Air conditioners come in many forms - for homes and businesses, air-cooled and water-cooled. This article compares air-cooled heat pumps with water-cooled heat pumps, variable-refrigerant-flow systems and air-source two-supply units.
Air-cooled vs water-cooled heat pump
Air-cooled unit: uses air to reject and absorb heat, producing chilled and hot water.
Water-cooled unit: uses water to carry away heat, producing chilled water.
1. Economic and technical comparison
The air-cooled unit initial cost is a little higher than the water-cooled unit, and its power per unit of cooling is also slightly higher. But its annual total cost is roughly level with, or slightly below, the water-cooled unit. The longer the annual run time, the more the air-cooled unit gains, and its payback against the water-cooled unit is shorter. The make-up water for the cooling tower strongly affects water-cooled cost; tighter maintenance that cuts water use is the main way to lower it.
Air-cooled suits small and medium systems in water-scarce areas. Longer run hours favour it, and its annual cost is lower than the water-cooled system. But if the water-cooled system is well managed and make-up stays under 3%, the extra air-cooled investment is hard to recover.
2. Features of the water-cooled heat pump
1. Wide application range and lower cost.
2. The most mature technology, and the most widely used air-conditioning system.
3. Heat and cold sources are usually centralised, so operation and maintenance are easy.
4. High summer cooling efficiency and a high energy-efficiency ratio.
5. Lower initial cost; with no insulated piping the material cost drops sharply.
6. Fewer noise sources than the air-cooled unit.
7. Low plant-room requirement - only normal ventilation is needed.
8. Much longer service life than the air-cooled unit.
9. Relatively small size and footprint.
3. Features of the air-cooled heat pump
1. Saves water, is eco-friendly and makes good use of the equipment.
2. Installed outdoors, it needs no dedicated plant room and saves building area and civil cost.
3. Cools in summer and heats in winter, removing the need for a boiler house; good for construction and landscape.
4. Removes the cooling-water system, cooling tower, pumps, piping and water treatment, saving that investment and running cost.
5. Saves electricity in winter heating; heat-pump heating uses about two-thirds less power than direct electric heating.
6. Available in several forms; low-noise units have less impact on surroundings.
7. Safety protection and automatic control sit in one unit, so it is reliable and easy to install and use.
8. The longer it runs, the better; maintenance cost is low.
4. Choosing the equipment
No fixed pattern exists for the central air-conditioning design of a large building; you must weigh the local conditions on site. Whatever equipment form you use, coordinate closely with the architecture, structure and other mechanical and electrical disciplines. Good design reads the project situation, balances every factor, and meets the owner intent under the premises of technical feasibility, safety, reliability, best use and economy.
Water-cooled and air-cooled each have their traits; choose by environment, space, required cooling capacity and region. The larger the building, the more you favour water-cooled; the smaller, the more you favour air-cooled.
Air-cooled is mainly for dry, water-short areas - its strengths are saved plant area and easy installation, but ambient temperature makes operation less stable. Water-cooled is for water-rich areas and runs stably, but the cooling water brings freeze-protection problems, so in winter it is troublesome. A cooling tower alone cannot heat in the north, so a water-source or ground-source heat pump is needed - its cooling and heating are better and it is the current best choice. Air-cooled heat-pump heating in the north is difficult and needs an electric auxiliary water-heating device.
In real cooling-design work, choose between air-cooled and water-cooled as follows:
1. Where water use is strictly limited, clearly favour air-cooled. The building structure must support the ventilation cross-section and floor load and meet the unit ventilation and heat-exchange needs.
2. If the building form or site forbids an outdoor cooling tower, the central system must work with the architecture and structure to use air-cooled, with the host room designed for load and ventilation and heat exchange.
3. With no such constraints, prefer a water-cooled host; this form and its technical coordination are very common and mature in the industry.
4. For combined design, a small air-cooled unit as an auxiliary to a water-cooled system is often a good choice.
5. In general, large loads, large cooling capacity or water-rich areas mostly use water-cooled.
Air-cooled vs VRF (variable-refrigerant-flow)
1. Principle
Air-cooled central system. The medium is usually an aqueous solution. The outdoor host makes chilled or hot water, piped to the indoor terminals; at the terminal the water exchanges heat with the room air, producing cooled or heated air that removes the room load. It produces cooling or heat centrally but treats each room load locally.
