16 December 2021
David Cooper, design manager at refrigeration contractor SURE Solutions, describes the satisfaction of being able to provide the answers to complex questions.
As a system designer, it is always rewarding to be involved with challenging projects. Refrigeration is often one of the last things to be considered when a new factory plan is presented. It starts with a definition of what the site is aiming to achieve and what success looks like to set the scene, no matter the scale. Then there is a list of criteria, wants, needs, expectations and so on. This quickly sets the boundaries to work within.
One of our recent projects, currently in the construction phase, was at a poultry processing facility for Cranswick. It presented plenty of challenges and, on reflection, ended up being a dream job from a design perspective.
The operational temperatures ranged between -43°C and +125°C and the system had a heating/cooling capacity in excess of 3MW with dynamic operational temperatures. The factory was designed to process for 20 to 24 hours per day all year round, resulting in long production shift patterns requiring zero downtime, especially on various freezing processes.
Close control process temperatures required consistent water at +1°C, while delicate production machinery susceptible to low velocity cold air meant managing warm humid air; ensuring condensation was not present in chilled areas. Additionally, no site services were permitted using synthetic refrigerant.
Over time the cooling and heating loads were determined and subsequently there was a requirement to have dual temperature operation in several areas. Depending on the production pattern, demand for any one of these areas could change. The system had to adapt and do so efficiently.
This had to be worked back into the plantroom to ensure the system was designed to accommodate a change in operational condition. The temperature swing was from evaporating temperature of -40°C to -10°C. When doing this on smaller loads it is most often achieved by regulating the evaporating pressure, although this would mean running higher temperatures inefficiently at a lower compressor suction set point. Once this was reviewed, it became clear that independent compressors and infrastructure for both suction set points would offer optimum efficiency.
The factory was designed to operate for long shift patterns. To achieve this, various critical areas needed special attention to enable long running hours. This resulted in sequential defrosting on one large spiral freezer with coil isolation to close the air flow circuit. Air coolers were designed with adequate coil frost thickness allowances, and the quantity of units selected permitted cooler downtime when defrosting. The inclusion of glycol mixing valve stations resulted in higher medium temperatures to prolong the requirement for defrosting.
The site had a considerable hot water demand for hygiene and processing purposes, anything up to 200m3 per day, with peak demand over three hours of 60m3. The design of this system was to recover any surplus heat from the refrigeration plant and then boost the temperature up using an ammonia heat pump. The intention was to set a design flow rate to ensure that all users had hot water at any given time and excess hot water would be filling up an insulated buffer vessel preparing for the hygiene window peak demand. When considering heat recovery from a refrigeration system, the load profile must be established and done so conservatively. It needs to be plotted out over a period of time to get your datum point for worse case heat recovery opportunities.
Commonly when considering heat pumps for hot water generation, the main refrigeration system is the greater power user; consequently, the aim should always be for the refrigeration system to do the least work possible. If there is too great a reliance of heat recovery and the refrigeration plant runs ‘harder’, then efficiency drops off very quickly. This was also an important aspect to the system design. Further to this, any low loads had to be evaluated to ensure that higher grade heat was available for recovering consistently. The heat pump would work harder when the refrigeration plants head pressure was optimised, when considering that the power absorbed by the refrigeration plant far exceeded that by the heat pump it was reasonable to design the heat pump compressor with a suitable differential head pressure to operate.
Heat was recovered from compressor oil cooling secondary circuit which operated at +48°C. This was used for warm glycol defrosting at lower mixed temperatures and also to preheat mains water up to +32°C and doing so equally at low compressor capacity loads. Mains water would then be supplied to the cascade ammonia heat pump unit, which boosted the water temperature up to +65°C. The heat pump was designed to maximise the heat recovery potential, there were three heat exchangers to achieve this: desuperheater, condenser and a subcooler.
The additional investment was financially appraised to demonstrate the concept had a respectable return. The gas usage using an industrial boiler to generate 197,000 litres/day equated to 12,610 kWh/day. At £0.03/kWh, this would have cost £378/day for gas. On the other hand, when considering the load profile of the main refrigeration plant the heat pump solution worked out at an average cost of £131/day. This demonstrated less than a 3-year payback on additional investment for the site, whilst reducing carbon footprint by around 1,774kg CO₂e per day.
There was an additional demand for higher grade heat for the office air conditioning when in heating mode, requiring water up to +55°C. A desuperheater in the main refrigeration system with a barrier closed circuit was designed within the system.
Upon reflection, this project was a designer’s dream job. It had everything and involved working with a forward-thinking customer who wanted to advance in technology whilst considering and minimising the environmental impacts of the operation.