Mike Briskman of LG Air Conditioning discusses the pitfalls of ventilation when the only place for the condenser to be sited is in a confined space
THERE are situations where the only viable location for condensers is within an enclosed space - the plant room.

This applies when an outdoor condenser may interfere with the appearance of the building or the neighbourhood. In other cases the occupier does not own the floor above or below the premises, and running pipes to the roof or to ground level may not be viable.

The plant room has to be well ventilated; otherwise the temperature would rise to the point where the air conditioner would first loose efficiency, and then fail to operate. In most cases it is necessary to introduce forced ventilation to achieve the required flow rates.

The ventilation rates can be very high. For example a 10kW condenser may typically have an air flow rate of 55m3/min. If placed in a 3x3x2.5m room, this would be equivalent to 150 air changes per hour. The ventilation rate of the room may need to be even higher if some of the air leaves the room without passing through the condenser.

It is important that by-passing (short circuiting) air is minimised by careful location of the ventilation inlet and outlet, and possibly by the use of baffles.

Fans built into condenser units change speed to regulate the flow according to load and ambient temperature. It is therefore essential to ensure the ventilation system does not force air through the condenser to the extent that the effect of internal fan speed control is negated.

More powerful

An alternative method is to duct each condenser to the outside. The condenser fan is not normally designed to cope with additional static pressure, and it therefore needs to be either replaced by a more powerful unit, or supplemented by an additional fan.

It can only be replaced if it can be ascertained that the fan speed control system within the condenser can handle the higher electric current.

If an added fan is used, then its speed must be regulated to match the condenser running conditions. One way is to use a proprietary fan speed controller which senses the condensing temperature, and adjusts the fan speed to maintain it at the required level (typically 450C-500C). (In the heating mode the fan speed controller needs to be disabled, and the added fan must stop when the condenser fan stops).

The LG Multi V Space is a VRF system where the condenser is specifically designed for indoor locations.

The inverter-driven fan can cope with up to 150 Pa of external pressure, which allows considerable flexibility in the indoor location of the unit. Indoor noise level is low, so that it can be sited in a plant room partitioned off from the office space.

When fitted against an outside wall, all maintenance can be carried out from the inside, making it suitable for high rise buildings. Condensers are available in cooling capacities of 14kW and 28kW. This product can avoid the design complexities and subsequent fine tuning associated with indoor locations.

Good practice in plant room location therefore involves the following:

· Conventional equipment is fitted into a ventilated plant room

· The ventilation air flow rate should exceed the sum of the flow rates through the condensers located in the room

· The design should ensure that the amount of air likely to by-pass condensers is minimised, and at the same time air is not forced through condensers in quantities that may negate the effect of the fan speed controller.

· Tests during commissioning should include a comparison between the temperature of the air entering condensers and the outdoor ambient temperature. Any rise in temperature would reduce efficiency and reduce capacity. A significant rise may stop the units from operating on hotter days. For example, if the design limit of the condenser is 400C, and the measured rise is 100C above ambient, then the units would exceed the design limit at outdoor temperatures above 300C.

· Pricing should allow for the additional design and commissioning work involved.

· Direct ducting of the condenser to outside
· The fan supplied with the condenser is unlikely to work against the additional pressure.

· The original equipment fan may be replaced by a more powerful unit only if it is known that the condenser system can deal with the higher currents.

· If an auxiliary fan is used, then its speed needs to be controlled to maintain the required condensing temperature. It may not be suitable for some multi-split systems.

Dedicated plant room system

A system designed specifically for indoor locations avoids the need for tailor made designs and additional fine tuning during commissioning. It can therefore provide a cost-effective solution where its specifications are suitable for the contract. The LG Multi V Space is specifically designed for this purpose, and it has the added advantage of low noise, making it suitable for siting in enclosures partitioned off from an office space or a living area. In high rise buildings there is no need for outdoor access for maintenance.

www.lge.com