The indoor terminal is usually a fan coil. Most fan coils adjust fan speed, or the water through the coil via a bypass valve, to regulate the cooling or heat sent to the room, so each room can be controlled separately and energy saving is good. But the chilled and hot water distribution takes some installation space.
VRF system. Its control system reads indoor comfort parameters, outdoor conditions and the refrigeration state, and by optimisation and comfort rules adjusts compressor capacity through inversion and controls fans, electronic expansion valves and all controllable parts, keeping the room comfortable and the system at its best working point. A VRF regulates the refrigerant flow to the indoor units to meet the load - a high-efficiency refrigerant air-conditioning system. It uses an inverter compressor, a multi-stage or unloader compressor, or several compressors together; it needs an electronic expansion valve or auxiliary loop to set the refrigerant flow to indoor units, and controls the indoor and outdoor fan speeds. After inverter and electronic-expansion-valve technology matured, VRF widely adopted inverter compressors and electronic expansion valves. The system works as ambient and load keep changing, and its parts interact, so its state keeps changing and the control must adjust capacity in time - a soft-regulation system.
Finding: the air-cooled unit transfers heat through a water secondary carrier, with some loss; the VRF exchanges heat directly through refrigerant, with no secondary carrier, so it is more efficient.
2. Equipment
1. Air-cooled system: outdoor unit plus auxiliaries (pumps, expansion tank) plus fan-coil terminals plus other materials and commissioning.
2. VRF system: outdoor unit plus indoor units plus installation.
Finding: the one-off investment of VRF and of the air-cooled unit are close.
3. Operating cost
1. Air-cooled: because of the water heat-exchange step, the outdoor host must start 40 minutes early and the indoor terminal 10 minutes early to pre-cool the room.
2. VRF: only the indoor unit needs to start 10 minutes early to pre-cool.
4. Service life
The air-cooled host is designed for about 26000 hours and the terminal about 16000 hours, so the whole system averages about 21000 hours, not counting duct and pipe life or corrosion effects. The VRF equipment is designed for at least 30000 hours, up to 40000. The VRF life is 1.4 to 1.9 times the water-cooled system.
5. Maintenance
The air-cooled system, besides cleaning indoor filters, needs periodic cleaning of the outdoor host water filter and descaling of the piping, or scale builds up and hurts flow and effect; maintenance is troublesome and needs a specialist firm. The VRF only needs periodic indoor-filter cleaning - simple, and the user can do it. The VRF is far better in convenience and cost of maintenance.
6. Other aspects
The air-cooled host uses a large screw compressor with no inverter control, so its starting current is very high and can disturb the grid and other appliances. Also, the chilled-water host and heating boiler each need a separate plant room, taking useful building space, usually in the basement to cut noise.
The VRF uses several small compressors plus inverter technology; its starting current is only 1 A and then rises gradually, so it never shocks the grid. The outdoor unit sits on the roof or at ground level and takes no interior space.
Finding: the VRF is simpler and more flexible to use than the air-cooled system.
Air-cooled vs air-source two-supply
Air-cooled chiller modules and air-source two-supply units both heat and cool, but they are not the same. The air-source heating unit that has drawn attention in the last two years also differs from the air-cooled module and the air-source two-supply. This article analyses the three products across three aspects: product standard, design conditions and component selection.
1. Different product standards
Table 1. Product standards
Product type | Air-source heat-pump heating unit | Air-source heat-pump two-supply unit | Air-cooled heat-pump chiller module | |
Standard No. | Domestic | GB/T 25127.2-2010 Low-ambient air-source heat pump (water chiller) units - Part 2: Units for household and similar applications | GB/T 18430.2-2008 Vapour-compression liquid chillers (heat pumps) - Part 2: Units for household and similar applications | GB/T 18430.2-2008 Vapour-compression liquid chillers (heat pumps) - Part 2: Units for household and similar applications |
Commercial | GB/T 25127.1-2010 Low-ambient air-source heat pump (water chiller) units - Part 1: Units for industrial or commercial and similar applications | GB/T 18430.1-2007 Vapour-compression liquid chillers (heat pumps) - Part 1: Units for industrial or commercial and similar applications | GB/T 18430.1-2007 Vapour-compression liquid chillers (heat pumps) - Part 1: Units for industrial or commercial and similar applications |
Different products follow the standards in the table.
