Technical Papers. 32nd Annual Meeting. International Institute of Ammonia Refrigeration. March 14 17, 2010

Transcription

1 Technical Papers 32nd Annual Meeting International Institute of Ammonia Refrigeration March 14 17, Industrial Refrigeration Conference & Exhibition Manchester Grand Hyatt San Diego, California

2 ACKNOWLEDGEMENT The success of the 32nd Annual Meeting of the International Institute of Ammonia Refrigeration is due to the quality of the technical papers in this volume and the labor of its authors. IIAR expresses its deep appreciation to the authors, reviewers and editors for their contributions to the ammonia refrigeration industry. Board of Directors, International Institute of Ammonia Refrigeration ABOUT THIS VOLUME IIAR Technical Papers are subjected to rigorous technical peer review. The views expressed in the papers in this volume are those of the authors, not the International Institute of Ammonia Refrigeration. They are not official positions of the Institute and are not officially endorsed International Institute of Ammonia Refrigeration 1001 North Fairfax Street Suite 503 Alexandria, VA (voice) (fax) Industrial Refrigeration Conference & Exhibition Manchester Grand Hyatt San Diego, California

3 Technical Paper #3 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Stefan S. Jensen, B.Sc.Eng. MIEAust, CPEng, NPER Scantec Refrigeration Technologies Pty. Ltd. Brisbane, Queensland, Australia Abstract 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 CO 2 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., NH 3 and CO 2. IIAR

4

5 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Introduction The evaporator arrangements described in this paper are generally suitable for facilities with refrigerated volumes from around 15,000 m³ (~500,000 ft³) and greater. This statement does not mean that the evaporator concepts could not be used in smaller facilities; it simply states the fact that the application of these arrangements to smaller facilities does not appear to occur frequently in practice, presumably for commercial reasons. Refrigerated warehousing facilities are generally designed for a large range of products and storage durations. For the refrigeration contractor this translates into a requirement for a great range of storage temperatures, storage humidities, temperature tolerances, defrost frequencies, air change and air movement requirements. Not unlike other industries, many refrigeration plant and evaporator concepts in the refrigerated warehouse industry are driven by vested interests of equipment suppliers and by the desire on the part of plant owners for low capital costs. A simple example of this is the construction of a cold store without an anteroom in a hot climate. The evaporator concepts described in this paper do not fall into the low capital cost category on the contrary, they have relatively high capital costs. The various principles shown are designed to maximize the long-term benefits to the facility owner. So what are the long term benefits to the facility owner when using a combination of appropriate evaporator arrangements and efficient refrigeration plant design? In summary they are: Minimization of energy consumption to around 35 kwh/m³*a (1 kwh/ft³*a) for large facilities Minimization of maintenance and repair costs by providing access to the evaporators and thus bypassing the refrigerated facility Minimization of disruptions to facility operations (maximizing up time) Minimization of workplace health and safety risks Technical Paper #3 IIAR

6 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Maximization of site utilization while remaining within the constraints of local building regulations, i.e., the maximization of pallet spaces Maximization of the quality of storage with respect to temperature, humidity and air movement. Dual Air Discharge, Dual Cooling Coil Penthouse Evaporators Figure 1 shows an early version of this arrangement; the concept is around 30 years old. Ease of manufacture and unit assembly was good. The base frame comprising cooling coils, drip trays, fans and air intake grid was factory assembled and the assembly was positioned within the refrigerated warehouse as a complete unit. The lobster back air diffusers for air discharge were fitted after installation of the insulated panel ceiling immediately prior to commissioning. The major disadvantages of the early version were: relatively high fan energy consumption due to the sharp edged air flow paths difficult and to some extent dangerous fan and coil access for repairs and maintenance relatively high risk of condensate drips from drip trays, air intake grid and fan cowls/housings sealing problems at the interface between evaporator housing and warehouse ceiling Figure 2 shows a further development [Reference 1]. This reduced the fan energy consumption to around one third of the fan energy consumption of the unit shown in Figure 1. A factor contributing significantly to this reduction is the relatively small air pressure drop throughout the air path. The air path pressure drop from the air intake point below the inlet grille to the air discharge point from the discharge duct is estimated at around 60 Pa (~0.24" W.G.) at design air flow. This pressure drop does not include the coil air pressure drop. 4 IIAR 2010 Technical Paper #3

