Low Temperature Ammonia Refrigeration Food Blast Freezing Application

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We have been asked to provide design and cost estimates for 270TR of -40F ammonia refrigeration for a series of large blast freezer cells. Requirement: Do so with lowest ammonia charge possible (less than 500 pounds in any case, to fall below CalARP inclusion threshold minimum). Four (4) 70TR coils selected to match the load, at 50% ammonia density, hold over 500 pounds each! This is without piping, liquid recirculation unit and high side ammonia charges included. What, please, are our best options to go back to the customer with regarding this challenge?