Engineered Refrigeration Systems
Few consulting engineers are specialized in refrigeration systems design. Unfortunately, this shortage means there is a lack of independent expertise to support clients in the food distribution and processing fields when it is time for them to...
Few consulting engineers are specialized in refrigeration systems design. Unfortunately, this shortage means there is a lack of independent expertise to support clients in the food distribution and processing fields when it is time for them to acquire a refrigeration system.
For this reason these clients often lean toward turnkey refrigeration projects, awarded through a performance-based call for tenders. However, with the global trend for better efficiency and the several incentives available from electricity utilities, it is more important than ever to analyze refrigeration systems and how they can be improved. Some improvements could lead to a 50% saving in the energy cost to operate a refrigeration plant.
This article shows the several features implemented in a large refrigerated facility and the role that needs to be played by consulting engineers during a performance-based call for tenders.
With annual sales over $11 billion and 65,000 employees, Metro is a leader in the food and pharmaceutical sectors and the largest food distribution company in Quebec. Across the province and Ontario Metro operates a network of more than 600 food stores as well as 250 drugstores.
To improve the delivery and distribution of its fruits, vegetables and dairy products in the Montreal metropolitan area, Metro built a large warehouse in Laval, just north of Montreal. The warehouse and docks facility covers 22,300 m² (240,000 sq.ft.), measuring 221 x 100 metres. Added to the main floor area are the offices covering an additional 2,150 m². The reception and loading docks have to accommodate 50 trucks at the same time.
To assist Metro with the project, Dessau was mandated to carry out a preliminary study, prepare a performance based call for tenders, as well as carry out site supervision.
Metro believes that it has a duty to respect and protect the environment and has been working for 15 years to continuously improve its environmental record. Each year the company conducts environmental audit in all its Quebec and Ontario warehouses. Energy efficiency in the new warehouse was therefore essential.
Conditions for storing
At the beginning of the analysis of a refrigeration project, the engineer must evaluate the several loads associated with the architectural, mechanical, electrical, stored products and operations in the facility, along with the different storage temperature requirements. The charts show the refrigeration load distribution per component and per building area.
In this case, the internal gains are the most important contributors of the total refrigeration load. They include heat gains from the lighting, handling equipment, evaporator fans, as well as occupants. The major portion of this load comes from the lighting and handling equipment.
The total refrigeration load of the building was evaluated at 3,343 kW (951 tons), including a 560-kW (160 TR) allowance required for 20 ripening rooms (11 double and 9 single) used mostly for bananas, as well as 210 kW (60 TR) to refrigerate the battery chargers room, which had to remain cool for logistical reasons. Once the several refrigeration loads were properly assessed, the design of a reference refrigeration system was developed.
Since each room is to hold specific categories of products, ambient temperature varies in the warehouse from slightly over freezing point at 0.5°C, up to a more elevated temperate of 13°C. Relative humidity ranges from 80% to 95%. Therefore, the temperature difference (TD) between ambient air and coil surface was selected carefully. A smaller TD would require additional coil surface, while a bigger TD means greater air dehumidification. Excessive dehumidification causes an increase in the defrost requirements, as well as potential losses for the company, which sells produce by the weight.
After a detailed analysis the design team chose to develop a dual loop system, with two distribution temperatures: −4 °C for colder rooms and 2°C for higher temperate sectors and ripening rooms.
Many options were possible in terms of refrigeration system configuration. The final choice was based on cost, overall plant efficiency and safety considerations. After carrying out several preliminary selections and energy modeling, the system developed for the performance-based call for tenders included two groups of compressors serving each loop, and discharging to a single condenser. The high temperature group would also be used to carry out sub-cooling for the refrigerant fed to the lower temperature evaporators.
Tender process allowed contractors to innovate
It is important to understand the role of the consulting engineers in the process of awarding a contract to a refrigeration contractor. For this project, we developed a reference design with all the details that should be included to achieve a performing building. Then, during the tender period, each contractor was invited to propose its own refrigeration design or follow the reference design. Once awarded, the winning contractor took the professional engagement to meet the performance requirement and liability for the proposed system.