7 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Through incorporation of a service platform between the coil air outlet and fan inlet, the access to the fans and fan motors was greatly improved. This service platform has a dedicated access door from the ceiling space. Although the risk of condensate drips was reduced somewhat with the layout shown in Figure 2, a weak point remained the timing of the hot gas defrost process. Excessive hot gas on time had a tendency to warm up the entire penthouse enclosure with the ceiling being the warmest area. Small frost particles on the underside of the penthouse ceiling would, as a result of the higher temperatures, thaw, liquefy and detach from the ceiling in the form of water droplets. Some of these droplets would fall down into the warehouse through the air intake grill and some would fall into the fan cowl and occasionally adhere at a point between the tip of the fan blade and the internal surface of the fan cowl causing fan overload trips. Many avenues were pursued in the attempts to solve these problems. One avenue was to employ dual valve stations so that the drip pans can be warmed up anywhere from 30 minutes to one hour before hot gas is supplied to the coil(s). This minimizes the duration of the hot gas flow to the coil to around 600 seconds or less and reduces the warming of the enclosure. It is a successful way of dealing with the issue, but the disadvantages are greater valve station complexity, greater valve station cost and increased operating expenses. Another avenue was the application of low temperature grease to the internal surfaces of the fan cowls. Although this also successfully prevents fan faults due to freezing of water on the inside of the cowls, the application of the low temperature grease needs to be repeated at regular intervals. A third avenue was very exact timing of the hot gas supply to the cooling coils combined with rotating the fans backwards intermittently at very low average speeds of around 2 5 Hz in an attempt to supply cold air from the warehouse to the penthouse enclosure and hence prevent thawing of the frost particles on the ceiling. But because most variable frequency drives are unable to operate at frequencies much below 10 Hz without risk of overheating, this effectively means stopping and starting the fans throughout the defrost process. This avenue was successful for Technical Paper #3 IIAR

8 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California moderate finned coil heights of around mm (4.6' 5.2'). For larger coil heights up to 2000 mm (6.6 ) and beyond, however, excessive warming of the penthouse ceiling was almost impossible to prevent, presumably due to a degree of chimney effect created when the cooling coils are warmed internally by the hot gas. The problems associated with the sealing of the penthouse enclosure against the ceiling of the warehouse were probably worsened with the concept shown in Figure 2 as compared with the concept shown in Figure 1. This is due to the angles of the discharge ducts and the practical difficulties encountered by the panel contractors to accurately miter the corners and achieve a perfect seal. The evaporator arrangement shown in Figure 2 requires a high level of coordination between the refrigeration contractor and the building contractor. This coordination needs to commence as early as possibly during the design stage because these penthouse units are a relatively heavy (up to 12,000 kg/26,000 lbs) operating mass for large units, and there is only a relatively narrow window of opportunity for site installation. Often the penthouse units need to be installed immediately after erection of the structural steel, but prior to installation of the roof. The roof may obstruct crane access such that the head of the crane used for positioning the penthouse assembly fouls on the roof sheets hence preventing a sufficiently high lift. This can then lead to additional site installation costs brought about by the requirement for special custom engineered lifting frames. Figures 3 and 4 illustrate different stages of an installation process with a lifting frame shown in Figure 3. A derivative of the penthouse concept shown in Figure 2 is the design for high humidity. Figure 5 shows the detail of a unit designed for a warehouse for storage of leafy vegetables. The high humidity is achieved by pulling the air stream through an adiabatic pad similar to those used in air cooled condensers with adiabatic precooling. Of the various penthouse concepts discussed herein, the dual coil, dual air discharge penthouse shown in Figure 2 is the one considered most suitable for incorporation of humidification pads. 6 IIAR 2010 Technical Paper #3

