ACUHS : Step 2

Ph.D Work of Jérôme Douzet (2007-2010)

The primary loop:
The condensing unit is connected to the hydrates generator via a refrigerant fluid (R-407C). The hydrates generator crystallizes the aqueous solution of TBAB by recovering the R407C vaporization enthalpy. The refrigerated unit is located outside the building to liquefy R407C at high pressure and ambient temperature.

The secondary loop:
The aqueous solution of TBAB replaces traditional secondary refrigerant fluids. The hydrates generator transforms the aqueous solution of TBAB into a slurry. The slurry is stored in an intermediate tank during the night (when electricity is cheaper). During the day, the storage tank is filled by the way of the pumps of the secondary loop and the slurry is melt in classical heat exchangers located in the room to condition.

Technological description of the installation
oThe condensing unit:
It is located outside the building. The condensing unit is a standard model from LENNOX ®. Its cooling capacity is 29,3kW (Electric-power: 13,8kW). It is classically composed of a scroll compressor and a condensing unit, i.e. a heat exchanger that condenses the R407C fluid at ambient temperature. Then the liquefied R407C fluid is directed to the hydrate generator after passing through an expansion valve.

oThe hydrates generator:
After passing through the expansion valve, the R407C fluid enters into the hydrates generator in which it controls the temperature of plate heat exchangers. The TBAB solution coming from the storage tank can crystallize on the surface of the plates. The hydrates generator is composed of 5 plates, that is to say 6 stages where the slurry forms. Every plate corresponds to a cooling capacity of about 5 to 6 KW. Crystals tend to accumulate on plates and are scrapped by a rotating scrapper. This hydrates generator was supplied to us by the company HeatCraft (France, 69 Genas) and is usually used to produce ice slurries in the refrigeration field (Ben Lakhdar et al., 2002 and Ben Lakhdar et al., 2005). The hydrates generator is 0,75m high and its diameter is about 1m (capacity: 200l). This hydrates generator is also called scraped surface heat exchanger.

oThe tanks:
The storage tank communicates with the hydrates generator. The liquid solution is sucked at the top of the storage tank where only liquid is present. In fact, the hydrate particles have a density higher than the liquid, and they sediment into the tank. Liquid remains at the top of the tank whereas solids accumulate at the bottom. This tank is approximately 2,5m of height and its volume is of about 2m3. It is entirely made of stainless steel. An "heating tank" is also installed in order to heat the hydrates generator if necessary.

oThe secondary circuit:
The secondary flow loop constitutes the distribution network of the slurry. The slurry is sucked from two different levels of the storage tank. The pump P1(volumetric pump) imposes the quantity to flow. Because it sucks the slurry at the bottom of the storage tank, the solid concentration can be important, up to 40-50% in volume. In order to dilute it, a secondary pump P2 sucks the liquid at the top of the storage tank and inject it just before the pump P1. It allows us to control the volume fraction of solid particles in the slurry of the secondary loop at a volume fraction of 20% so that viscosity is moderate.
The secondary flow loop is made of brass pipes of 40 mm (in cooper) of diameter at the exit of P1. The length of this loop is approximately 100m. These pipes will to go through the air-conditioned rooms. Connections of 10 mm of internal diameter are branched to every heat exchanger. A flowmeter (coriolis flowmeter from Micro Motion © F) is installed on this circuit, just before the return of the slurry in the tank.

oThe pumps:
Their roles have been previously explained. We give here some technical characteristics of pumps P1, P2 and P3.
Pump P1: lobes pump from INOXPA type TLS 1-40. Power 1.1 kW. Maximum flow rate: 8m3/h.
Pumps P2 and P3 are identical: centrifugal pumps from INOXPA model ESTAMPINOX EFI 2003, power 0,37 kW. Measured flow rate at 30Hz: 3m3/h.

oThe heat exchangers:
The heat exchangers are standard models from LENNOX ®. They are connected on the secondary flow loop with brass pipes of internal diameter of 10mm, using a little pump installed on each of them. The heat exchangers cool down the air of the room by melting the slurry. 3 floors are conditioned, by the use of 8 heat exchangers (3 on the 1st floor, 2 on the 2nd floor and 2 on the 3rd floor). One of these exchangers is instrumented in order to take measures and to model heat exchanges.

oThe sensors:
- Temperature sensors:
They are PT 100 type. These were screwed together with brass bases, themselves brazed in the copper pipes, in a way that the end of the sensor is located in the pipe.
- Pressure and differential pressure sensors:
Our differential pressure sensors are piezoresistives transmitters made by Keller. They were chosen according to their measuring range in function of the place where they are located, (?P from 500 mbar to 50 bars). The sensors are connected with the piping through brass pipes (ø 10mm) brazed on the circulation pipes, and screwed on the sensors. Differential pressure sensors allow to measure the pressure drop during the slurry's circulation. It allows us to estimate the hydrate fraction in the slurry (Darbouret et al., 2005).
Two absolute pressure sensors are also made by Keller and are fixed to the piping in the same way as the temperature sensors. The sensor located near the storage tank will allow us to check the slurry height, and the other sensor located on the primary loop will be compared to the differential pressure sensors located also on this primary loop.

System control
For practical reasons, the generation of the hydrate slurry is controlled manually and operated during the night. It is to say that we have to operate ourselves and simultaneously the starting up of the different units: refrigeration unit, hydrates generator and pump P3. An optional pump is also installed on the tank in order to mix the slurry from the bottom to the top of the tank.
The distribution of the slurry in the rooms is controlled by end-users. In each room, only one of the heat exchangers is accessible to the users (in the case where there are several heat exchangers in one room). This heat exchangers is called "master heat exchangers" and the others "slave heat exchangers". When an end-user activate one of the master heat exchanger, it activates simultaneously all the slave heat exchangers of the room.
The air conditioning request of one of the rooms activates automatically the activation of the two circulation pumps.
The cooling power of each heat exchanger is controlled by the way of:
-The rotation speed of the internal ventilators (3 positions knob).
-The flow rate of the pumps that are connected to the heat exchangers and connected to the secondary flow loop (3 positions, maximum flow rate: 2m3/h).
Both the internal ventilators and pumps are controlled via a temperature probe in the master heat exchanger.