critical systems now use them widely.

VSDs on pumps in cleanroom systems are both safe and cost-effective, often with payback periods under one year. Chilled and condenser water pump flows vary significantly. These systems typically require minimum flows of 50–75%. According to the cube law, small reductions in flow yield large energy savings—20% less flow equals ~50% power savings.

Most modern chilled water systems use constant primary/variable secondary pumping with VSDs. VSDs require two-way valves on all chilled water loops to be effective.

New facilities can use variable flow primary pumping systems, eliminating the need for secondary pumps and reducing capital costs. This simple system varies chilled water flow based on load and is widely endorsed by chiller manufacturers and organizations like ASHRAE.

10. Centrifugal Compressors

Upgrading air compressors can yield major energy savings. Centrifugal compressors are oil-free and more efficient than screw types, but they perform poorly under low loads (>30%). The best approach is a hybrid: centrifugal compressors for base load, smaller screw compressors for peak load. Heat recovery systems should be included.

Alternatively, use a central high-efficiency centrifugal system with large air storage and piping for buffering. This stabilizes plant-wide loads, reduces equipment wear, and minimizes energy waste.