9 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications The penthouse unit type shown in Figure 2 may be fitted with variable speed fans for energy conservation. Good air movement and temperature uniformity may be achieved within the refrigerated facility even at greatly reduced fan speeds. Figure 6 illustrates a relatively large 85 x 55 m (279 x 180 feet) consolidation area serviced by two penthouse units. The air velocities and air temperatures were measured at 25% fan speed (240 rpm) at various measurement points around 1 m (3') above the floor level, refer Figure 7 for measurement point pattern. The 25% fan speed represents an average coil face velocity of 1 m/s (197 feet/min.). The room temperature and air velocity measurement results are shown in Table 1. The design discharge velocity from the penthouse unit discharge duct is 4.5 m/s (886 feet/min) at 100% fan speed. The relatively high temperatures at measurement points F39, F40 and F50 are due to a dock door opening directly to ambient conditions at the time. Despite the imperfections of the concept shown in Figure 2 it has been implemented widely by a range of warehouse operators, consulting engineers and refrigeration contractors in Australia and selected Asian countries since its introduction to the market in Single Air Discharge, Single Cooling Coil Penthouse Evaporators Where the dual coil, dual discharge evaporator arrangement described in the preceding chapter is unsuitable for automatic ambient air defrost, the single coil, single air discharge penthouse evaporator is designed specifically with this type of defrost in mind (Figure 8). Of course, this particular arrangement may also be defrosted by other means, but then there are other less capital-intensive penthouse evaporator concepts available to suit that requirement. The support steel/coil/fan arrangement shown in the lower part of Figure 8 fits into the panel assembly shown in the upper part. The coil/fan assembly is arranged for induced draught through the coil; the fans discharge through the flap valves into the angled air supply duct. Technical Paper #3 IIAR

10 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Single air discharge penthouse evaporators have been designed and built with capacities up to around kw (57 63 TR), see Figure 9 for a unit in that capacity range. The construction of units with greater unit capacities is theoretically possible, but there are certain practical limitations that must be taken into account. These are: 1) Coil face dimensions. An evaporator coil with a capacity around 200 kw will be around 2000 mm (~6.6 feet) finned height and 5250 mm (17.2') finned length. The structural base, the penthouse enclosure size, the defrost door sizes, the penthouse unit mass etc. all increase proportionally with coil size and the positioning of very large penthouse units below the warehouse roof becomes increasingly difficult as unit size escalates, 2) Practical finned length. With very tall and relatively long evaporator coils for low temperature freezing applications, the refrigerant circuiting becomes increasingly difficult unless the designer resorts to multiple liquid supply and wet return headers on the coil. Multiple headers complicate the valve station and increase installation costs and very long evaporator coils increase shipping costs particularly for overseas container shipping, 3) Structural requirements within the building structure. Figure 9 illustrates some elements of the surrounding building structure and the relative size of these. It is desirable for most warehouse operators to minimize the amount of columns present within the warehouse. This in turn creates a general trend towards longer and longer truss spans and hence increasing difficulties associated with supporting very large evaporator assemblies. The single air discharge, single coil arrangement shown in Figure 8 further minimizes the risk of condensate drips within the refrigerated space. It does this by positioning the fans in an upright position well away from the air supply point to the warehouse, by separating the penthouse on the air side from the refrigerated space during defrost and by incorporating a drain tray throughout the internal floor of the penthouse enclosure. The smooth air path introduced with the concept shown in Figure 2 is 8 IIAR 2010 Technical Paper #3

11 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications retained in this arrangement, but the angled air outlet duct is not introduced until downstream of the fan discharge. All other things being equal this increases the capital costs of the penthouse enclosure and lengthens it at the same time. During defrost the automatic pivoting baffles at the air inlet and the air outlet sides of the penthouse are automatically shut. The horizontally sliding air defrost doors are then opened and the direction of rotation of the fans is reversed. Via the defrost air intake door the fans draw air from the warm ceiling cavity and push this air through the cooling coil. Once the air has passed through the cooling coil it turns 90 horizontally and is discharged through the horizontally sliding defrost air discharge door into the air discharge compartment. Once inside the air discharge compartment, the air stream turns 90 upwards and is discharged to ambient through the vertical defrost air discharge duct. The defrost air discharge compartment and the valve station compartment are combined; any condensate drips from the valve station are captured by the penthouse floor pan. Refer to Figures 10 and 11 for images of the defrost air discharge duct and the pivoting baffles during defrost. Another fundamental difference between the arrangements shown in Figure 2 and Figure 8 is the structural frame. The single coil, single air discharge penthouse unit is assembled on the ground on a structural frame. Figure 12 shows the frame prior to installation of coil, drip tray and fans. Following assembly, the entire unit comprising coil, fans, drip trays, insulated panels and frame is hoisted into position, (Figure 13). More importantly, however, between the floor of the penthouse assembly and the insulated panel ceiling of the warehouse is a void of mm (2' 3'). From a practical point of view this means that the penthouse unit assembly can be suspended from the warehouse overhead steel structure while the insulated panel ceiling is being erected below. Figure 14 shows a finished penthouse unit in position. The openings for air return and air supply are not cut until towards the end of the warehouse construction. This has some advantages in terms of co-ordination of trades and site safety, but it also simplifies the task of sealing between the air ducts and the penthouse enclosure. Due to potential movement between the panel ceiling Technical Paper #3 IIAR

