Design and specification of refrigeration and air conditioning systems
Start with the load
A credible design starts from a transparent cooling-load calculation: transmission, product and respiration load, infiltration/air changes, internal gains (people, lights, motors, fork-lifts), fan and defrost allowance, plus a modest safety margin. Candidates must be able to explain psychrometric processes - sensible and latent heat, apparatus dew point, room RH - and why oversizing gives poor dehumidification, short cycling and a low part-load efficiency.
Temperature difference and efficiency
Choosing the design temperature difference (TD) is a commercial as well as a technical decision. A large TD means smaller, cheaper coils but a lower evaporating temperature, a higher pressure ratio, more defrost and lower COP. Comfort cooling typically uses 8–10 K air-on to evaporating; chilled water coils 5–6 K approach; high-humidity cold stores use 5–6 K to hold RH and limit product weight loss.
Selection, standards and specification
Specify to EN 378 (charge limits, safety groups, ventilation), F-Gas rules, Ecodesign/energy labelling, and any client or BREEAM requirement. Cover: refrigerant choice and GWP, pipe sizing and oil return, oil management, defrost method, redundancy (N+1), part-load performance, noise, controls and BMS interface, access for maintenance, and the commissioning and handover requirements written into the specification.
Worked example
A produce store at +2 °C designed with a 6 K TD runs at an evaporating temperature of −4 °C. With a 32 °C ambient and a 12 K condenser approach, condensing is 44 °C. Raising the evaporating temperature by 1 K typically improves COP by roughly 2–4%, so a 2 K reduction in TD is a real energy saving - but watch RH and coil cost.
Traps
Confusing TD with approach; forgetting fan and defrost heat; ignoring part-load operation; specifying a refrigerant without checking its GWP phase-down position.