Retail Chain Air Conditioning Systems
In retail stores , space conditioning is required both for human comfort and for proper operation of refrigerated display cases. The AC unit should introduce a minimum quantity of outside air, either the volume required for ventilation based on ASHRAE Standard 62 or the same to maintain slightly +ve pressure in the space, whichever is larger...
The proper air conditioning of retail stores and supermarkets is a difficult task and offers mechanical designers many challenges. It is one of the most complex applications for heating, ventilating, and air conditioning systems.
In supermarkets, indoor air can be too warm, too dry, too wet or too cold. Any of these conditions can lead to numerous problems for building owners and their customers. One ideal condition does not usually apply to the entire store. If conditions are too hot — products spoil, too dry — products shrink, too wet — products mold, too cold — customers complain.
Some areas, such as meat preparation, must be cold, about 50°F, to help reduce spoilage. Air too dry is good for freezers but not so good for packaged meats, produce and people. Areas that are too damp promote mold, mildew and bacterial growth; and besides concerns with health, these growths produce offensive odours.
During the cooling season, moist outdoor air infiltrates the store to oppose systems that try to maintain the proper conditions.
Therefore, enough conditioned make-up air must be supplied to maintain positive pressure in the store. In big multi-use stores, it is extremely difficult to keep moisture in the air below a 50°F dew point in the summer.
Most supermarkets are large buildings with numerous openings to outside. As the moisture difference between the outdoor and indoor air rises, the force driving the moisture indoors increases. On a 95°/78°F design day, one might expect a 56°F dew point when using a DX cooling system.
There is definitely an optimal space dew point to aim for in the typical supermarket. Tests performed by one chain show that the ideal store dew point is around 53°F. Anything less than this is not desirable if maximum store profits are to be earned.
Cooling Load Estimates
Cooling loads should be calculated using the methods outlined in data for calculating loads caused by people, lights, motors, and heat-producing equipment as specified in the equipments.
In retail stores, space conditioning is required both for human comfort and for proper operation of refrigerated display cases. The air conditioning unit should introduce a minimum quantity of outside air, either the volume required for ventilation based on ASHRAE Standard 62 or the volume required to maintain slightly positive pressure in the space, whichever is larger.
Many supermarkets of a large chain owned or operated by a single company. The standardized construction, layout, and equipment used in designing many similar stores simplify load calculations. It is important that the final air-conditioning load be correctly determined. Refer to manufacturers’ data for information on total heat extraction, sensible heat, latent heat, and percentage of latent to total load for display cases. Relative humidity above 55% at 75°F and 72 gr/lb absolute humidity) substantially increases the load; reduced absolute humidity substantially decreases the load, as shown in Figure 1.
Figure 1: Refrigerated Case Load variation with store air humidity...
Trends in store design, which include more food refrigeration and more efficient lighting, reduce the sensible component of the load even further. To calculate the total load and percentage of latent and sensible heat that the air conditioning must handle, the refrigerating effect imposed by the display fixtures must be subtracted from the building’s gross air-conditioning requirements (Table 1).
Modern supermarket designs have a high percentage of closed refrigerated display fixtures. These vertical cases have large glass display doors and greatly reduce the problem of latent and sensible heat removal from the occupied space. The doors also require anti condensation heaters to minimise condensation and fogging. These heaters should cycle by automatic control.
Air-conditioning systems must compensate for the effects of open refrigerated display equipment.
Design considerations include the following:
• Increased cooling requirement because of removal of large quantities of heat, even in summer.
• Net air-conditioning load after deducting the latent and sensible refrigeration effect. The load reduction and change in sensible latent load ratio have a major effect on equipment selection.
• Need for special air circulation and distribution to offset the heat removed by open refrigerating equipment.
• Need for independent temperature and humidity control.
Energy costs may be extremely high if the year-round air-conditioning system has not been designed to compensate for the effects of refrigerated display equipment.
Heat Removed by Refrigerated Displays
The display refrigerator not only cools a displayed product but envelops it in a blanket of cold air that absorbs heat from the room air in contact with it. Approximately 80 to 90% of the heat removed from the room by vertical refrigerators is absorbed through the display opening. Thus, the open refrigerator acts as a large air cooler, absorbing heat from the room and rejecting it via the condensers outside the building.
Occasionally, this conditioning effect can be greater than the designed air-conditioning capacity of the store. The heat removed by the refrigeration equipment must be considered in the design of the air conditioning systems – because this heat is being removed constantly, day and night, summer and winter, regardless of the store temperature.
Multishelf refrigerated display equipment requires 55% RH or less. In the dry-bulb temperature ranges of average stores, humidity in excess of 55% can cause heavy coil frosting, product zone frosting in low-temperature cases, fixture sweating, and substantially increased refrigeration power consumption.
Cooling from refrigeration equipment does not preclude the need for air conditioning. On the contrary, it increases the need for humidity control.
