Unitary air conditioner systems have a largest voluminous product in the market segment throughout the world. As the business volume is high and very less people are involved in selection, sales and execution, the product knowledge and its awareness towards the right choice of selection and application are limited to fresh engineers. All are working as product sellers.
Now that we understand the conceptual arrangement of air-conditioning cooling systems, the distinction between the local DX and central chilled water systems is critical from a mechanical, architectural and energy management perspective. Let’s analyze the key factors that determine the selection of (DX) direct expansion system. The write up is for the benefit of fresh engineers for sales and design of AC system.
Check out this statement “DX system is suitable for a single thermal zone application”. What does this mean?
Why is it so?
To answer this, first understand the concept of thermal zone. A thermal zone is referred to a space or group of spaces within a building with heating and cooling requirements that are sufficiently similar so that desired conditions (e.g. temperature) can be maintained throughout using a single sensor (e.g. thermostat or temperature sensor). Each thermal zone must be ‘separately controlled’ if conditions conducive to comfort are to be provided by an HVAC system. Few examples below illustrate and clarify the concept of a zone.
- In a building, the perimeter areas with large glazing & exposure are prone to larger solar radiation. Such areas shall experience higher heat load than the indoor core spaces and must be separately controlled.
• In a commercial building, the space containing electronic processing equipment such as photocopiers, fax machines and printers see much larger heat load than the other areas and hence, it is a different thermal zone.
• A conference room designed for 50 people occupancy shall experience lower temperatures when it is half or quarterly occupied. The design, thus, shall keep provision for a dedicated temperature controller for this zone.
• In an airport, a smoking room shall be categorized as an independent zone for health and safety reasons. A good air-conditioning system should not allow mixing of smoke contaminants with return air of other public lounges.
• A 1000 seat theatre shall be treated an independent zone than the entrance concourse or cafeteria as the dynamics of occupancy are different.
• A hotel lobby area is different from the guest rooms or the restaurant area.
• A hospital testing laboratory, isolation rooms and operation theatre demand different indoor conditions or pressure relationships than the rest of areas and thus shall be treated as a separate zones.
• A control room or processing facilities in industrial set up may require a high degree of cleanliness/positive pressure to prevent ingress of dust/hazardous elements and thus may be treated as separate zone.
In nutshell, any area that requires different temperature, humidity and filtration needs or is prone to huge variations in thermal loads shall be categorized as an independent zone. The reason that most modern offices interiors have low partitions is not to do only with aesthetic and spacious looks; it has relevance to keep air-conditioning simple and effective. Zoning may very well be categorized as an architectural responsibility since it requires a good understanding of building function and schedules.
Let’s check out why DX systems are only suitable for single thermal zone application. The reasoning is as follows:
- DX systems do not provide modulating control. The capacity control in DX system with fully hermetic sealed compressor is, generally, accomplished by cycling the compressor ON and OFF in response to the signals from a thermostat. What this means is that the DX system will only have one point of control – typically a thermostat. Thus, two rooms with thermostat controllers set at say 22°c and 28°c shall conflict with each other or in other words the two rooms cannot achieve the set conditions unless the rooms are served with independent units. Semi-hermetic compressors offer the benefit of being able to unload pairs of cylinders within a single compressor. For instance, a compressor with six cylinders can be staged to operate at 100%, 67% and 33% capacity by operating on six, four, or two cylinders respectively. These provide only limited step modulation. Presently, multiple scroll compressors are installed ductable split models to take care of part load in place of semi hermetic reciprocating compressor, which also aims at reducing the compressor power load.
The issue of system control leads to the concept of HVAC zoning just like architectural zoning. Active HVAC system may be designed to condition a single space or a portion of a space from a location within or directly adjacent to the space.
- DX systems cannot be networked conveniently. The refrigerant piping plays a key role in connection of various components in terms of size, length and pressure drop. Split units installation is restricted by distance criteria between the condensing unit and the evaporator, which is usually 30 to 40 feet for smaller units and around 100 to 120 feet for larger units. For large buildings consisting of multi-zones, DX system may be viewed as collection of multiple independent units placed at different locations in a distributed network with each unit working in isolation.
Each DX system is, thus local self-contained unit consisting of its own compressor/s, evaporator coil, fan, condensing unit and filtration unit. Depending upon the capacities required and areas served the DX system could be room air conditioners, split air-conditioners or package air conditioners. All these serve a single thermal zone and have its major components located in one of the following ways:
- Within the zone
• On the boundary between the zone and exterior environment
• Or directly adjacent to the zone
Newer DX Configurations/Options
Newer technology has found ways to combat the above weaknesses if not fully at least substantially.
