Refrigerated Storage Facilities-Aspects of Energy Efficienclr Design , Installation and Operation

Refrigerated Storage Facilities Aspects of Energy Efficienclr Design, Installation and Operation Leelananda Rajapaksha The practice of refrigerated storage design and fabrication has been subjected to many subtle changes over the time due to concerns on energy and environment, improvements of thermal and fire properties of insuiation material, advancements in fabrication technology, energy efficient space planning and issues of personal/property safet1,. This paper highlights certain key aspects of planning and design of cold storages r,r'here changes with respect to properties/construction of insuiation panels, refrigeration system selection, ail tightness, in-use energy efficiency and environment considerations etc. have exerted considerable influences on the overall practice of cold room design installation and operation.


'1,. Introduction
A refrigerated storage facility (commonly known as coid room) is an enclosed space in which the temperature and storage conditions are mechanically controlled. Food and agrarian industries depend very much on cold rooms to store products and raw materials. For example dairy products such as ice cream and yoghurt, processed meat, seed potato, and vegetable seeds such as 'kaaupi'. Depending on the product stored, the operating temperatures of cold rooms would be different.
The operating temperature of a cold room depends on many factors; thermal properties of the materials being stored and the expected storage durations are just two considerations. In general there are two basic types of storage facilities; those operated above 0.C (coolers) and those operated below OoC (freezers). According to American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRAE) a broader classification would be, i) controlied atmosphere for long-term storage of fruits and vegetables, ii) coolers at temperatures of 0'Candabove,iii) freezers at-2to -3"Civ) low-temperature storage rooms for frozen products, usually maintained at -23 to -29o C, with a surplus of refrigeration for freezing products received [1].
Cold rooms use rnechanical refrigeration systems to maintain controlled conditions inside, remove heat from the stored product and to offset any heat gains from the surroundings; through insulation panels, floor panels and doors etc. The refrigeration plant is usually a separate system linked rvith the cold room through a refrigerant circuit (direct system) or a secondary heat transfer fluid circuit such as glycol (indirect system). Figure 1 shows hardware arrangement of a direct system. a -Liquid receiver, b-Compressor(s), c-Condenser, d -Cooling tower, e -Evaporators In ihe usual arrangement, the refrigerated storage is located inside a purpose-br-rilt building and the plant room in a separate section or a room depending on how the plant layout is planned.
The art of cold rooms fabrication has been subjected to many changes over the time with advancements of insulation material, panel construction and assembly technologies, increasing concerns on energy and environment, safety aspects (personal and property), and concerns about satisfying stringent insurance requirements (for protection against accidents including,fire). As a result, the focus of the construction of cold rooms has shifted towards making more air tight, better insulated facilities that are with relatively higher energy efficiency and could withstand fire at least for a specific period of time. This article presents a qualitative look at few selected aspects associated with design, fabrication and operation of cold storage facilities.

Basic design considerations
Fabrication of thermally insulated enclosure of a cold room and design of the refrigeration system involves many technical and operational considerations, some of which are interconnected. Among those some of the main considerations are i) rate of product coming in to the storage ii) resident time of the product, in case of foods ihis depend on the product texture and purpose of storage iii) allowable cooling rate that depends on nature of the product, thermal properties, moisture content and concerns about preserving flavour and freshness etc. iv) product leaving temperature after being stored v) effect of relative humidity on stored product vi) temperature distribution, a uniform distribution is desired in many cases vii) length of coid air piume and impingement on stored product viii) traffic and human movement in and out of storage area ix) effect of air circulation on both the product and the employees x) conditions of controlied environment (ventilation and total pressure if necessary), xi) product entering temperature, and xii) nature/thermal properties of packing material of the product.

2|1. Location of site
In selecting a location for a cold storage facility a number of conditions shali receive attention. These include access from main highway, space for movement of truck/long vehicles/iorries, room availability for futule expansion, land cost, availability of power and water supply, waste and sanitary water disposal provisions, fire protection requirements of the local authority, noise of equipment, external appearance, tax and insurance requirements, plant security, undersoil bearing conditions and surface drainage etc.
In general, cold room facilities are preferred to be located away from congested areas where the cost of increased transportation could be offset by better piant lavout possibilities, while taking the advantage of lor.t'er-priced labour supply, or other economies of operation.