12 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California and penthouse ducts, the sealing problems are not completely eliminated and often the facility owner needs to check the seals between the ducts and the panel ceiling as part of regular maintenance procedures. A potentially weak point of this evaporator arrangement is the void between the underside of the penthouse and the insulated panel ceiling of the warehouse. Although the void is convenient for the positioning of drains (Figure 15), there is a risk of condensation. Unless this void is ventilated or unless the underside of the penthouse floor is trace heated, condensation is likely to occur in this area either on the underside of the penthouse enclosure or on the top of the ceiling panel or in both places. An interesting detail of the arrangement is the incorporation of a hinged fan, (Figure 16). This is included in order to gain convenient access to the void between the fans and the air leaving side of the evaporator coil. The air supply openings to the warehouse are visible on the right. The automatic defrost doors external to the penthouse enclosure require particular attention during commissioning. Prior to defrost and following completion of defrost it is necessary for these doors to open and close 100% reliably. During normal operation, it is necessary for the doors to seal perfectly in order to avoid unnecessary infiltration. Leaking door seals may also give rise to frost formation at the seals and this may jeopardize reliable automatic opening and closing. There are no special door requirements these may be standard good quality cold room doors with automatic actuators which can respond to signals supplied from the central PLC. Figure 17 shows a defrost door with actuator and limit switch. Failure, however, to perfectly align the doors prior to fitting the automatic actuators can cause very irritating and time consuming rework in practice. As with any automatic door system, there are potential risks to personnel. The automatically operated defrost doors in penthouse units with ambient air defrost are 10 IIAR 2010 Technical Paper #3

13 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications no different. Unless precautions are taken, the situation may arise where a person enters a penthouse unit during defrost. This is possible through the open defrost air intake door. If the defrost door then proceeds to close automatically behind the person, a potentially dangerous situation can occur. A very simple and effective preventative measure is to combine the automatic door with a manually operated gate. The manually operated gate is installed in the door frame and is accessible only when the defrost door is open. To gain access through an open defrost door, the person must first disengage the gate. The gate opens outwards through the defrost door opening and once open, the gate automatically blocks the automatic defrost door in the open position. An attempt by the defrost door to close automatically against the open gate will cause an overload and the automatic defrost door then simply needs to be reset once the person has left the penthouse enclosure and reengaged the gate in the closed position. Dual Air Discharge, Single Cooling Coil Penthouse Evaporators The air throw from a single coil, single air discharge penthouse unit as described in the preceding chapter is limited to around 80 m (~260'). This air throw is of course influenced by practical factors such as duct discharge velocity, discharge duct design, obstructions in the air path after discharge etc., but given the best design efforts in all these areas, practical experience has indicated that beyond a horizontal distance of 80 m (260') from the duct discharge point, the air movement becomes <0.5 m/s (100 feet/min). The air throw limitations of the single coil, single air discharge penthouse unit led to the development of the single coil, dual air discharge penthouse unit, (Figure 18). The single coil is furnished with a divider plate in the centre. Through one half of the evaporator coil, the air stream flows East-West, through the other half it flows West-East when viewed in plan. The evaporator is serviced by one refrigerant valve Technical Paper #3 IIAR