1. The standards above are for reference only - they are the ones generally sent for third-party testing. In real design, makers consider that radiator heating may be used, so the air-source heating unit and the air-source two-supply unit reach an outlet temperature above the standard, 55°C or higher; the air-cooled module usually does not exceed 50°C, so it cannot serve radiator heating.
For the air-source two-supply unit, the market now splits by region into low-temperature and normal-temperature units; the low-temperature unit also follows GB/T 25127.1 or GB/T 25127.2, and the normal-temperature unit follows the table.
2. Different design-condition standards
Table 2. Product design conditions (note: values marked * vary by maker)
Product type | Air-source heat-pump heating unit | Air-source heat-pump two-supply unit | Air-cooled heat-pump chiller module | ||
Design condition | Used side | Heating | Standard outlet 41°C; max 50°C*; flow 0.172 m³/(h·kW) | Standard outlet 45°C; max 50°C*; flow 0.172 m³/(h·kW) | Standard outlet 45°C; max 50°C*; flow 0.172 m³/(h·kW) |
Cooling | Standard outlet 7°C; max 15°C; flow 0.172 m³/(h·kW) | Standard outlet 7°C; max 15°C; flow 0.172 m³/(h·kW) | |||
Heat-source side (or heat-rejection side) | Heating | Standard dry-bulb -12°C; range 21°C to -20°C | Standard dry-bulb 7°C; range 21°C to -7°C | Standard dry-bulb 7°C; range 21°C to -7°C | |
Cooling | Standard dry-bulb 35°C; range 21°C to 43°C | Standard dry-bulb 35°C; range 21°C to 43°C |
By design principle all three follow the reverse-Carnot cycle. The air-source heating unit stresses low-temperature heating performance, and to reach it uses vapour-injection technology, variable-speed compressors, two-stage compression or even transcritical refrigeration. Each maker decides which technology to use by its technical stock, cost control and market grasp, to raise low-temperature heating performance and the m/p ratio.
By the same logic, the air-source two-supply unit and the air-cooled module, because of different regions and function focus, generally use single-stage reverse-Carnot or variable-speed compressors; but to raise competitiveness a maker may also adopt the heating unit technology. All units follow the vapour-compression reverse-Carnot cycle.
As the market matures, the three products concepts and technologies will keep converging, but they will still split by region into low-temperature and normal-temperature units.
From design conditions, the air-source heating unit stresses heating, shown by clear low-temperature heating effect and saving versus gas, coal or electric boilers; its reference standard is therefore GB/T 25127.1 or GB/T 25127.2, with a stated -12°C ambient. The other two have wider use - shown by their use of GB/T 18430.1-2007 or GB/T 18430.2-2008, they may also use GB/T 25127.1 or GB/T 25127.2; they consider cooling performance first and heating second, and where heating is insufficient they usually add electric heating.
The air-source two-supply unit mainly follows the normal-condition standard; if it follows the low-temperature condition it is grouped with the air-source heating unit.
By terminal: the air-cooled module heats mainly through fan coils, while the air-source heating and two-supply units must serve both fan coils and full radiant floor heating, so they are stricter on design conditions, outlet temperature and control precision; the two-supply unit therefore resists high pressure, high differential pressure and high temperature better than the air-cooled module.
By electrical control, the three products are basically the same and need no special requirement.
3. Different component selection
Table 3. Main component selection
Component | Air-source heat-pump heating unit | Air-source heat-pump two-supply unit | Air-cooled heat-pump chiller module |
Compressor | Low-temperature vapour-injection heat-pump compressor | Heat-pump dedicated compressor | Standard air-conditioner compressor |
Condenser | Plate, shell-and-tube or shell-in-tube heat exchanger | Shell-and-tube heat exchanger | Plate or shell-and-tube heat exchanger |
Expansion device | Electronic or thermostatic expansion valve | Electronic or thermostatic expansion valve | Electronic or thermostatic expansion valve |
Evaporator | Finned heat exchanger | Finned heat exchanger | Finned heat exchanger |
Economiser | Plate heat exchanger | ||
Auxiliary expansion assembly | Electronic or thermostatic expansion valve |
At present, besides the traditional four major parts, the air-source heating unit usually adds an intermediate economiser or flash tank for the two-stage compression loop; the other two generally have only the four major parts. But raising low-temperature performance adds new technologies or combines several, for example a regenerator; for two-stage compression the loop is quite different from the traditional single-stage one, and a single rule cannot cover it.