With increases in store humidity, heavier loads are imposed on the refrigeration equipment, more defrost periods are required, and the display life of products is shortened. The dew point rises with relative humidity, and sweating can become so profuse that even non refrigerated items such as shelving superstructures, canned products, mirrors, and walls may sweat. Lower humidity results in lower operating costs for refrigerated cases.
There are three methods to reduce the humidity level:
1. Standard air conditioning, which may overcool the space when the latent load is high and sensible load is low
2. Mechanical dehumidification, which removes moisture by lowering the air temperature to its dew point, and uses hot-gas reheat when needed to discharge at any desired temperature
3. Desiccant dehumidification, which removes moisture independent of temperature, supplying warm air to the space unless post cooling is provided to discharge at any desired temperature.
Each method provides different dew-point temperatures at different energy consumption and capital expenditures. The designer should evaluate and consider all consequential tradeoffs. Standard air conditioning requires no additional investment but reduces the space dew-point temperature only to 60 to 65°F. At 75°F space temperature this results in 60 to 70% rh at best. Mechanical dehumidifiers can provide humidity levels of 40 to 50% at 75°F. Supply air temperature can be controlled with hot-gas reheat between 50 and 90°F. Desiccant dehumidification can provide levels of 35 to 40% rh at 75°F. Post cooling supply air may be required, depending on internal sensible loads.
The air-handling equipment and distribution system are generally recommended for both cooling and heating using heat pump air conditioning system in extreme summer and winter cities like northern region in India. However, this needs not be applicable over 70% of the balance country mostly close to coastal areas and with higher relative humidity always present.
For small retail outlets from 1000 to 2000 sqft area, most of the super markets in India prefer to install ducted split air conditioners, which are compact, space saving and need no floor space. The capacity varies from 15 TR to 30 TR depends on the heat load and the city ambient conditions. These split units have separate outdoor condensing units.
In such systems fresh air can be injected directly behind the evaporator units’ location drawn from the adjacent wall openings. This is the traditional Indian system followed due to space between the ceiling and false ceiling acts as negative pressure area. The size of the wall opening depends upon the fresh air make up and air changes required. Most of these designs follow one air change per hour similar to other comfort air conditioning system.
View of the Ducted Split Model
For the large retail outlets of 5000 sqft and above, have allocated floor space for installing floor mounted packaged air conditioners or an air handling equipments connected to roof mounted air cooled condensing units.
Air-cooled condensing units are most commonly used in supermarkets. Typically, a central air handler conditions the entire sales area. Specialty areas like bakeries, computer rooms, or warehouses are better served with a separate air handler – because the loads in these areas vary and require different control than the sales area.
Most condenser installations are made on the roof of the supermarket. If air-cooled condensers are located on the ground outside the store, they must be protected against vandalism as well as truck and customer traffic.
The operation of these air conditioners are mostly simplified using micro processor controls, easily readable and operatable by the customer.
Designers overcome the concentrated load at the front of a supermarket by discharging a large portion of the total air supply into the front sales area. The air supply to the space with a standard air-conditioning system is typically 1 cfm per square foot of sales area. This value should be calculated based on the sensible and latent internal loads. The sensible heat factor based on the lighting and equipment loads installed considerably and according to the SHF, the circulating air quantity plays important role to reduce internal heat loads.
The amount of human customers’ presence will add latent as well the sensible loads in the stores area. Hence, the detailed heat load analysis declares the correct air conditioning system selection.
The choking of frequent filtration system normally does not happen in high humidity stores with positive pressure maintained in the conditioned areas. However, in dry goods areas with exposed powdered materials will lead to high dust level in filtration system, which needs to be taken care. Treated fresh air system will also reduce the entry of fresh air to low dust levels with proper filtration of entry fresh air.
The retail operators have not yet established the power consumption levels and control requirements in India – due to recent entry in to this business. However, the indigenously available split air conditioning units are designed and developed to meet the overall other market segment requirements, such as high EER, low power consumption etc, which will be the common usage factor for these products.
Cold air being denser, the open refrigerated display cases settles to the floor and becomes increasingly colder, especially in the first three feet above the floor. If this cold air remains still, it causes discomfort and does not help in cooling other areas of the store that need more cooling. Cold floors or areas in the store cannot be eliminated by the simple addition of heat. Reduction of air-conditioning capacity without circulation of localised cold air is analogous to installing an air conditioner without a fan. To take advantage of the cooling effect of the refrigerated display cases and provide an even temperature in the store, the cold air must be mixed with the general store air.
Very Large Retail Store cum Hyper Market
Large spaces can be centrally controlled with large chiller units integrated with air handling equipments. These units can be fully equipped with cooling – and humidification and also continuously monitored for temperature and humidity through building management system. Terrace spaces are used for air cooled chillers and cooling tower in case of water cooled system.