The tender process left latitude to the different contractors to propose innovative designs for the refrigeration process. During the tender period, Metro received a total of nine different proposals for the refrigeration system, including almost every technology and configuration possible:
• Single stage, cascade, direct or indirect systems;
• Single or dual loop, using direct expansion refrigerant, pumped refrigerant or pumped secondary fluid;
• Ammonia, carbon dioxide or R-507;
• Multiple or centralized, dry or evaporative condensing units.
The amount of important data to manage makes the process of awarding the contract a hard task for an inexperienced person, who may lean toward choosing the least expensive proposal. This is where the owner needs to rely on experienced consulting engineers to help in selecting the best option to suit its needs. The chart on page 29 shows relative prices and operating costs relating to the proposals from the different contractors.
Analyzing and selecting
The analysis of the several alternatives was not an easy task considering the impact of each system on the supervision requirements, maintenance cost, total refrigerant charge, operating conditions, defrost method, the durability of the installation, the reliability of the compressors, the potential energy efficiency improvement, and financial subsidies, etc.
In the end and following Dessau’s recommendations, Metro selected neither the cheapest nor the most energy efficient system, but the one that scored the best with regard to the selection criteria.
Single stage ammonia refrigeration with
pumped liquid CO2
The selected design is a single stage ammonia refrigeration system with pumped liquid CO2. This configuration has the advantage of a lower ammonia charge (compared to a pumped liquid ammonia system) and the system is confined within the mechanical room. This aspect reduces greatly the risk of a potential leak, which was a major concern for Metro.
Two liquid CO2 circuits, −4 °C and 2°C, are pumped in the evaporators supplying cold in the different sectors of the warehouse. The liquid CO2 is cooled by a shell and plates, condensing the CO2 on one side and evaporating the ammonia on the other side. Mono screw compressors reject compressed gas at 1,144 kPa (and 32 °C condensing temperature). In order to get the best out of the selected design in terms of energy efficiency, the compressors are divided in two groups (low and high temperature) and the plate heat exchangers were oversized to minimize the temperature difference between what is required inside the evaporators and the saturated suction temperature (SST), which has a positive impact on the compressors&rs
What contributed most
to energy savings?
The most interesting energy efficiency improvement that was implemented is the adjustment of condensation pressure along with outside temperature. Out of a total 10,700,000 kWh consumption for a standard plant, this energy efficiency measure generates savings estimated to be 1,000,000 kWh. Another major improvement is the liquid sub-cooling achieved at a higher evaporation pressure through a secondary suction port on the screw compressors. In this case, liquid sub-cooling generates savings ranging around 500,000 kWh.
The most important contributor to energy savings is the selection of more efficient compressors. For a particular application some compressors could have a coefficient of performance (COP) 50% higher than non-efficient compressors. It is very important to ask for performance data from compressor manufacturers. This concept, together with an improved lighting performance in the building, generates a 5,560,000 kWh saving for the whole building. The reference building’s expected consumption is 14,000,000 kWh, while the expected energy consumption of the new Metro warehouse is 8,440,000 kWh. It is an energy savings of 39.7% when compared to the reference building.
Benefits of final concept
The final concept shows better energy efficiency and uses natural refrigerants, which help to minimize greenhouse gases emissions related to energy production and fugitive emissions. Metro is also acting as a responsible citizen in that the selected system minimizes the potential risk inherent to keeping hazardous substances in facilities.
The new facility meets Metro’s expectations and the performance-based call for tenders gave the owner a chance to peek into different engineers’ heads before choosing the manufacturer/contractor who would design its refrigeration system.
In retrospect, contractors also benefit through this process, since the consulting engineers had the ability to identify the value added in the different proposals. This approach allows the contractors more room for innovation, instead of being condemned to develop designs strictly based on price. Even without doing the actual design of the system, Dessau as the consulting engineer had a major impact toward the success of this project, thanks to its expertise in refrigeration systems.cce
Jean-Philippe Morin, ing., P.Eng., CMVP-IT and Laurier Nichols, ing., P.E., AP LEED BD+C, ASHRAE Fellow, are both with Dessau in Montreal.
Prime consultant, refrigeration:
Dessau (Laurier Nichols, ing., Jean-Philippe Morin, ing.)
Refrigeration contractor: CIMCO
Architects: TLA architects
HVAC consulting engineer: Equation
General contractor: Vergo Construction (including JCF architects, Meconair, CLA)
To see the graphics for this article, please see the digital edition.