14 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California station only. The ambient air defrost principle remains the same as for the single air discharge unit. Figure 19 shows the door positions during defrost and maintenance. The single coil, dual air discharge penthouse concept is suitable for refrigerated warehouses where the length exceeds approximately 90 m (~300 feet). Such warehouses may of course also be serviced by two single air discharge, single coil units, but the capital costs will increase and the return on the additional capital investment will be difficult to justify. Single Air Discharge, Single Cooling Coil Alcove Evaporators An evaporator arrangement gaining popularity is the single air discharge, single cooling coil alcove evaporator shown in Figure 20 [Reference 2]. During normal operation, air is drawn from the warehouse through the opening at low level. The air stream then turns 90 up through the evaporator coil, travels through the fans and is discharged back into the warehouse at high level. During defrost, the top and bottom openings facing the refrigerated warehouse are closed automatically, the rotational direction of the fans is reversed, the defrost doors facing the exterior of the alcove enclosure are opened and warm ambient air is forced vertically down through the evaporator coil. Figures 21, 22, 23 and 24 show various practical installations. Alcove evaporators can be designed for very large capacities up to 400 kw (114 TR). The main limitation is the size of the coil, but alcove evaporator arrangements with two coils mounted end to end are not uncommon. In practice, however, the element usually dictating the footprint of the alcove evaporator is the cold store racking. If the alcove is erected within the cold store (see Figures 21 to 24 inclusive), it is necessary to sacrifice pallet spaces. From the warehouse operator s point of view it makes no sense to sacrifice half spaces so usually the length allocated for the alcove unit is determined as multiples of whole pallet spaces. Secondly, the alcove cannot for operational and safety reasons protrude into the aisle(s) of the warehouse there 12 IIAR 2010 Technical Paper #3

15 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications also has to be sufficient space for safety rails/bollards at the front of the alcove facing the warehouse to protect the defrost doors and the door mechanisms against damage from forklifts. The depth of the double cold store racking minus allowances for insulated panel, doors and bollards therefore usually dictates the finned width of the evaporator coil. The single air discharge, single cooling coil alcove evaporator arrangement has several advantages. These are in summary: 1) Simplicity and relative ease of installation, 2) The refrigerant pipelines may be installed externally to the building at the same relative level as the evaporators in new installations it is generally possible to avoid wet return risers and ensure gravity drainage of the wet return lines the entire distance back to the accumulator, 3) Significantly reduced structural requirements in relation to the building, 4) Elimination of refrigerant pipelines and hence potential condensate drips in the ceiling cavity of the facility condensate drips from the refrigerant pipelines down on the insulated panels have been the cause of extensive insulated panel damage in many refrigerated warehouses, 5) Simplified evaporator circuiting for low temperature applications without having to resort to multiple headers and circuit orifices, 6) Simple and convenient access to evaporator coil, fans, valve station, drip tray, defrost door mechanisms and condensate drain. There are, however, also features of the alcove evaporator arrangement that are less advantageous. These are: a) If alcove evaporators are used in warehouse extensions where the existing evaporators are serviced by pumped liquid and wet return lines situated in the ceiling cavity of the warehouse, it is almost always necessary to incorporate Technical Paper #3 IIAR

16 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California a satellite accumulator at ground level in order to avoid very tall wet return risers from the alcove evaporators, b) The fan power consumption of alcove evaporators calculated as a percentage of the gross cooling capacity of the evaporator has a tendency to be somewhat greater (15 25%) than in the equivalent ceiling mounted penthouse units described in the preceding chapters. This is due to the generally greater air pressure drop in the air path of an alcove evaporator, c) In those cases where the alcove evaporator is situated within the warehouse, pallet spaces will be sacrificed, d) In those cases where the alcove evaporator is situated outside the refrigerated warehouse (Figure 20) and the air is ducted in/out of the cold store, the floor area of the alcove evaporator is counted as part of the building for the purposes of obtaining Development Approval. In most jurisdictions, authorities nominate the percentage of the total land area the building may occupy and if the alcove foot print is included in the building area this then effectively reduces the number of pallet spaces the building can store, e) In those cases where the alcove evaporator is situated outside the refrigerated warehouse and the air ducted in/out of the cold store, the air intake to the alcove evaporator may in part be blocked by pallets. Summary of Features, Benefits, Advantages and Disadvantages The evaporator concepts described here are not necessarily universally practical in all refrigerated warehouse applications. In extensions to existing facilities, existing plant infrastructure may dictate a certain evaporator concept. In new installations cold store racking, engine room location, engine room relative elevation, site security, site boundaries, room footprint, etc., may dictate another evaporator concept. Assuming all evaporator types described can be applied at will to a certain refrigerated 14 IIAR 2010 Technical Paper #3

