Primary air and water condensate cooled systems
Shops in town and city heritage districts
by Francesco Gori*
While the installation of air-cooled (VRF) variable refrigerant flow systems is fairly widespread in Italy, the use of water-cooled VRF primary air systems however, as the two cases described below, is something quite uncommon.
Water-cooled VRF systems offer an appreciable potential application in heritage districts, for two reasons: very low noise level and the fact that the equipment is installed indoors. In addition, they make it possible to achieve appreciable coefficient of performance (COP), which results in containing operating costs and contributing to decreasing indirect emissions of greenhouse gases (CO2) in central areas.
The following paragraphs describe two installations using water cooled and heat pump VRF systems, manufactured by Mitsubishi Electric, and recently installed is as many Conad Group supermarkets: The first, in the heart of Florence, and the second in Viareggio, also in a central area.
These installations differ in the fact that they use two different solutions to cool the condensers. In fact, in the Florence store, well water has been used which is then flushed into the sewer system. Whilst at the Viareggio store, there is partial recovery of the heat generated by the food refrigerators to improve performance of the heat pump used in the air conditioning system. The two VRF units are fed by water delivered from the cooling refrigeration unit condenser, before it enters the cooling tower. In this way, the amount of heat supplied to the VRF units and heat pump systems contributes significantly to raising the COP ratio and achieving higher energy efficiency levels.
Another feature that I think is important to point out is the fact that both installation have primary air handling units with direct expansion fan coils connected to the VRF system. This was achieved by the fact that it was possible to connect the Mitsubishi Electric condensation units to suitable direct expansion units as if they were indoor units, with the same control connection features. It is, in my opinion, a most interesting step forward in the possible applications of VRF systems.
Well-water VRF air conditioning systems
The store in the centre of Florence, is arranged on two floors, with storerooms, changing rooms etc.., it covers a total area of just over 400 square meters, with a volume of just over 1,500 cubic meters. The air conditioning system consists of a variable refrigerant flow system for use in both summer and winter. The system is condensed with well water and the condensing unit is installed inside the technical utility room in the basement. The air is exhausted via an existing light well at ground level by means of a mechanical extractor. The indoor units are wall-mounted, each equipped with infrared remote controls to adjust operational modes.
Direct expansion handling unit
The plant is also equipped with two primary air handling units located in the technical utility loft above the retail area. The equipment is equipped with heat exchangers to recover sensitive and latent heat, free-cooling by-pass dampers and an air handling direct expansion fan coil, fed with R407C refrigerant, which is part of the VRF system. The unit is equipped with a fan to exhaust air, an air delivery fan and an electronic handling and control card connected to a data transmission bus. The two units serve one floor each. The air recovery ducts have been fitted with calibration shutters to reduce exhausted airflows and keep the right degree of overpressure in the room.
The primary air delivery and exhaust system in the basement maintains equivalent pressure in the storerooms and very slight overpressure in the remaining zones, including the changing rooms, so that the existing exhaust system is able to achieve equal pressure in all relative zones.
The air intake and exhaust distribution ducts consist of circular rectangle ducts made from galvanised steel. The pair is delivered through dual fin vents.
Condensation circuit with well water
Well water is used for the condensation of the air conditioning system and the food refrigeration system. It has been installed inside the technical utility room; its depth is estimated to be 10 m and it is fitted with a submerged pump which delivers the amounts of water required. The well pump is controlled by a pressure switch installed on the delivery pipe and is equipped with an Inverter control to modulate the flow of water to the system. A minimum operating pressure of at least 250 kPa is maintained on the delivery line, downstream of the sand filter, to guarantee regular filter operations. A separate pipe is used to discharge the condensation water from the refrigeration units and is inserted in the exhaust duct just before the connection to the municipal sewer system, by means of a non-return valve. The condensation system pipe is fitted with an aqueduct suction fixture.
Control and adjustment of systems
The adjustment system foresees the control of air conditioning systems using a timer card that starts and stops the air conditioners, extractors and electric heaters in bathrooms at set times.
To ensure efficient management of indoor units and air handling units which are part of the air conditioning system, we have installed four remote control units, respectively two for the air handling units and two for the indoor units serving the retail area. As far as the management of the latter is concerned, it has been divided into payout desks and sales areas, both with independent centralised controller. The control panels have been wall-mounted. Each unit is supplied with its own infrared remote control for individual management.
VRF air conditioning with water tower
This store in Viareggio, in the central city street Via dei Fratti, is slightly larger than the former, with an area around 570 square meters and a volume of about 1870 cubic meters. The air conditioning system is, once again, a variable refrigerant flow model, with a heat pump and two condensing units to ensure air-conditioning for summer and winter. Unlike the previous example, this system is condensed with tower water and achieves a partial recovery of heat from the refrigeration unit condenser of the cooling system.
Each outdoor unit is connected to four box type indoor units which are 4-way blow, ceiling mounted, an indoor wall-mounted type and a ceiling supported air handling unit.
