01 November 2021
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DESIGN IN DETAIL - SPONSORED BY COOL DESIGNS
Design
Most hotel bedrooms that are air conditioned have a single ducted fan coil unit within a dropped bulkhead supplying air through a grille.
The selection is made on sensible duty (sensible duty is king) with corrections for room and ambient temperatures, summer and winter, and refrigerant piping lengths. Heat recovery VRF is used mostly.
Whilst we are careful to ensure the temperature in the space is as design, we need of course to think about sound levels.
The design needs a fixed point. We settle on a fan coil unit that is capable of the calculated sensible duty (corrected at steady state – all units running) with a set airflow, often based on medium speed or high speed and we cross reference the sound curves.
Flow selector box
The room remote controller instructs the fan coil unit, and its flow selector box, to heat or cool the space depending on the requirements of the guest. In very basic terms the flow selector box diverts either hot or cold medium to the fan coil unit.
The flow selector box can make a small `whooshing` noise whilst changing, which, although no issue in offices, can cause a small concern in a hotel bedroom the dead of night. These therefore are always mounted outside the room in either the corridor or riser.
Published data and site readings
The mass-produced fan coil unit is supplied with sound data logged, at design and factory testing. It is then installed in a myriad of applications with hard or soft surfaces/areas, fabric or other noise generators have an effect in the room. Three main factors, external to the unit, often require focus.
1. The mechanical `attachments'. Air discharge grilles are usually not supplied by the manufacturer, nor the duct links from the unit to the grille. Grille selection will have bearing on the resultant room noise level.
2. The `fabric` of the room itself. Soft surfaces absorb noise and of course hard surfaces the opposite. For example, in a recent commissioning test, 1dbA was improved by removing the shipping plastic from the mattrass in the new room.
3. Noise pollution external to the room. Perhaps other guests or road noise can tip the scales above the required levels.
The published fan coil unit noise ratings are of course at the unit, but readings are often required at the `bed head'.
Filter effect to sound and duty
The cleanliness of the filter has a huge effect upon all the desired design requirements and commissioned data. mcp∆T gives us the cooling duty based on the Airflow set (m). Dirty filter has the effect of both reducing this mass flow rate and increasing noise levels. `Double whammy'.
Example airflow results from the test shown in the images had airflow as follows:
Summary
Whilst within the initial design and flow box location we meet the building design criteria, the impact of mechanical services and ongoing maintenance is key.
