Harris Environmental - Design Guide page 5

Refrigeration Design

Refrigeration Design Refrigeration systems for environmental rooms are profoundly different than systems adequate for most cold storage applications. These differences are apparent in four areas: response to transient loads, controls, noise, and safety features to protect the contents of an environmental room.

Response to transient loads
Since environmental room loads are not constant, the cooling system must rapidly remove peak loads while maintaining control. This is accomplished by running the refrigeration compressors constantly.

At low-load, the refrigerant gas bypasses the air cooler, but continues to circulate through the compressor and condenser. When loads suddenly increase in the room, the refrigerant gas is instantly metered to the cooling coil in proportion to the increase in load. In storage rooms, this is not an issue--the refrigeration system simply turns on and off when the room temperature goes above the setpoint, resulting in relatively large temperature differences throughout the room. When fast response is necessary, designers typically specify continuous compressor operation with “hot-gas bypass” capacity control.

Refrigeration controls
There are three common methods of controlling the operation of refrigeration compressors.

• • On-off based on suction pressure
Cold storage rooms use this method. The compressor is cycled on and off based on the refrigerant pressure in the cooling coil. The system responds slowly, and a narrow-range chart recorder will show the “saw-tooth” pattern generally acceptable in storage areas. It is the least expensive form of control.

• • On-off based on air temperature
Environmental rooms with specified uniformity greater than ±0.5ºC and gradients larger than 1.0ºC often turn compressors on and off in response to air temperature in the room. The time constant of the system is shorter than with suction pressure control, so heat load changes can be dealt with more quickly. Older high-quality environmental rooms used this control with reasonably good results.

• • Time-proportioning PID control
Formerly used only in the most sophisticated industrial processes, PID control is now nearly the same cost as on-off air temperature controls. (Less than $250 difference)
Microprocessors calculate the rate of temperature change, and cycle the refrigeration equipment in very small time increments (less than one second). This provides exceptionally close temperature control--well within the classic 0.5ºC uniformity and 1.0ºC gradient as long as the equipment has enough cooling capacity and air has been properly distributed throughout the room.

Noise
In storage rooms, noise is not generally an issue, so the complete refrigeration system is often mounted on the room wall. This design is usually too noisy for environmental rooms. A typical cold storage room system generates 90 decibels at a distance of one foot from the fan. In contrast, an environmental room system seldom generates more than 70 dBA.

The decibel scale is logarithmic--an increase of 10 decibels represents a ten-fold increase in sound-power. In other words, the cold storage room system generates about 100 times more sound power than a well-designed environmental room. Normal human conversation generates about 70 dBA--shouting generates about 90 dBA.

The designer can ensure a quiet room through specifying four features: remote-mounted, vibration-isolated compressors, low speed fans and manual-override fan speed controls.

Placing the compressors in a remote mechanical room moves the noise away from the room, and vibration isolators reduce their structure-borne noise.

Specifying fan speed to be no greater than 1140 rpm reduces the noise produced by the fan tips, which travel through the air much faster than the motor shaft.

Manual-override fan speed control allows the room occupant to further reduce fan speed. A lower speed--less air--can be perfectly acceptable when the sensible heat loads in the room are below the maximums used for system design.

Inexpensive safety components
Often the designer must be concerned with the durability of the refrigeration system since the room temperature will rise if the system is down for repairs. Several inexpensive system components can be specified to increase system reliability.

• • Refrigeration filter-driers
Inside the refrigerant piping, dirt and moisture eventually corrodes the system and damages components. A small filter-drier removes this contamination and can double the life of a compressor.

• • Suction line accumulator
The compressor is a gas pump--not a liquid pump. Since liquid cannot be compressed, it can burst the compressor seals or break the connecting rods on the piston when it enters the compressor. A suction line accumulator is a small tank located in the suction line--the piping between the cooling coil and the compressor. It prevents liquid refrigerant that may have bypassed the cooling coil from being pulled into the compressor. Liquid often bypasses the coil when loads fall rapidly, preventing all the refrigerant from evaporating to a gas inside the coil. If a system is constantly losing compressors, this is often the reason. The problem is prevent with a low-cost suction line accumulator.

• • Multiple independent sensors
In cold storage rooms, a single temperature sensor controls the refrigeration system, the temperature recorder and alarms when these are provided. This has the advantage of low cost and avoiding confusion between sensors.

Environmental rooms, however, are not normally specified with such risk in sensors. If a single sensor fails or moves out of calibration, the system does not control, the alarm system is ignorant of the problem and the chart recorder provides a false sense of security. Generally, environmental rooms are specified to contain separate, independent sensors for temperature control, recorders and alarms. This minimizes the consequences of sensor or instrument malfunctions.