Refrigerant Sub Cooling is a reliable energy-saving alternative to increasing the cooling capacity of air-conditioning systems, refrigeration systems, and most anything that uses refrigerant. Reduces the condenser energy consumption.   To learn more on Federal Alerts Visit: U.S Department of Energy

The Sub Cooling technology is one energy-saving technology to emerge and not only provides for additional cooling capacity but will reduce compressor power, leading to greater overall system efficiency. The Federal Government has issued several alerts concerning this technology.


Refrigerant Subcooling

Technology for improving cooling system performance

Refrigerant subcooling is proving to be a reliable energy-saving alternative to increasing the cooling capacity of air conditioning systems in many facilities. As shown below, the technology modifies a standard direct-expansion, vapor-compression refrigerant system with the addition of a heat exchanger in the liquid line of the system.


This Federal Technology Alert

(FTA), one of a series on new technologies, describes the theory of operation, energy-saving mechanisms, range of applications, and field experience for the refrigerant subcooling technology. Featured is a subcooling device with an external heat sink. One such subcooling device, called the Fisher Tri-Temp System (FTTS), is patented by Ralph H. Fisher.


Energy-Saving Mechanism

A refrigerant subcooling unit provides additional cooling capacity and can also reduce energy consumption for increased overall system efficiency. It works best where year-round outdoor temperatures are high and constant. Effectiveness of the technology is based on the properties of refrigerants, which absorb latent heat until they vaporize. The subcooling increases refrigerant cooling capacity, and use of an external heat sink (either minicooling tower or ground-source water) reduces compressor power. Although detailed operation and maintenance data are generally lacking, the technology is proving particularly applicable in direct-expansion vapor-compression air-conditioning equipment, especially where old units are being replaced or where new construction/expansion or new installation is planned.


Technology Selection

Refrigerant subcooling is one of many energy-saving technologies to emerge in the last 20 years. The FTA series targets technologies that appear to have significant untapped Federal sector potential and for which some Federal installation experience exists. New technologies were identified through advertisements for technology suggestions in the Commerce Business Daily and trade journals, and through direct correspondence. Numerous

Eva orator – Desuperheat – Condensing – Subcooled – Evaporating – TEV – Condenser - S9506041.1 – Condensing - Evaporating

Superheat – Desuperheat – TEV - Evaporator Condenser - Heat Exchanger – Subcooled – Superheat - Normal System System with Subcooler


Federal Technology Alert

A publication series designed to speed the adoption of energy efficient and renewable technologies in the Federal sector Prepared by the New Technology Demonstration Program

The U.S. Department of Energy requests that no alterations be made without permission in any reproduction of this document. responses were obtained from manufacturers, utilities, trade associations, research institutions, Federal sites, and other interested parties. Technologies suggested were evaluated in terms of potential energy, cost, and environmental benefits to the federal sector. They were also categorized as those that are just coming to market and those for which field data already exist. Technologies classified as just coming to market are considered for field demonstration through the U.S. Department of Energy s Federal Energy Management Program (FEMP) and industry partnerships. Technologies for which some field data already exist are considered as topics for FTAs. The refrigerant subcooling technology was found to have significant potential for federal-sector savings. Potential Analysis of a large sample (nearly 25% by floor area) of Federal facilities indicates a major, untapped energy conservation potential in the Federal sector. The subcooling technology not only provides for additional cooling capacity but can also reduce compressor power, leading to higher overall system efficiency. Besides saving energy, subcooling benefits the environment through reduced emissions of sulfur and nitrogen oxides and carbon dioxide associated with power generation.



Qualitative field testing and theoretical analyses have shown the subcooling technology to be technically valid and economically attractive. The technology is generally applicable to direct expansion vapor-compression equipment with or without head pressure control. Potential Federal-sector applications for refrigerant subcooling include direct-expansion vapor compression air-conditioning equipment. The subcooling technology with external heat sink is especially useful under the following conditions:


For high-temperature(a) applications, generally in conjunction with air-conditioning and heat pump systems (split      or packaged systems) or reciprocating, screw or scroll chillers


Where chillers, split systems or packaged systems are to be replaced (equipment that is 15 years or older or where new construction/expansion or new installation is planned). External heat sink subcooling devices (such as FTTS) are not recommended for the following applications:


as add-on devices

large centrifugal chillers

off-peak cold storage

low-temperature applications

insufficient space for a minicooling tower

in some cases a system with a water-cooled or evaporatively cooled condensers may be as effective as external    heat sink subcooling devices.


Field Experience


More than 12 systems have been fitted with external heat sink subcooling devices, seven of these in the Federal sector (IRS headquarters in Washington, D.C.). Detailed performance of the technology has not been monitored at any of the sites. Building owners/ operators have noticed reduced electric consumption after the retrofit. The operator at the IRS site is satisfied with the performance of the retrofits and is considering installing another one at that site.


Typical installation cost is $700/ton (approximately $200/kW) of cooling capacity. A typical yearly maintenance cost for the air-conditioning system is about $25/ton to $35/ton, and for the mini-cooling tower (external heat sink) it is about 2% of the cost of the cooling tower. When an external heat sink subcooling device, such as an FTTS, is installed with the air-conditioning system the outdoor unit is downsized; therefore, the general maintenance of the

Air-conditioning system decreases and there is an additional maintenance associated with the external heat sink (mini-cooling tower). In general, reduction in cost of maintaining the air-conditioning system is offset by the additional cost for maintaining the minicooling tower. The general maintenance of the air-conditioning system with an external heat sink subcooling device is similar to a conventional system. The mini-cooling towers will need periodic maintenance, such as checking the pre-filter on the makeup water and cleaning the sump.