As the technology is getting more advanced day by day, new eco friendly refrigerants are getting replaced. The operating parameters are not the same; the selection and installation are more complicated. The older refrigerant models have become absolute and customer goes for long lasting air conditioners due to its cost. Hence, a lot of care is desirable.
Variable Air Volume (VAV) Units for Ducted Package Systems
Variable Air Volume (VAV) components can be fitted on the air distribution ductwork, thus affording good control of conditions within the respective thermal zone. Variable air volume system (VAV) delivers a constant temperature of air and responds to changing thermal loads by varying the quantity of supply air.
Generally, such a fitment on the whole system means a large increase in cost. In a limited mode, like for instance just one cabin to be zoned out in a full floor – one can install a VAV diffuser for the cabin. Such a device has a motorised damper fitted on the air outlet and the damper operates automatically in response to a thermostat. In other words, the diffuser admits or restricts supply air to the cabin in response to the command of a thermostat. Such devices cost about Rs.15, 000 approximately for a 400 cfm size diffuser.
Variable Refrigerant Flow (VRF) System for Multiple Evaporators
The term variable refrigerant flow (VRF) refers to the ability of the system to control the amount of refrigerant flowing to the multiple evaporators, enabling the use of many evaporators of differing capacities and configurations connected to single condensing unit.
The arrangement provides an individualized comfort control, and simultaneous heating and cooling in different zones. This refrigerant flow control lies at the heart of VRF systems and is the major technical challenge as well as the source of many of the system’s advantages.
Many zones are possible, each with individual set point control. Because VRF systems use variable speed compressors with wide capacity modulation capabilities, they can maintain precise temperature control, generally, within ±1°F (±0.6°C), according to manufacturers’ literature.
VRF system being the split installation is restricted by distance criteria between the condensing unit and the evaporator. Although few manufacturers’ literature states the refrigerant lines can be as long as 500 feet, but when you read the fine print, after the first ‘Tee’ from the condensing unit, you are limited to 135 feet to the furthest unit. Other than the restricted distance criteria between evaporator and condensing unit, there are some legitimate concerns that need to be addressed.
- VRF systems are complete, proprietary systems from the controls right up to the condensing units, refrigerant controllers, and all the system components other than the refrigerant piping. That means users do not have the flexibility to use anybody’s building control and automation system to run these systems. You’ll need a BacNet or Lonworks black box to connect from your building DDC system to the VRF system, and you can only monitor what it’s doing, you can’t control it.
• As the system has a larger spread, the refrigerant pipes traverse long lengths – hence their pressure testing and protection becomes critical. Long refrigerant piping loops also raise concerns about oil return.
• Long refrigerant lines also raise the potential of refrigerant leaks, which can be a safety hazard. The refrigerant leak, especially, if the system serves small rooms can cause oxygen depletion. So, you need to limit the system size within reasonable limits based on smallest room area served. For e.g. if the room area is 100 sq-ft, you would need to limit the refrigerant quantity under less than about 30 lbs. Contractors are concerned about long refrigerant piping runs for multiple evaporators.
In these systems, compliance with ANSI/ASHRAE Standard 15, Safety Standard for Refrigeration Systems, is difficult;
- Currently, no approved ARI standard exists for a performance rating of VRF systems. Consequently, manufacturers need to apply for waivers from the Department of Energy to market their products in the US. Although these waivers have been granted, new applications need to be submitted for new product groups;
• VRF systems are expensive and complex. The complicity involved in VRF/VRV is continuous and have to be dependent on the vendor who has supplied for life of equipment.
A unit with two equally sized fully hermetic compressors may operate at 100% and 50% capacity by starting or stopping one of the two compressors. Unequally sized compressors provide greater staging flexibility, for instance, a 30-ton unit with three compressors rated at 10 tons and 20 tons will have capacity stages at 33%, 67% and 100%.
- The DX systems are suitable for small or medium sized buildings free of multiple thermal zones and demanding 100 TR or less of air-conditioning. For big areas such as mall type stores requiring say 200 TR of refrigeration, DX system may be viewed as 4 units of 50 TR each subject to availability of space and aesthetics.
• DX systems are more effective for the services requiring low temperature and low humidity conditions. The application includes the grocery stores, fruit & vegetable stores, meat processing units, instrument rooms, laboratories, bio-medical labs, critical manufacturing and process facilities.
• DX systems can be applied along with central chilled water system for areas requiring 24 hours operation such as server rooms, data centres etc. DX systems can be also be applied for augmenting the HVAC needs in the existing central HVAC systems necessitated due to expansion or addition of more equipment.
Factors Favouring DX System
- One of the most common reasons for selecting a DX system, especially, in smaller buildings is the lower installed cost than a chilled-water system because it requires less field labour and has fewer materials to install.