17 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications warehouse project, it may be of interest to both facility owners and contractors to analyze which evaporator concept is optimal for the job. Table 2 illustrates such an analysis. The analysis allocates a point score of 1 to 10 to each evaporator feature with the highest number being the best score. The analysis is not universally applicable because each facility is different and some features may not be considered relevant to a certain facility. However, the analysis can easily be adjusted by the designer and the facility owner to suit a particular project and the most favorable evaporator concept can be determined accordingly. Ambient air defrost for penthouse and alcove evaporators has been used in all parts of Australia including Melbourne (latitude 37, 47'S penthouses) and Perth (latitude 31, 57'S alcoves). By comparison San Diego, California is latitude 32, 44'N. The lowest winter temperature in Melbourne is 2 C (35.6 F), the lowest night time temperature in Perth is 9 C (48.2 F). In the case of penthouse units, the defrost air is provided from the ceiling cavity. During the daytime, the air temperature in the ceiling cavity is generally higher than the ambient temperatures due to sun radiation. Most refrigerated warehouse facilities in Australia are constructed as an insulated chamber covered by a weather roof manufactured from corrugated steel sheets. The ceiling cavity is in this context the space between the weather roof and the top of the insulated chamber. Ambient air defrost has been used in facilities with design warehouse temperatures down to 30 C ( 22 F). The energy efficiency benefits of ambient air defrost as compared with hot gas defrost may be visualized using a simplified example. Assume a dual coil, dual air discharge 135 kw (38.6TR) penthouse evaporator with the finned coil dimensions length by width by height 4200x550x1320 mm (165.4x21.6x52.0") and a total surface area of 1205 m² (12,970 ft²). If the evaporator is covered by 2 mm (0.079") of frost throughout with an average density of 200 kg/m³ (12.5 lbs/ft³) then the energy required to melt this frost is approximately 45 kwh (153,550 BTU). If hot gas is supplied to the evaporator for say 1/3 of one hour during defrost and all this hot gas condenses in the coils while rejecting the latent heat to the frost on the evaporator Technical Paper #3 IIAR

18 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California coils as well as to the air surrounding the evaporators, then the energy supplied to the coils will be approximately 96 kwh (327,570 BTU). Unless the evaporator is separated from the refrigerated warehouse during the defrost process, then the excess energy over and above that required to melt the frost on the coils will enter the refrigerated warehouse. To maintain warehouse temperatures, this excess defrost energy will need to be removed by the refrigeration system. Supposing the refrigeration system has a coefficient of performance (COP) of 1.6 then the electrical energy required to remove the excess defrost energy is around 32 kwh per defrost. In a plant with three evaporators and daily defrosts this additional electrical energy consumption accumulates to around 35,000 kwh annually or ~2% of the total annual refrigeration plant energy consumption. This example is simplified and the energy efficiency gain of ambient air versus hot gas defrost depends on how well (or how badly) one or the other system has been designed and commissioned. It is, however, a common experience that coils do not defrost uniformly hence necessitating continued supply of hot gas after the majority of the frost has melted. The score of evaporator concept IV supports the recent trend experienced in several practical applications. Discussion The evaporator arrangements described in this paper are not characterized by low capital costs. On the contrary, they have relatively high capital costs. Despite this, they have all experienced increasing popularity in a significant number of large-scale refrigerated warehouse facilities constructed in recent years in Australia and the Far East. This provides evidence that facility owners are in a position to assign positive operational value to the concepts described and also that facility owners are able to justify the additional capital costs on the basis of an acceptable return on investment. 16 IIAR 2010 Technical Paper #3