Direct expansion handling unit
The system has two outdoor primary air handling units connected to the direct expansion unit, like in the case above. The machines are equipped with heat exchangers to recover sensitive and latent heat, free-cooling by-pass dampers and an air handling direct expansion fan coil, fed with R407C refrigerant, which is part of the VRF system. The unit is equipped with a fan to expel air, an air delivery fan and an electronic handling and control card which can be connected to a data transmission bus. The machines are positioned above the ceiling, in the stock and sales area. In particular, the unit named UTA2 serves the fruit and vegetable sales area, with the possibility to adjust the intake air temperature independently.
Manual calibration shutters have been installed on the machine recovery air ducts to reduce the exhaust air flow level and maintain the correct degree of pressure in the store.
Air Distribution
In the fruit and vegetable sales area, the air is delivered by high induction diffusers installed in the suspended ceiling and connected to the air distribution duct by means of double wall insulated hoses.
Both the delivery and exhaust of air is foreseen with equivalent pressure in the environment, both in the meat processing department and the fruit and vegetable area. In winter, the flow of hot air in the meat processing department can be intercepted using a manual shutter fitted on the air duct.
Condensation system with tower water
In order to replace and overhaul the existing condenser system used to cool food and to obtain the condensation fluid for the air conditioning system, the designers decided to install a cooling tower in the basement.
The tower is a centrifugal fan type with a belt drive and pulleys and three-phase dual pole asynchronous motor with a lowered profile and a ducted air delivery and exhaust systems, made it possible to integrate with the existing situation. The total amount of heat exhausted by the tower (THR) is 185 kW, with at an water intake and output temperature of 29/35°C and outdoor air temperature of 24.1° C.B.U. The water flow rate is 7.35 l/s and the air flow rate is 18,000 m3/h.
This type of system generates significant energy savings during the winter. In fact, the heat input from the refrigeration circuit to the condensation circuit is partially offset by the removal of heat from the air conditioning condensing units.
The condensation water network is made of galvanised steel and has been installed entirely within the building. The whole condensation pipe circuit was laid at an altitude lower than the cooling tower return circuit fitting to prevent the circuit from emptying when the plant system is shut down.
Three-way motorised diverter valves have been installed on the condensation circuit motocondensing unit, which shall exclude the same units when the system is stationary. The valves are controlled directly from the motocondensing units via an internal relay. When the system is stationary, the valves divert the flow of condensation water, by-passing the motocondensing unit heat exchangers.
Management System
The control system used for the air conditioning system is a digital electronic type system, consisting of a programmable controller with keyboard and LCD display (G50), which is able to manage the following functions:
Turning on and off the air conditioning system at set times. The function is controlled by the internal clock. As for the adjustment of climate parameters within the retail areas, these are configurable via remote control panels installed in individual rooms. There are also some independent control panels for air-handling units with a sensitive element in the environment;
Setting the water tower functions: on the basis of temperature read on the probes fitted on the water intake and exhaust fixtures, the fan (dual polarity) is activated at Speed 1, Speed 2 or halted. The system will manage the post-heating fan coil installed on the tower exhaust duct when certain outdoor climatic conditions occur, detected by the outdoor temperature probe. Fan coil operations are blocked water tower fan is at a standstill.
Management of the condensate water circuit pump: the operational status of each individual pump is checked, the automatic reversal of operations is realised and, when certain conditions occur, it guarantees that this system will be switched ON and OFF.
* Dott. Ing. Francesco Gori, Studio ESA, Consulenze, Firenze.
Water-cooled COP, environmental quality and noise levels
The choice of condensing water, as an alternative to air, is fundamental, where feasible, from both an energy and environmental standpoint.
At an energy level, the running of the air conditioning and refrigeration systems (as in the case of food supermarkets ) generates significant increases in the COP of machines and, therefore, proportional reductions in energy consumption which lead to lower emissions of CO2 into the atmosphere . All this is accompanied by a proportionate reduction in operating costs, due to lower energy consumption.
Water condensation systems have a number of significant advantages, especially for food refrigeration: the activities described above account for 50/55% of all electricity consumptions.
Water condensation systems, the performance of a compressor on a low temperature (LT ) system can reach COP values of 1.9 to 2.0, instead of the typical values of 0.7 to 0.8 of an air based system. In main units running at normal temperature (NT), the difference is more or less the same: An air condensation system produces a COP of 1.3 - 1.5, while a water system can reach COP values of close to 2.5.
The COPs clearly rise by up to 100%, depending on the circumstances. Moreover, for refrigeration systems which must run continuously round-the-clock, this generates a saving of over half the energy consumptions it would otherwise use. As a result, CO2 emissions are also halved, generating marked drop in environmental impact.
Another positive aspect is the low noise features of the system, especially at night, when the condensers serving the refrigerator cells and counters continue to operate and emit noise into the surrounding environment. Nothing solves this problem better than water condensation systems.