16 December 2021
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DESIGN IN DETAIL - SPONSORED BY COOL DESIGNS
By Andy Bradison, specification manager, Cool Designs Ltd
Overview
Electrically-driven options for AHU heating and cooling are now considered more and more, due to the legislation change and direction towards net zero carbon.
Utilising direct expansion (DX) on AHU coils allows for a simple, efficient and cost-effective heating and cooling method.
Heat pump (Based on Toshiba)
Heat pumps use the AHU coils part of their own refrigeration circuit, delivering either hot gas when the AHU DX coil becomes the condenser during heating requirement, or, when the cycle is reversed, delivering cool suction gas when the AHU DX coil becomes the evaporator (or cooler). It's all about control.
The air-cooled heat pump contains the refrigerant pump, the compressor. It also contains one of the heat exchanger coils. This coil is the condenser whilst cooling and the evaporator whilst heating. The unit has a reversing valve (heat or cool) so, by utilising the heat exchanger (DX) coil in the AHU we have our main refrigeration circuit components.
Controlling the air
The compressor has inverter drive. Inverter is controllable. Until recently the supply duct temperature was difficult to control as the available system controls at the time relied on return air/room air and adjusted to suit.
With the advent of the FDP3 `gismo from the gods` the inverter compressor (and therefore quantity of heating or cooling) can be controlled from a 0-10v signal.
The RBC AHU1 control box is mounted at the AHU Coil and contains all necessary boards etc for the operation of the refrigeration circuit. The FDP3 0-10v gismo sits within the panel. It can also be supplied loose for fitting in the BMS panel or other. One RBC AHU1 control box and one FDP3 is needed per refrigerant circuit
Multiple circuits
To achieve the heating and cooling duty, and to have an element of resilience, more than one refrigerant circuit is often installed.
The first example below shows two heat pumps serving a single AHU coil. Note the two RBC control boxes mounted on the AHU, at the coil. They need to be in close proximity as sensors lead from this control box to the DX coil itself, ensuring correct operation.
The second example below shows five `twin-fan` heat pumps serving a single AHU coil for a hospital project. The two smaller units on the right are serving cassette split systems.
Duty and number of circuits
The heat pumps are from the Digital and Super Digital range of heat pumps from Toshiba. Nominal duty per unit ranges from 5kW to 25kW approx.
The second example above, with the five units, had a DX coil requirement of 120kW. Five units were supplied, with five RBC AHU1 control boxes, each with an FDP3 gismo within.
Efficiency
- Cooling EER between 2.7 and 3.7 over the range
- Heating COP between 3.4 and 4.1 over the range
Summary
Efficient operation and simplistic approach allow for many applications of AHU DX using heat pumps.
Seamless 0-10v inverter control on a wide range of heating and cooling loads, at AHU coils, can be achieved using multi refrigerant circuits.