• DX systems tend to be distributed for larger buildings that increase reliability; a building conditioned using DX system may have a dozen or hundred of individual and independent units located throughout the building. Failure of one or two of the units may not impact the entire building. On a smaller scale this may be viewed as a disadvantage unless standby is provided.
• If the tenants are paying the utility bills, multiple packaged DX units may make it easier to track energy use, as only the specific unit serving that tenant would be used to meet the individual cooling requirements.
• DX systems are not complicated by interconnections with other units. Maintenance of local systems tends to be simple and available through numerous service providers.
• In buildings where a large number of spaces may be unoccupied at any given time, such as dormitory, small hotels etc. the local DX systems may be totally shut off in the unused spaces, thus, providing potential energy savings.
• For small areas within full scale offices like communication rooms or server / computer rooms, where it is necessary to have 24 hour air conditioning – it is possible to have independent split, ancillary AC units exclusively for these areas.
• DX systems can be installed quickly and their operation is relatively simple. Often short delivery schedules are generally available as factory standard off the shelf unit. Easy to install and replace. Compact and require a smaller footprint than alternatives.
• As a self contained system, a DX system may provide totally individualized control options, for instance, if one room needs heating while an adjacent one needs cooling, two local systems can respond without conflict.
• DX unitary systems are ideal for retrofitting applications. These may be used to supplement areas of inadequate service by a building’s existing central system.
• Air cooled condensers can be located on the roof of a building or even within the perimeter wall of the building. Cooling unit is available in wide variation of floor, wall as well the ceiling suspended unit.
Limitations of DX System
- DX systems cannot benefit from economies of scale. Capital costs and the operating costs generally tend to be higher for larger setups requiring 100TR or more. The building designer must thoroughly evaluate all pertinent installation, operating, and maintenance costs to make an informed decision
• DX systems cannot be easily connected together to permit centralized monitoring or energy management operations. These can be centrally controlled with respect to on-off functions only
• DX units have capacity control limitations; compressor unloading systems are generally step devices, which limit capacity modulation. At low load conditions, the compressors will cycle and unconditioned air will pass through the system during the off cycle, which may cause temperature swings (i.e. hot and cold spots) in the conditioned space;
• The coefficient of performance (COP) of a DX system is low. Unitary systems consume more power (kW per ton) compared to central systems of same capacity;
• Lack of interconnection between units also means that loads cannot be shared on a building wide basis. Central HVAC systems deliver improved efficiency and lower first cost by sharing load capacity across an entire building;
• One cannot have a zone within a zone. As an example in a general office, air conditioned by a DX system – if there is a cabin or two – these cabins cannot have individual independent controls (unless variable air volume (VAV) units are considered);
• Multiple DX systems using window or small capacity split units may spoil the exterior elevations and aesthetics of the building;
• For distributed DX systems, although the maintenance may be relatively simple, such maintenance may have to occur directly in occupied building spaces;
• DX systems may not be suitable for the applications requiring high air delivery rates and the areas requiring significant positive pressurization (unless the DX systems are engineered). The standard unitary systems provide 400 cfm of air delivery capacity per ton of refrigeration;
• DX systems are not suitable for areas requiring high degree of cleanliness unless the systems are custom built. The standard units, generally, provide fan static pressure of 2 to 3 inch water gauge, which may not be sufficient to cope up the resistance of high efficiency filtration.
• DX systems installation many a time requires plumbing arrangements with in the conditioned area if the cooling unit is placed indoors. The design should take into account the condensate removal required from the conditioned space and the possibility of leakage.
• DX window or small split-air conditioners are free air discharge units and are non- ducted. Multiple units or package unit shall be needed to optimize air distribution where the span of building (length or width) exceeds 12 feet.
• Smaller split units with cooling (evaporator) unit located indoors in conditioned space are 100% re-circulation units. They do not provide ventilation, so a separate ventilation system is necessary.
• Split DX systems are constrained by distance limitation of approximately 30 to 100 feet between condensing unit and evaporator. Chilled water systems are not constrained by any separation distance criteria between chiller and the cooling coil.
• Special requirements of surface coating may not be available on the condensing equipment placed outdoors in harsh corrosive/saline environment. The condensing unit will, therefore, have a shorter life span.
• Multiple DX systems for large area applications shall require larger footprint of mechanical room or quite a number of mechanical rooms.
The HVAC industry largely human is dependant in the field in case of selection, design, installation as well as service. If all have been done perfectly, the desired cooling required by the customer can be achieved. The customers satisfaction mostly manufacturer dependant and not with the engineer or technician concerned.