2500kw Ammonia / Glycol plant

Our client made the decision to replace their central ammonia system in 2017 and Seward Refrigeration Ltd were invited to present our company and were interviewed by sites project management team. Seward Refrigeration were successful and made the tender list for the new installation project, taking our design and sales team approximately 4 months to complete a number of variations and options in line with the tender specification document.

Seward Refrigeration were then selected as the chosen refrigeration contractor for the new installation project, following a further two months of variations and options. Due to our clients budget, many plant items were removed from the original scope of works, including standby HT compressors, automatic ammonia purging system, additional low temperature glycol circuit and additional evaporative condenser to name a few.

Reducing energy consumption is always our primary objective, and our first choice will always be natural refrigerants for any new installation project. Using Ammonia as the primary refrigerant for a project of this stature was always going to be our first choice. Ammonia has no effect on global warming, is efficient, cost effective and green, a perfect option as we move forward into the future.

The refrigeration system we proposed and installed was a 2500kw two stage ammonia / glycol system (Coolflow DTX), which would be installed over two phases. The low stage feeds direct pumped ammonia to a cold store, with the high stage feeding a gravity fed intercooler vessel / plate heat exchangers, evaporating at -16.5ºc, supplying glycol at -15ºc.

The low stage plant consists of two inverter driven Grasso V300 reciprocating compressors (Run and standby) and LT pumped surge vessel (Run and standby pumps), feeding two ceiling mounted draw through evaporators in a cold store, operating at -20 degrees c. The high stage plant consists of two inverter driven, Grasso SP1 screw compressors, a gravity fed intercooler vessel complete with 3 x plate heat exchangers, 1 x evaporative condenser, HP float and 3 x inverter driven KSB glycol pumps (2 x run, 1 x standby). The cold glycol circuit feeds two AHU’s, evaporators in holding chill & production areas, a large despatch chill, all operating between +2/+5 degrees c, including blast chillers. In total there were 45 evaporators installed and all manufactured by Coolers & Condensers. All coolers feeding chilled areas were installed with EC fans, providing energy savings during low load conditions.  

To reject the total THR of the compressors, we installed a BAC evaporative condenser, condensing at +32 degrees c / +21.5 degrees c wet bulb. The condenser was  equipped with fan speed inverters, wet bulb sensor and electronic control system that automatically floats the discharge pressure dependant on the ambient wet bulb temperature and discharge pressure. This technique ensures that the HT compressors in operation will absorb the minimum amount of power throughout the fluctuating range of the discharge pressure. The condenser was coated with a Baltibond hybrid coating for protection against a harsh costal environment, and fitted with an access ladder and walkway to facilitate access to the eliminators and spray nozzles for cleaning. Motor davits were also installed for the ease of removing fan motors, if required in the future. The condenser also came equipped with a split cooling circuit for compressor oil cooling via an independent glycol circuit.

The defrosting method for all glycol coolers is achieved via a warm glycol ring main, consisting of two inverter driven, suction flooded KSB glycol pumps, (Run and standby) reclaiming free heat from a discharge line partial desuperheater / condenser. The warm glycol circuit is controlled at a setpoint of +20 degrees c via a modulating three port valve, providing a very efficient defrosting process. Both cold store evaporators are defrosted via hot gas supplied by the main discharge line fed from the two HT screw compressors.

The refrigeration duty for the 1st phase totals 1700kw, with a further 800kw accounted for in the design to cater for future cooling capacity. To provide the future cooling duty, an additional HT screw compressor, evaporative condenser, HP float and cold glycol pump will be required, taking the total plant duty to 2500kw.

The four Grasso compressors are all controlled via independent OMNI controllers, controlled in sequence, and all parameters are networked to the main system PLC, providing historical data and graphs for monitoring purposes. To control and monitor the plant we installed an RDM PLC, complete with Data manager. All roof void control panels, controlling the glycol evaporators each have independent HMI screens and all RDM TDB controllers are networked back to the central RDM Data manger in the switch room. An RDM 3G modem was installed allowing our staff to monitor the system remotely if required and from our offices we can check set points, reset alarms and determine if a service call is required.

We installed a TQ leak detection system for the plant room and Coldstore, four ATEX rated extract fans, each inverter controlled, and extract hoods above each cold store ammonia valve station. A ventilation system supplies ambient fresh air in the plant room and a pressurised ventilation system / split air conditioning system was installed for the switch room.

To conclude, there was over 4500 meters of pipework installed and the installation project took 14 months to complete. Each evaporator fed from the pumped ammonia / glycol circuits was introduced to the system one by one, whilst working around sites live production facility. Seward Refrigeration offered our client the best compromise, between capital investment, running costs, maintenance and environmental impact with the new low charge ammonia / secondary glycol system. Our customer was pleased to have their existing, high charged ammonia plant finally replaced. Not only has the new system provided our client with peace of mind, they’re delighted with the performance, in particular with the blast chills achieving ideal core temperatures in 2-3 hrs (dependant on product type), surpassing the performance of the existing blast chillers.

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