The use of full fresh air free-cooling dispenses with the need for mechanical DX cooling or water/glycol-based cooling. This provides an advantage where there is a desire to eliminate the presence of refrigerants, preferring to maximise on natural cooling methods.

The general arrangement for such equipment is shown in Fig 1.

Fig 1:Full fresh air free-cooling unit

These units simply comprise the following key components and are available in upflow or downflow configuration:
· Fresh air damper
· Sound attenuator section
· High efficiency air filters
· Plug fans
· Insulated and sound attenuated casing

There are two modes of operation:

1) Recirculation mode during start-up (fig 2)

Fig 2: Full circulation

On start up, a fan is energised and the outside damper is driven to 50% which enables the outside air to be monitored to ensure the rate of change of temperature within the room is less than 0.5ºC/min averaged over five minutes before opening fully. In downflow configuration the re-circulated room air is drawn through the grilles in the top of the unit. Fresh air is drawn in from the back of the unit and mixes with the re-circulated air, which then passes into the room via the floor void.

2) Normal operation (fig 3)

Fig 3: Full fresh air

The outside air damper is motored fully open to allow full fresh air to be drawn through the louvre and discharged into the room via the floor void. Room exhaust air is then expelled via pressure relief dampers / flaps which are external to the unit. This alleviates over-pressurisation of the room and expels accordingly to maintain the desired room pressure. These are weighted dampers, which can be set to develop a specified room pressure.

Since the fresh air temperature is subject to wide variation over a 24-hour period and across seasonal variations, it is essential to vary the amount of fresh air being supplied to the conditioned space in order to maintain the setpoint space temperature. The plug fans are fed via multi-tap transformers which allow them to be run in low or high speed mode via the controller.

The unit is controlled by a remote room temperature sensor, which monitors the next-generation switch room condition and a fresh air temperature sensor mounted in the rear of the unit. The combination of both sensors operates the fresh air damper and sequences the fan stages on or off.

Fan operation is as follows:
Plug fan 1 Speed Plug fan 2 Speed Stage 1 On Low Off Off Stage 2 On High Off Off Stage 3 On High On Low Stage 4 On High On High

The unit controller has a sequence operation to ensure equal fan run hours. These packaged close control units are designed to 'clip on' to exterior walls of containers or Portacabins containing telecommunications switch equipment. These units comprise full free cooling, incremental and full mechanical DX cooling capability and function in generally the same way as fresh air with supplementary DX cooling units described above.

The three modes of operation are shown in figs 4, 5 and 6.

Fig 4: Full free cooling

Fig 5: Incremental

Fig 6: Full mechanical DX cooling

 

With thanks to Mike Creamer of Business Edge who revisits his Masterclass series of articles, updating and adding to the information which proved so useful to readers when the series was first published over ten years ago. In this reincarnation, the series will cover both air conditioning and refrigeration and serve as an on-going source of technical reference for experienced personnel as well as providing a solid educational grounding for newcomers to our industry.