19 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Some installations incorporating the evaporator penthouse and alcove arrangements have, however, not been as successful as all stakeholders could have wished. Unfortunately, once a facility has been completed, fundamental design and installation errors are usually very difficult, if not impossible to rectify. Some of the more serious problems that have been experienced in practice are: a) Poor sealing of the penthouse/alcove enclosure. The construction of the penthouse/alcove enclosure normally forms part of the building trade, i.e., the insulated panel subcontract. Lack of experience on the part of the panel contractor, inadequate attention to the details of the enclosure design or inadequate site supervision can cause these problems. The consequences are condensate drips, short intervals between defrosts and poor energy efficiency. b) Poor sealing of the penthouse/alcove doors. Refer to comments under a). c) Inadequate ceiling height within the penthouse evaporator enclosure. It is a common mistake to let the finned coil height dictate the internal ceiling height of a penthouse enclosure. This can cause difficult service access throughout the life of the unit. If the finned coil height is relatively shallow, the coil needs to be installed on a flashed support stand to ensure adequate height for personnel within the penthouse enclosure. d) Poor internal lighting. Poor lighting within the penthouse units making service access difficult and at times dangerous. e) Very tall wet return risers from alcove units. Wet return risers up to 8 m tall (26 ) from the alcove evaporator relative level to the main wet return line in the ceiling cavity have been seen in extreme cases. Of course this eliminates the need for a satellite accumulator, but the very high refrigerant pressure drop in the riser will penalize energy efficiency throughout the life of the installation. f) Poor and unreliable door automation in penthouse and alcove units with ambient air defrost. Technical Paper #3 IIAR

20 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California g) Inadequate structural supports for penthouse units causing constant sealing problems at duct entries to the warehouse. The building structure, the insulated panel ceiling and the penthouse enclosure do not move concurrently and by the same distance. h) Poor internal flashing within the penthouse/alcove unit that causes air to bypass the evaporator coil(s). i) Excessive noise levels due to inappropriate fan and/or face velocity selections. j) Air bypass through the drain pan of penthouse units particularly in dual air discharge, dual coil penthouse units at reduced fan speed. k) Insufficient fall in the condensate drain pipe(s) from the penthouse unit to the condensate drain point. The condensate drain pipe(s) from the penthouse unit to the edge of the building is (are) usually installed above the insulated panel ceiling of the warehouse. It is often necessary for the condensate to travel m ( ') above the ceiling before arriving at the dropper. If the penthouse unit is not installed at a sufficiently high level, then drainage by gravity becomes difficult. It then becomes necessary to pump the condensate away and this can easily jeopardize reliability. l) Condensate drain points of inadequate capacity to accommodate the very large and sudden quantities of condensate being discharged from a large penthouse unit during defrost. Refer also the point above. Often the condensate falls more than 10 m (33') to the condensate drain point at ground level and the instantaneous flows from a large 200 kw (57TR) evaporator can become so large that condensate drain points at ground level need to be 150NB (6") to accommodate the flow without condensate backing up in the dropper or even causing condensate pipe leaks or ruptures. m) Poor defrost in dual air discharge, dual coil penthouse units caused by inappropriate hot gas defrost arrangements resulting in stagnant cold or subcooled liquid refrigerant remaining in the bottom of the evaporator coil(s). 18 IIAR 2010 Technical Paper #3

21 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications This problem is often sought to be eliminated by leaving the hot gas on for too long leading to the melting and re-freezing problems described in the body of the paper. n) Excessive fan power consumption due to excessively high coil face velocities occasionally combined with poor duct design. Coil face velocities >3 m/s (590 feet/min.) are generally considered excessive in this context. o) Inadequate air throw due to incorrectly designed discharge ducts particularly from alcove units. After leaving the coil, the air stream needs to travel vertically (often more than 9 m or 30') and then turn 90 to enter the warehouse and flow horizontally upon entry. Inappropriate duct design around the bend and downstream of the bend causing duct turbulence can jeopardize the throw available following air discharge. All of the issues raised above relate to design and project coordination and can as such be eliminated during the design phase and with appropriate site supervision. As is commonly known many refrigerated warehouse projects are often a result of a group of consultants, contractors, managers and quantity surveyors working together. It is also often a fact that these project participants by and large do not know each other prior to the project and may not work together again after completion of a particular job. This is how the design and supervision issues raised above often fail to receive the attention they deserve, and this is how project teams are often unable to apply the lessons learnt on a project in future installations. The facility owner should play a significant role in this context, but often the focus of facility owners is not on the technical details of the warehouse construction, but more on the logistical functions that a warehouse is constructed to satisfy. This paper attempts to summarize for stakeholders what traps to be aware of when applying the evaporator arrangements that have been described. Technical Paper #3 IIAR

22 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California References Jensen, S.S Large Scale Cold Stores, An Innovative Design Approach, Proc. AIRAH Conference, Melbourne, Australia, March Jensen S.S Dry Expansion Feed in Dual Stage Ammonia Plants: Operating Experiences in a Large Refrigerated Distribution Centre. Proc IIAR Ammonia Refrigeration Conference and Exhibition, Reno, Nevada. 20 IIAR 2010 Technical Paper #3