Case Study

Aqualitative analysis was performed on the basis of whole-building utility billing information from one private sector site. Southeastern University (SU), in Washington, D.C., has a 22-year-old, 100-ton (352-kW) chiller system with a cooling tower. This system was replaced with two 20-ton (70-kW) remote condensing units, each fitted with an external heat sink subcooling device, including two minicooling towers. Comparison of utility bills from pre- and post-replacement periods indicated savings in demand charges and energy charges of $5,819/year. On the basis of whole building utility billing data alone, it is difficult to quantify the actual savings. Because there was no other change to the building during the one year period (peak demand and energy consumption remained unchanged

during winter months), significant portion of the energy and demand reduction can be attributed to the replacement system with an external heat sink subcooling device.


Implementation Barriers

There are no known barriers for implementing the subcooling technology. Federal energy managers who are familiar with refrigerant subcooling systems are listed in this FTA. The reader is invited to ask questions and learn more about the technology.


(a) Low-temperature application refers to applications with evaporator temperatures less than -10ˇF, medium-temperature refers to -10ˇF to 30ˇF evaporator temperatures, and high-temperature refers to evaporator tempertures greater than 30ˇF.


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Strategic Environmental

R&D Program

The Strategic Environmental Research and Development Program, SERDP, cosponsor of these Federal Technology Alerts, was created by the National Defense Authorization Act of 1990 (Public Law 101-510). SERDP’s primary purpose is to "address environmental matters of concern to the Department of Defense and the Department of Energy through support for basic and applied research and development of technologies

that can enhance the capabilities of the departments to meet their environmental obligations." In 1993, SERDP

made available additional funds to augment those of FEMP, for the purpose of new technology installations and


Federal Energy Management Program

The Federal Government is the largest energy consumer in the nation. Annually, in its 500,000 buildings and 8,000 locations worldwide, it uses nearly two quadrillion Btu (quads) of energy, costing over $11 billion. This represents 2.5% of all primary energy consumption in the United States. The Federal Energy Management Program was established in 1974 to provide direction, guidance, and assistance to Federal agencies in planning and implementing energy management programs that will improve the energy efficiency and fuel flexibility of the Federal infrastructure. Over the years several Federal laws and Executive Orders have shaped FEMP’s mission. These include the Energy Policy and Conservation Act of 1975; the National Energy Conservation and Policy Act of 1978; the Federal Energy Management Improvement Act of 1988; and, most recently, Executive Order 12759 in 1991, the National Energy Policy Act of 1992 (EPACT), and Executive Order 12902 in 1994. FEMP is currently involved in a wide range of energy-assessment activities, including conducting New Technology Demonstrations, to hasten the penetration of energy-efficient technologies into the Federal marketplace.

About the Federal Technology Alerts

The Energy Policy Act of 1992, and subsequent Executive Orders, mandate that energy consumption in the Federal sector be reduced by 30% from 1985 levels by the year 2005. To achieve  this goal, the U.S. Department of Energy s Federal Energy Management  Program (FEMP) is sponsoring a series of programs to reduce energy

consumption at Federal installations nationwide. One of these programs, the New Technology Demonstration

program (NTDP), is tasked to accelerate the introduction of new energy saving technologies into the Federal

sector and to improve the rate of technology transfer. As part of this effort, FEMP, in a joint venture with the Department of Defense s Strategic Environmental Research and Development Program (SERDP), is sponsoring a series of Federal Technology Alerts (FTAs) that provide summary information on candidate energy-saving technologies developed and manufactured in the United States. The technologies featured in the Technology Alerts have already entered the market and have some experience but are not in general use in the Federal sector. Based on their potential for energy, cost, and environmental benefits to the Federal sector, the technologies are considered to be leading candidates for immediate Federal application. The goal of the technology Alerts is to improve the rate of technology transfer of new energy-saving technologies within the Federal sector and to provide the right people in the field with accurate, up-to-date information on the new technologies so that they can make educated judgments on whether the technologies are suitable for their Federal sites. Because the Technology Alerts are cost-effective and timely to produce (compared with awaiting the results of field demonstrations), they meet the short-term need of disseminating information to a target audience in a timeframe that allows the rapid deployment of the technologies a n d ultimately the saving of energy in the

federal sector. The information in the Technology Alerts typically includes a description of the candidate technology; the results of its screening tests; a description of its performance, applications and field experience to date; a list of potential suppliers; and important contact information. Attached appendixes provide supplemental information and example worksheets on the technology. FEMP sponsors publication of the Federal Technology Alerts to facilitate information-sharing between manufacturers and government staff. While the technology featured promises significant Federal-sector savings, the Technology Alerts do not constitute FEMP s endorsement of a particular product, as FEMP has not independently verified performance data provided by manufacturers. FEMP encourages interested Federal energy and facility managers to contact the manufacturers and other Federal sites directly, and to use the worksheets in the Technology Alerts to aid in their purchasing decisions.


For More Information FEMP Help Desk  (800) 363-3732 International callers please use

(703) 287-8391


General Contact

Ted Collins

New Technology Demonstration Program

Program Manager

Federal Energy Management Program

U.S. Department of Energy

1000 Independence Avenue, SW, EE-92

Washington, DC 20585

(202) 586-8017

Fax: (202) 586-3000


Steven A. Parker

Pacific Northwest National Laboratory

P.O. Box 999, MSIN: K5-08

Richland, Washington 99352

(509) 375-6366

Fax: (509) 375-3614


Produced for the U.S. Department of Energy

by the Pacific Northwest National Laboratory

Reprinted March 1999

(originally printed November 1995)

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste

Refrigerant Sub-Cooling Technology Alerts

Please Visit:

Email your questions and comments to:

Store Location  / I-75 Business Park


I-75 / Georgia Exit 69

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Chula, GA   31733

(904) 424-9773