Heating, Ventilation, Air Conditioning, and Refrigeration

Quick Tip: The efficiency of today’s standard roof top units has improved by more than double over the last 20 years. 

General Description
HVAC & R systems are widely used for human comfort in homes, businesses, and commercial buildings.  There are some very important industrial uses for these systems as well.  Many Arkansas plants have heating equipment for comfort heat during the colder months, while a few others have significant energy use from cooling their processing plant area(s). Furthermore, some plants require ventilation or temperature/humidity control to maintain process quality. Refrigeration is especially important to the food processing industry where cold-storage, blast freezing, and other refrigeration processes are essential to maintain the quality and safety of the product.

 

The industrial uses of HVAC systems are fundamentally the same as the commercial uses, and therefore the same principles apply.  We can apply many of the same EE ACTIONs to industrial systems than we learn from large commercial buildings.  Refrigeration systems are large energy users and there are several EE ACTIONs that can lead to significant utility savings.

Potential Energy and Cost Savings Opportunities:
The below energy conservation opportunities or energy efficiency actions (EE ACTIONS) are provided as a partial list of potential savings opportunities in your plant.  They are grouped as no-to-low cost, moderate cost, and long-term cost investments.  Consider each for your plant and feel free to contact us for clarification or any assistance you may need in assessing specific projects.

No-to-low cost investment

EE ACTION (HVAC): Night-setback and/or computer controlled HVAC systems
Many industrial offices started using programmable thermostats long before they became popular in residential buildings. Building controls should be set to provide a comfortable environment for workers when they are present, but they should go into conservation mode at night and on weekends.  The same strategy may, or may not, be applicable to process area HVAC as well.  If the plant is not operating during nights and weekends, then night-setback controls can be used.  Large make-up air systems can consume significant amounts of natural gas and electricity, so look for ways to reduce operating times and/or temperature set points.  Note that the term ‘setback’ is used as a generic temperature setpoint adjustment – either lower during heating season or higher during cooling season.  For Arkansas, the greatest savings potential for setback strategies is during the wintertime/heating season.

EE ACTION (HVAC&R): Preventive Maintenance
Changing filters, cleaning coils, checking for refrigerant leaks, ensuring tight seals in duct work, and other simple measures can save large dollar amounts on large systems.  Proper preventive maintenance can save 5-10% on operational costs of an industrial HVAC system, which is typically several thousand dollars a year.

EE ACTION (HVAC&R ): Floating condenser pressure
Condenser pressure will increase with outdoor ambient conditions.  The pressure should be allowed to drop when it is cool outside, to a set point that still allows the expansion valve to function properly.  Lower condenser pressures mean the compressor doesn’t have to work as hard, which saves energy and extends equipment life.

EE ACTION (HVAC&R): Raise suction pressure
The compliment to reducing condenser pressure is increasing the pressure in the evaporator.  The low differential pressure across the compressor means it is doing less work. This project is simple, yet has significant savings potential.

EE ACTION (HVAC&R): Eliminate parasitic loads
Heat leaks into your system should be eliminated.  Inadequate seals, failed insulation, and open doors all contribute to the heat load that your system sees.  These parasitic loads reduce your system’s capacity and efficiency.  Find them and fix them. 

EE ACTION (HVAC): Reduce chiller usage with cooling tower “free cooling.”
There are periods each year (typically spring & fall) when the wet-bulb temperature falls below the chiller set point.  When this occurs, an opportunity exists to use the cooling tower water as the heat sink.  Required system configurations will vary. 

Moderate investment

EE ACTION (HVAC&R): Use Adjustable Speed Drives
Adjustable speed drives can be used for screw compressors, evaporator fans, condenser/cooling tower fans, re-circulating pumps, etc.   Furthermore, the varying loads of HVAC&R system are perfect applications for variable speed drives.

EE ACTION (HVAC): Free Cooling with Economizer units
Air-side Economizers use temperature and humidity sensors to determine when it will require less energy to condition the outside air than to re-condition the return air from the conditioned space.   The higher humidity in Arkansas means that AC systems often have to do more dehumidifying than in dryer climates, but we in Arkansas can still achieve moderate savings from “free cooling” several months out of the year, and reduced loads at night even in warmer months.

EE ACTION (HVAC): Use variable air volume systems
Whether replacing a failed unit or specifying a system in new construction, VAV systems can save significant energy compared to traditional constant-volume systems.  Use a life-cycle cost analysis to compare the two, since VAV systems can have a slightly higher first cost. 

EE ACTION (Refrigeration): Waste heat recovery from industrial refrigeration systems
Discharge temperatures from ammonia refrigeration systems are warm enough for low-grade heat recovery.

EE ACTION (Refrigeration): Efficient compressor sequencing
Multiple compressor systems have many things in common with compressed air systems.  The multiple compressors must be controlled so that they do not short cycle or run at excessively low loads.  Time intervals when compressors are running at less than 70% load should be minimized.

EE ACTION (Refrigeration): Efficient defrost sequencing
Controls should allow evaporator coils to defrost on demand rather than on a set timer.  Avoid hot gas dwell times in excess of 15 minutes. 

Long-term investment

EE ACTION (HVAC): Replace failed roof-top units and chillers with high efficiency systems
Replacing an air-conditioning system is a very expensive capital project.  Replacing failed systems, or those requiring significant and continual repair, with high energy efficient units can allow energy savings throughout the unit’s lifetime.  Current federal incentives for energy efficient systems can also help.  Within Arkansas, it is typically not economically feasible to replace operable 15-20 year rooftop units (RTUs).  Economics can be based on the cost differential between standard and high efficiency units.  The incremental cost for purchasing high efficiency versus standard efficiency after unit failure is recommended.

EE ACTION (Refrigeration): Variable volume index compressors
Consider using a variable volume index compressor in major refrigeration system retrofits.  Much like a variable speed drive, these compressors can adjust to changing load conditions and significantly reduce energy consumption.