Optimization of hot-gas defrost in industrial refrigeration systems
This project examines the possibility for optimizing hot-gas defrost in industrial refrigeration systems. The focus is on optimizing the design of the system and also adding a compressor for production of hot-gas. The purpose is to minimize the losses during defrost and to lower the condensing temperature for the rest of the system. The expectation is that an overall yearly saving of 14% can be accomplished.
The evaporation temperature – and thereby the evaporator surface temperature – in refrigeration plants operating below 5-8 °C will be below 0 °C. This will lead to frost formation on the evaporator surfaces and periodic defrosting is necessary to maintain the cooling capacity. Hot-gas defrosting is commonly used in large industrial refrigeration plants like blast freezing tunnels. To get a high temperature and pressure of the hot gas used for defrosting many plants run with a relatively high minimum condensing temperature. This leads to unnecessary energy consumption since the air temperature in Denmark in most part of the year is low enough to decrease the condensing pressure further. This project will explore the possibility for reducing the condensing pressure by optimizing the design of the evaporator, system and control strate-gy but also by introducing a serial compressor for defrosting, so that only the necessary amount of hot gas is lifted to the desired temperature level, allowing the rest of the plant to run at a lower condensing temperature. This is expected to lower the total yearly energy consump-tion of the system by about 14 %.
Industrial refrigeration systems running with ammonia as refrigerant often run with evaporator temperatures below 0 °C. The ice that forms on the evaporator surface is typically removed with a hot gas defrost process. This project investigated two different methods to control the hot gas defrost experimentally and numerically.
The first method, the pressure control method, operates with a fixed evaporator pressure throughout the defrost cycle. It accumulates liquid refrigerant in the beginning of the defrost process and allows gas flow through the evaporator during the whole operating period. The second method is based on draining the evaporator for liquid refrigerant instead. This reduces the initial pressure gradients and minimizes the hot gas consumption upon completion of the ice removal.
An experimental campaign was carried out simulating the operation of a flooded ammonia evaporator and the associated hot gas defrost system. These measurements quantify the differences in energy consumption for the two defrost methods and deliver valuable information for the sizing of defrost systems.
The measured evaporator pressure and mass flow rates were used to validate a dynamic model of the evaporator and the connected valves and pipes. The calibrated model could generate new insights like the impact of the hot gas pressure on the defrost time and the accumulated mass of refrigerant inside the evaporator, which is much lower for the liquid drain method.
The self-adjusting nature of the liquid drain method was found to save 15% of energy per defrost operation compared to a pressure regulated defrost cycle. The savings increase with increasing defrost time since the pressure control method consumes more energy after removal of the ice. For an increased defrost duration of 30 min instead of 20 min, this leads to 30% energy savings for the liquid drain method.
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Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Teknologisk Institut | ||
| Danfoss A/S | ||
| JOHNSON CONTROLS DENMARK ApS | ||
| Claus Sørensen A/S | ||
| Ringsted Slagteri |