23 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Table 1. Air Temperature and Air Velocity Distribution for Consolidation Area Temperature Air velocity Temperature Air velocity Location [ C] [m/s] [F] [feet/min] F F F F F F F F F F F F F F F F F F F F F F F F F F F Technical Paper #3 IIAR

24 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Temperature Air velocity Temperature Air velocity Location [ C] [m/s] [F] [feet/min] F F F F F F F F F F F F F F F F F F F F F F F Mean Standard deviation IIAR 2010 Technical Paper #3

25 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Table 2. Summary and Rating of Evaporator Features Evaporator concept I II III IV Feature Energy efficiency Air path pressure drop Air distribution Maintenance access Workplace Health and Safety Risk of condensate drips into warehouse Risk of condensate drips onto ceiling Coil circuiting General ease of installation Automation Ease of Defrost Variable speed fan integration Refrigerant detection Refrigerant leak management Ease of commissioning Pallet space maximization Building structural requirements Refrigerant pipeline erection (ease of) Refrigerant pipeline location Refrigerant valve station design Reliability Evaporator Legend: I: Dual Air Discharge, Dual Cooling Coil Penthouse Unit II: Single Air Discharge, Single Cooling Coil Penthouse Unit III: Dual Air Discharge, Single Cooling Coil Penthouse Unit IV: Single Air Discharge, Single Cooling Coil Alcove Unit Technical Paper #3 IIAR

26 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 1. Typical dual air discharge, dual coil penthouse unit of the early design Figure 2. Typical dual air discharge, dual coil penthouse unit of 1998 design 24 IIAR 2010 Technical Paper #3

27 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 3. Penthouse Unit during Installation Figure 4. Penthouse Unit during Installation Technical Paper #3 IIAR

28 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 5. Penthouse Unit for High Humidity Applications Figure 6. Penthouse Units Servicing Consolidation Area 26 IIAR 2010 Technical Paper #3

29 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 7. Floor layout and Measurement Points, Consolidation Area Technical Paper #3 IIAR

30 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 8. Single Coil, Single Air Discharge Penthouse Unit with Ambient Air Defrost 28 IIAR 2010 Technical Paper #3

31 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 9. Single Coil, Single Air Discharge Penthouse Unit, Nominal Capacity 200 kw (57TR) Figure 10. Defrost Air Discharge Compartment during Ambient Air Defrost Technical Paper #3 IIAR

32 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 11. Penthouse Horizontally Pivoting Supply Air Baffles during Ambient Air Defrost Figure 12. Single Coil, Single Air Discharge Penthouse Frame prior to Coil and Fan Installation 30 IIAR 2010 Technical Paper #3

33 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 13. Single Coil, Single Air Discharge Penthouse Unit being positioned Figure 14. Finished Single Coil, Single Air Discharge Penthouse Unit in Position Technical Paper #3 IIAR

34 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 15. Void under Single Coil, Single Air Discharge Penthouse Unit Figure 16. Hinged Fan in Single Coil, Single Air Discharge Penthouse Unit 32 IIAR 2010 Technical Paper #3

35 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 17. Horizontally Sliding Defrost Door with Automatic Actuator and Limit Switch Figure 18. Dual Air Discharge, Single Coil Penthouse Unit Technical Paper #3 IIAR

36 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 19. Dual Air Discharge, Single Coil Penthouse during Defrost and Maintenance 34 IIAR 2010 Technical Paper #3

37 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 20. Single Coil Alcove Unit Figure 21. Horizontally Sliding Automatic Defrost Air Discharge Door for Alcove Unit Technical Paper #3 IIAR

38 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Figure 22. Horizontally Sliding Automatic Defrost Air Intake Door for Alcove Unit 36 IIAR 2010 Technical Paper #3

39 Evaporator Arrangements for Large Scale Cool and Cold Storage Applications Figure 23. Automatic Warehouse Air Intake Door for Alcove Unit Figure 24. Internal Alcove Unit Door in open Position Technical Paper #3 IIAR

40 2010 IIAR Industrial Refrigeration Conference & Exhibition, San Diego, California Notes: 38 IIAR 2010 Technical Paper #3