Evaporator Arrangements for Large Scale Cool and Cold Storage Applications
Author: Stefan S. Jensen, B.Sc.Eng. MIEAust, CPEng, NPER
The evaporator arrangements in large refrigerated warehouse facilities need to satisfy a range of objectives, some of which may be conflicting. Focusing on the facility owner, these objectives generally relate to facility utilization, energy efficiency, workplace health and safety issues, operability, temperature uniformity, temperature flexibility, consistency of air movement, capital costs and operating costs including maintenance. In modern large scale cool and cold storage facilities being constructed in Australia, the use of ceiling-mounted induced draught finned air coolers is generally limited to smaller areas, some batch blast freezers and loading docks. Ceiling-mounted induced draught evaporator arrangements will therefore not be discussed. For the main cool and cold storage areas, a variety of penthouse unit and alcove unit evaporator designs have evolved over the last two to three decades. This paper will describe the evolution of a range of such evaporator arrangements. It will attempt to detail the practical experiences associated with each design and focus on the type of design improvements that were made each step of the way, how successful these improvements were in practice and what other improvements can be made in the future. Due to the climatic conditions in most parts of Australia and due to a desire to conserve energy, there has been a general trend away from hot gas defrost and towards automatic ambient air defrost in the evaporator arrangements described in this paper. The ambient air defrost concept employs several automatically operated doors and shutters to direct warmer ambient air over the cooling coil to effect the frost removal while at the same time separating the cooling coil from the refrigerated space. Electric defrost is practically no longer used in large scale refrigerated warehouses in Australia with volumes greater than approximately 15,000 m³ (500,000 ft³) except in some CO2 applications. This is mainly due to energy efficiency and reliability issues and electric defrost will therefore not be discussed. Although the evaporator arrangements discussed in this paper are suitable for most refrigerants including synthetic substances and secondary refrigerants, the applications shown mainly employ natural refrigerants, i.e., NH3 and CO2.