Building size and configuration
Building configuration and size considerations of a cold storage faciiity shall include the mode of transport in receiving/dispatching the goods, seasonal variation of product volumes and the possibility of multiple use of the facility, stacking arrangement, lot size of the product, the nature of the pellets and the product, movements of the product within the storage, requirements for personal and office facilities. However, in case of in-house storage, where product is manufactured and stored at a single location, the above may change depending on the requirements of the whoie plant.
Most cold rooms are made with singie-story structures. Small columns on wide centers permit palletized storage with minimai lost space. A modified one-story design is sometimes used to reduce horizontal traffic distances and land costs. An alternative is to locate non-productive services (including offices and the machinery room) on a secondfloor level, to permit full use of the ground level for production and storage.
Mechanization must be considered as well. In areas where land availability or cost is a concern, a high-rise refrigerated storage building may be a viable option. Designs that provide minimum overall costs restrict office facilities and utility areas to a minimum Products or material that needs be kept in a cold room are usually stacked preferably on racks. If the package itself is not suitable for stacking, pellets are usually used. A pellet is a platform of sufficient strength, usually made out of wood or plastic, on which the product is piled to regular shape such a cube, to facilitate movement of the load using a pellet truck for example. Different racking layouts, as shown in The floor area in a facility where diverse merchandise is to be stored can be calculated on the basis of L30 to 160 kg per gross cubic metre to allow about 40% for aisles and space above the pallet stacks [1]. In special-purpose or production facilities, products can be stacked

Insulation material
Insulation is essential for low-temperature storage in order to reduce the refrigeration demand. The ideal insulation panel is light weight, structurally strong, with minimum thermal conductivity and 100% resistance to moisture and other weatirer phenclmena. A variety of insulation materials is available, with the options of prefabricated panels, or board form. In addition, certain insulation material come as foam applied directly to the surface to be insulated to obtain a monolithic insulation without joints.

Panel construction
Prefabricated board form insulations used in the fabrication of refrigerated storages are often called sandwiched panels. The panels are made of insulation layers of sufficient thickness and protected using metal sheets on either side, forming a sandwich. The metal sheet, which is also referred to as the cladding, is usu,ally made of aluminum or steel and coated with some protective powder/paint. The cladding is corrugated to produce a groovy surface as shown in Figure    In all three insulation materials, EPS, PUR and PIR, have certain permeability such that the thermal performances are affected by moisture.
Therefore it is very important to maintain a good vapour barrier at every possible place where vapour could migrate into the panel, which increases the thermal conductivity, leading to increases in both the demand on refrigeration and energy consumption.
During the installation of services and refrigeration equipment, insulating panels have to be damaged to allow cable trays, refrigerant pipes etc. to pass through the panels. in all such cases, the damages should be treated properly and the insulation restored using standard methods, preferable using foam insulation injected to the damage area so that the possibility of vapour migrating into the exposed insulation is prevented (restore the vapour barrier). In the present context, use of environmentally friendly foam and blowing On the contrary, a package system encompasses all the hardware of a vapour compression refrigeration system, including the evaporators on one platform; Figure 6, and come as a precharged unit ready to use. Figure 6 -Package based cooling system The cold room is connected to the refrigeration system using a secondary heat transfer fluid pipe network (commonly referred to as glycol circuit). Separate coolers are located in the cold rooms in place of the evaporators of a direct system. A water cooled package will have a cooling tower in addition to the above hardware.

Otherconsiderations
In addition to the design considerations given under section 2, there are certain specific issues that need the attention of refrigeration system designer in the context of pattern and purpose of use. In its regular operation of a cold room it is more likely that the cooling demand shows seasonal variations. Outside air entering a cold storage reflects negatively on the refrigeration system as this air represent additional latent and sensible heat loads, which could be proportional to the opening duration of door. This infiltration cooling load is rather substantial in magnitude and could be up to 25% -30% of the total refrigeration load [5]. Sensible part is due to temperature of air, u'hich is higher t\an that in the cold room, and latent part is due to the moisture in outside air that finally will end up on the cooling coil surface as frost. This situation results in relatively longer defrosting durations, r,r'hich may not be desirable depending on the refrigeration system capacity, required air movement pattern in the cold room and with respect to maintaining homogeneous product temperature.
In large cold rooms lvhere a number of doors are available and door openings are frequent, the additional energy consumption due to the heat load associated with outside air migration can be substantial and represent a considerabie energy bill. On energy saving perspective, a scheduled opening pattern of the doors are preferred where in each opening the duration is kept to a minimurn by careful study of the use pattern. Further, installation of strip curtain or air curtain at entrances help reduce additional infiltration load coming into the cold room through opened doors.

Defrosting and removal of moisture
Defrosting too represent a considerable refrigeration load, however, relatively smaller than infiltration load due to door openings [2]. During defrosting the ice formed on cooling coils are melted and the resulting water is left to be collected into the defrost pan of the coil. Depending on the temperature inside the cold storage, this water could freeze again in the pan or itr the drain lines that drain the collected de{rost water out of the cold room. It is important to remove this water to reduce the chances of fungi and bacterial growth inside the cold room.
To complete the defrosting process satisfactorily, tire defrost pan and the drain lines are heated. Defrost pan could be heated using hot refrigerant vapour or using electrical heaters. The drain lines are usually electrically heated where heating element is wrapped around the drain line. Due to this arrangement, proper control of drain line heaters/heating process is very important with respect to potential fire risk.

Conclusions
The article attempted to highlight areas which a designer of cold storage shall take into account in the approach of cold room design. The importance of consumption were highlighted.
How the practice of cold room usage could affect the overall energy consumption and satisfactory performance of the refrigeration plant were also highlighted as the would be useful for a young practicing engineer.