We present an interesting case of integration of the air conditioning system with the refrigeration system carried out in a supermarket.
by Ing. Ernesto Sanguinetti R. - Ing. José Lescano D.- COLD IMPORT SA - Lima - PERU *
Initially the markets had self-contained refrigeration equipment in each refrigerated cabinet where perishable food products were displayed and sold to the public. Having the self-contained equipment means that the complete equipment is part of the furniture and that the heat that evacuates the condenser is delivered directly to the environment of the place where the public makes the purchases.
Subsequently with the increase of the population and the demand of foods the Supermarkets and Hypermarkets appear with the consequent increase of the dimensions and the quantity of refrigerated furniture. The use of refrigerated furniture with self-contained equipment had to be discarded because the heat evacuated by each condenser would significantly influence the thermal load of the furniture itself and in the comfort of customers by raising the temperature of the room.
We opted for the most logical thing: that each refrigerated unit keep its Evaporator or Evaporator Unit and its Thermostatic Expansion Valve inside each piece of furniture, but that the Condensing Unit formed by the compressor and the condenser is installed in an off-site engine room , making the connection between them by means of copper pipes. This machine room could be located on the first floor or on a second floor but outside the sales area, which made it possible for the condensers to be better ventilated and the heat they evacuate goes directly to the outside atmosphere.
Large pipeline lengths and greater amount of coolant began to be used, but the advantages were achieved. To store larger quantities and to replace what was sold in the refrigerated furniture of the stores began to use cold rooms and also process rooms in the back rooms. Always with its own complete refrigeration equipment: Unit Evaporator or evaporator inside the chamber and Condensing Unit in the Machine Room.
For many years a Condensing Unit was used for each refrigerated cabinet and cold room, having therefore a large number of Condensing Units in the engine rooms.
Over time, several problems were observed:
- When the ambient humidity inside the shop or shop was high, all refrigerated furniture had to inevitably draw moisture from the environment, so those above 0 ° C had to drain large amounts of condensed water (they had to remove heat sensitive) and those that were at 0 ° C and less freezing moisture on the evaporator coils (they had to remove sensitive heat and additional latent heat) which in turn caused increased thaw cycles. In both cases there was a higher energy consumption to maintain each design condition because the thermal load of each piece of furniture had to add the thermal load to condense and / or freeze the ambient air from the store.
- Condensation was also observed on the surfaces of the refrigerated furniture and that the water droplets that were formed presented an unpleasant aspect to the view, then when they slipped and reached the floors near each piece of furniture they showed a "wet" floor also with an unpleasant aspect but with the aggravation of being an imminent danger that the customers or the employees of the establishment could slip and / or fall, in addition, there was detected the appearance of bad odors under the furniture and premature oxidation of metallic surfaces.
-Additionally it was observed that the glass covers and doors were fogged with the condensation of the humidity, layers of ice formed on the surface of the frozen products or on its wrapping or its packaging. Another unpleasant aspect to the sight and touch of the customers.
All of the above prompted the search for a solution and the best solution was to install air conditioning systems to air conditioning the entire commercial premises because it achieves two things:
- Check the temperature and humidity of the air to minimize and, if possible, avoid the problems mentioned above.
-Becind to customers a pleasant environment whose permanence to these conditions is instinctively longer (especially in summer) and invites to make more purchases.
It soon became necessary to analyze the consumption of electric energy in a supermarket and found that the distribution was approximately the following (see graph 1):
Installing parallel compressors
After the previous analysis, we entered the stage of seeking the improvement of energy consumption to lower those percentages or to optimize consumption. We entered the stage of eliminating the Condensation Units for each refrigerated cabinet. In place of several Condensing Units, parallel compressors were started, divided into medium temperature compressors called the "medium temperature group" (for cold storage units and chambers of fresh produce) and low temperature compressors is called a "low temperature group" (for furniture and chambers for preserving frozen products).
This set of parallel compressors began to be called a "rack of compressors" that were mounted on a single metal structure or chassis that is located in a smaller machine room and that all unloaded to a single air cooled condenser is installed on the roof of the Supermarket or Hypermarket.
With the use of parallel compressors, the efficiency of the food cold system in supermarkets was improved because, by modulating its operation, we achieved that the demand of the system adjusted to the cold demand, for which strategies were applied using compressors with unloaders, then speed variators and digital compressors. Hand in hand with these technologies was the development of electronic controllers that allowed to apply logic of operation achieving significant energy savings.
As mentioned, in a supermarket the refrigeration and air conditioning systems represent approximately 70% of the total consumption of electrical energy, this leads us to continuously search for improved systems. One option is to integrate the air conditioning system with the cooling system and this can be achieved by developing the following designs:
1. A central unit with a rack of compressors that have a single discharge to a condenser but have three independent "suction temperature groups": Air Conditioning, Medium Temperature and Low Temperature.
Advantages: Lower cost of installation and operation because we centralize the refrigeration and air conditioning systems in a machine room.
Disadvantages: Having a common discharge means using only one refrigerant for all three suction groups. R134a refrigerant can be used for high and medium temperature groups, however, it is not efficient to use it for the low temperature group. In contrast, if we use R449 or R507 have good performance in low temperature, but not so in high and medium temperature.
2. Separate and separate the air conditioning system (Central Air Conditioning with parallel compressors) and take advantage of the coolant of its circuit to subcool the refrigerant liquid of the "medium and low temperature group" of the cooling system of Cold Food), or if we have an air conditioning system with "ice water", we can take advantage of the water, also to subcool the cooling liquid of the cold food system. In the graph 2 can be observed schematically both systems:
- We take advantage of the air conditioning system which has a higher evaporation temperature to subcool the cooling liquid of the refrigeration system, which increases the cooling capacity (we improve the cooling effect in the thermodynamic cycle) by which the compressors perform less work and this allows us to select smaller capacity compressors.
- It improves the coefficient of performance COP of the refrigerating system as it achieves a saving of energy when consuming less electrical energy to operate the compressors.
- With refrigerant liquid reaching lower temperature to the expansion valve we also reduced its size.
- Lower cost of installation and operation since both plants will be located in the same engine room. Reducing spaces, we have greater ease and access for system assistance (preventive maintenance, corrective maintenance).
- At night in supermarkets turn off the air conditioning equipment and if the system is integrated, food cold will not be harmed as long as the parallel compressor plant has speed variators or other digital device to ensure the cooling demand.
- The heat rejected in the heat exchanger by the coolant of the cooling system is absorbed by the air conditioning system. Therefore, it is very important to determine the optimum liquid subcooling (TA-TB in the previous chart) as we can increase the energy consumption in the air conditioning system too much.
Determining the optimum subcooling point ensures an energy advantage in the integrated system (refrigeration and air conditioning), which will depend on the condensation temperatures considered at each installation site.
In Lima - Peru in the 2016 installed an integrated system in an 4,500 m2 Supermarket approximately, achieving good results. We are developing similar projects to be installed now in the 2017.
* Engineers Ernesto Sanguinetti R. and José Lescano are part of Cold Import SA of Lima - Peru. They can be contacted at firstname.lastname@example.org or email@example.com