The air change remains similar to other small areas with one air change minimum. The power supply and distribution design is the maximum in such large area air conditioning solutions. Roof top air conditioners integrated duct system can be used for this application with floor space saving. However, such units are not yet readily available in our country.
Most supermarkets, except large chains, do not employ trained maintenance personnel, but rather rely on service contracts with either the installer or a local service company. This relieves store management of the responsibility of keeping the air conditioning operating properly.
New Focus Areas on System Design
As the technology is growing fast in HVAC industry, it will be better to consider the following concepts for energy efficient air conditioning system performance.
Energy Savings Using Ventilation Control in Retail Stores and Supermarkets
It is important to know if rising AC use is one of the reasons for such an increase and if energy efficiency measures are being used effectively to limit the power required by AC systems. These systems exist primarily for the comfort of customers and staff, also for stock in case of food, but the same effect can be achieved with much less energy use, particularly in hot climates. Typical measures in the retail sector may include more efficient use of display lighting, more and no arbitrary setting of cooling temperatures combined with self closing doors or air curtains.
The example store has 10,000 ft2 of floor space and is occupied for 18 hours per day, seven days per week. The savings grow as the store size increases and the occupancy period is extended. At this savings rate, the CO2 sensors pay for themselves within a couple of months.
Ventilation Control adjusts the economizer's outdoor air intake based on the measured CO2 levels to maintain the proper ventilation rate (10 cfm of outside air/person). The picture below shows you a typical occupancy pattern for a retail store or supermarket. With Ventilation Control, the outdoor air intake follows the occupancy pattern. As customers come and go throughout the day, the CO2 sensor adjusts the amount of outdoor air entering the store accordingly. Your current ventilation system thinks the store is always fully occupied.
Taking Control of Air Conditioning
Controlling electrical components in an air-conditioning system is vital to ensure correct and efficient operation. It has become even more important to increase energy savings. It is also an essential expectation of customers that they have the opportunity to ‘control’ their working environment and, theoretically at least, the functioning of their air-conditioning systems.
Two types of control are involved — the functioning of the equipment electronically and the manipulation of the conditions within the occupied space.
Since the advent of cheaper and highly reliable electronics, today’s air-conditioning systems rely internally more and more on complex electronic control rather than the electro-mechanical control of old. Externally, a wide variety of systems are provided to enable air-conditioning equipment to interface with users.
A directly driven compressor operates at fixed speed and does not compensate for the reduction in load as the set point is reached, resulting in constant stop/start operation.
Fan motors inside air-conditioning equipment are critical to the function. The transfer of heat away from the coil is vital. The outdoor fan is required to run at different speeds for different modes of operation under different outdoor conditions to maintain efficient system operation. The indoor fan is also important, as it delivers the cooling as per user requirement. In the past, such fans were mostly controlled via taps for the different speeds. This approach was very effective, but the speed change was sometimes audible for the customer. The degree of control was also limited to the amount of taps or relays that could be fitted to the control PCB.
Other components in the air-conditioning system have benefited from advances in electronics. In many systems refrigerant expansion was, and still is, managed via mechanical expansion valves and capillaries. However, some systems now use electronic expansion valves. These produce greater efficiency, as the degree of opening can be controlled via sensors in the unit and predetermined control algorithms in the software on the micro-controller.
Such components have become very important in air-conditioning systems to increase energy-efficiency ratios and coefficients of performance. No single individual component makes a massive difference but the combination has a profound net effect.
With regard to external or room control, electronics is once again to the fore, with central control of several separate systems being popular. Remote Internet control, energy-monitoring data and innovations such as touch screens or key pads are the latest attractions to the end-user. Infra-red remote room control has become common place in the last few years.
Interfaces with building-management systems are commonly offered by manufacturers like Toshiba, carrier, daikin etc, but open-protocol technology is also offered by companies like Siemens, Johnson controls, Honeywell etc; which are recognised as the future of customer control technology.
To provide centralised control, a Japanese A.C company, for example, uses a wide range of controllers that can be adapted to complex control scenarios — from relatively simple central controllers with limited connectivity to systems for on/off control and basic reporting of faults.
When more complex control solutions are required, toolbox of software solutions that are easily implemented and fully tried and tested — allowing fully functional seamless links to all major BMS manufacturers.
This software acts as a fully-functional supervisor which can interface with other items of building plant via the digital input/output modules.
This type of software also provides reportable alarm management, with dedicated reporting to a wide range of other sources. There are many generic versions available, with dedicated diagnostic software routines and many functions that allow programming related to specific equipment functions.
A good example is the input from a fire alarm to stop and restart the whole large retail store.
The overall design considerations in a supermarket or a retail stores, should have more focus on energy efficiency, controlled indoor air quality environment, modern temperature and humidity control systems with precise control through a data system, will be the need of the day for efficient and economic operation of the air conditioning system operation.
AUTHORS CREDIT & PHOTOGRAPH
R MURALIDHARAN IYENGAR