Cold Link Africa July/August 2019 - Page 26

FEATURE INCORPORATING COLD CHAIN a secondary coolant, limiting the R290 charge to the plant room area. “In Europe, large R290 systems offer a relatively simple alterative for MT applications and it seems set to increase significantly in future,” says La Grange. A concern in our local market is that R22 (an HCFC) is still almost half the price of most HFCs in spite of the fact that it is a refrigerant with an ODP and illegal to use in new installations, says La Grange. One factor seems to be the very low purchase cost of R22 from the manufacturers in the east by refrigerant importers. REGULATIONS Ammonia refrigeration plants are common options for a cold store, but come with strict regulations and requirements in terms of health and safety. The type of product being stored will define the requirements of the refrigeration system. efficient in future (providing this was not originally addressed during design and commissioning),” he says. Large DC or storage facilities have traditionally often been R717 (ammonia) installations, says La Grange. “Ammonia as a refrigerant has few equals.” Ammonia has no ozone depleting potential (ODP) and a global warming potential (GWP) of 0. With its excellent thermodynamic properties, it makes for a very energy efficient solution that has no direct adverse effects on the environment. R717 solutions operating at low temperature are at some disadvantage in terms of COP though, says La Grange. If you use reciprocating compressors on a R717 system, they typically need two-stage compression to operate at low suction conditions, adversely affecting COP. The MT side of the installation could also be done using a secondary coolant. In such cases the secondary coolant (water and glycol mixture) is circulated from the machinery room to the room where the heat absorption is required. With a good design, there is no electrical defrost required for the air-coolers (unlike an evaporator with an expansion device). The saving in coil defrost energy is significant, says La Grange. With a secondary coolant, it is also possible to 26 modulate the cooling capacity of the air cooler effectively. This is done with a three- way “mixing” valve, meaning no defrost is required in many cases. Some large DC/industrial applications favour R717/R744 (CO 2 ) cascade solutions. The R744 refrigerant is a very good alternative for LT applications and the R717 has a good COP at MT conditions, explains La Grange. The R744 systems heat rejection forms part of the R717 system heat load. In these cases, the R717 system is always required to operate for the R744 system to be operational. That is suitable in most cases, but it could be a limiting factor in others. In retail applications, the R717/R744 cascade solution uses a secondary coolant circulated to take care of the MT load, explains La Grange. That way, the R717 system has a low charge and the charge is contained in a machinery room with all the required safety measures in place. With the R744 system being sub-critical, it functions very similar to an HFC system but for the higher operating pressure. A cascade system with hydrocarbons (HC) like R290 and R744 also offers many advantages. In such cases, the R290 system is less costly than the R717 to take care of the MT load, according to La Grange. The MT side could be done with You can’t simply build a cold store whichever way you’d like. There are various regulations that need to be followed to obtain an Occupation Certificate for a building which insurance companies are starting to demand on new build projects, explains Van der Merwe. Some of these regulations include: • SANS 10400 (structural steelwork, building works, building efficiency, and fire engineering) • SANS 10147, SANS 347 (refrigeration installations) • SANS 10142 (electrical installations). There are also various product-dependent regulations, explains Ford. For example, pharmaceuticals and flammable products must adhere to strict health and safety regulations, including that electrical work must have a certain amount of protection to avoid sparks should a malfunction occur. Temperature logging and control is also key to ensure the product doesn’t go off, making temperature monitoring (especially for long term storage) very important. Also, shelving comes with its own regulations, and weight and volume need to be taken into consideration so as not to affect flows and even temperature throughout the cooling space. INSTALLATION DO’S AND DON’TS So, you’ve selected your system – now it’s time for the installation. La Grange says it’s important to look at more than just initial cost. “Before considering your options, apply some business acumen.” What would the expected payback period of the project be? In most cases the project is financed and the operation would need to recover sufficient benefit from the installation in a short enough time period for the project to be financially viable. A more costly installation might save significantly on running cost (energy consumed to operate) though, he says. The ratio of the installation cost vs. operating cost vs. maintenance cost is becoming more widely used to compare installations. Use well-established guidelines like the Total Equivalent Warming Impact (TEWI) to compare alternatives, advises La Grange. This approach factors in direct as well as indirect global warming www.coldlinkafrica.co.za factors. With a lower index total, the installation is also going to have a lower running cost. Should the energy supply be complemented with a renewable option like solar, the TEWI index could be significantly reduced. “Another crucial factor to your project is to ensure your heat load calculations are correct,” says La Grange. “Few things can save or waste more energy than heat load calculations that don’t consider all relevant factors.” For instance, what would the potential payback period be of using better, but more costly insulation? Few consider that. Size also matters – especially when it comes to the refrigeration plant. If you are too conservative and the plant ends up being oversized, you might adversely affect the viability of the project, explains La Grange. Too small and your customer’s room will struggle to reach operating set point during the warmer ambient periods. A system that is oversized for the application might end up being less reliable when it is required to operate a part load conditions in the winter. Compressors are designed to operate for prolonged periods of time, so at part load conditions they could short cycle (stop and start regularly) resulting in a shortened life cycle of a very costly piece of equipment. Before getting started, La Grange also advises that you review the potential payback period of each system and make sure you have facts and figures to back up your project planning. Get the recorded weather data (wet bulb and dry bulb) from the area where you intend to do the installation. That way your condenser or evaporative condenser selection process for the site is informed properly. “Verify your hypothesis with sound facts. Cross-check your work with existing operations if possible. Good case study principles need to be applied.” Van der Merwe emphasises the importance of looking at the total cost of ownership of a refrigeration system with these tips: • Budget for regular maintenance. • Consider installing mobile racking to ensure that the size of cold store is minimised for the pallet holding capacity with the subsequent reduced running costs associated with a smaller cold store. • Be careful of going for the lowest cost solution without considering the possible differences in equipment capacities/specifications and the effect on the running cost. The short- term benefit of a low first cost will be quickly overcome by high running costs and poor plant performance or production or product losses. • Consider the contractor’s track record and the calibre of clients that they serve regularly. Ford advises: • Either you know how to do it or you don’t – don’t overcommit. COLD LINK AFRICA • July/August 2019