5: CONTEMPORARY SOURCES FOR CHILLED WATER

Section 2 discussed the types of chillers often found in larger commercial and institutional cooling systems, specifically:

• air-cooled chillers
• water-cooled chillers

There are many electrically powered air-cooled and water-cooled chillers available in the 10- to 500-ton range (1 ton = 12,000 Btu/hr). These chillers are designed to operate on 3-phase electrical power, which is usually available in urban areas, as well as areas designated for commercial building development. However, 3-phase power may not be available in rural areas. This doesn’t mean that buildings requiring commercial-capacity cooling cannot be sited in such areas, which may only have single phase utility power available.

One solution could be an array of 2 or more single phase chillers operated in stages. In this context, the word “chiller” could represent a device that only provides chilled water, or a heat pump that can provide chilled water for cooling and warm water for heating. Single phase chillers are available with capacities up to about 5 tons. Thus, a 20-ton cooling load could be handled by four 5-ton chillers operated in stages.

The use of multiple smaller-capacity chillers has advantages and limitations. One advantage is redundancy — if one of the chillers is down for maintenance, the other chillers can still provide partial load capacity.

Another advantage is load matching. An array of four fixed-speed 5-ton chillers could provide four incremental steps of cooling capacity. This allows instantaneous cooling capacity to be better matched to variable cooling loads. In some systems, it may also eliminate the need for a buffer tank.

When multiple chillers are used, they are typically controlled by a system that uses a closed-loop PID (proportional-integral-derivative) control algorithm that’s configured for a specific chilled water target temperature to the load. The controller varies the number of operating chillers to “steer” the delivered water temperature as close as possible to that target temperature.

One potential drawback of multiple chillers is that they typically require more piping and valving relative to a single large chiller. Another is that the “footprint” required for multiple chillers is usually more than a single chiller of equivalent capacity.

An array of two or more single phase electric chillers (or heat pumps) can be piped very similar to how multiple boilers are piped. Figure 5-1 shows an example based on using three monobloc-style air-to-water heat pumps as chillers.

The system in Figure 5-1a uses individual circulators to create flow through each heat pump. Each circulator operates only when its associated heat pump operates. Each heat pump has combination isolation/purging valves and unions, allowing it to be isolated from the remainder of the system and serviced, or even temporarily removed if necessary. Each heat pump also has reinforced flexible hose connectors that reduce vibration transfer to rigid piping. A spring-loaded check valve is installed in the piping at each heat pump to prevent reverse flow when some heat pumps are on and others are off. A QuickSetter™ automatic balancing valve in the branch piping to each heat pump allows flow rates to be balanced.

All the heat pumps connect to low head loss (generously sized) headers that lead to a hydraulic separator equipped with a magnet for capturing dirt and iron oxide particles. The entire system operates with a propylene glycol antifreeze solution sufficiently concentrated to prevent freezing in any exterior piping. All piping is also insulated to prevent condensation and heat gain during cooling, as well as to reduce heat losses during heating mode.

The system in Figure 5-1b uses a single variable-speed, pressure-regulated circulator to provide flow to all heat pumps. Each heat pump branch is equipped with a high Cv zone valve (or motorized ball valve) that opens when its associated heat pump is operating. The variable-speed circulator operates in constant differential pressure mode and automatically adjusts speed based on the number of operating heat pumps. FlowCal™ balancing valves automatically maintain a pre-set flow rate through each heat pump when it operates. This piping configuration would have a lower electrical power demand in comparison to use of individual circulators for each heat pump.

The piping shown in Figure 5-1 assumes that heat pumps — rather than dedicated chillers — were selected so that the system can provide heating and cooling. The piping at the hydraulic separators is optimized for heating mode operation (e.g., heated fluid passed through the upper portion of the separator while cooler fluid passes through the lower portion). If dedicated chillers were used for a system that only provides cooling, the coldest fluid leaving the chillers should pass through the lower portion of the separator. The slightly warmer fluid returning from the cooling load would pass through the upper portion of the separator. The warmer fluid would have a slightly lower solubility for dissolved gases, and thus enhance the air-separating function within the upper portion of the SEP4™ separator.

Figure 5-2 shows an array of six nominal 5-ton air-to-water heat pumps that supply chilled water for cooling a small industrial facility equipped with chilled beam terminal units (to be discussed in section 7). These heat pumps also supply warm water for heating.

It’s important to understand that the piping shown in Figures 5-1a and 5-1b allows any of the heat pumps to operate, in the same mode, as required by the load, but does not allow some of the heat pumps to operate in cooling mode while others simultaneously operate in heating mode. Mixed mode operation is useful in some buildings, especially during swing season conditions. On a cool day in fall or spring, perimeter areas of a building may require heating, while core areas of the building require cooling.

Simultaneous heating and cooling from a multiple heat pump array is possible, but requires more piping, valves and controls. Figure 5-3 shows one example of piping that could be used with an array of three air-to-water heat pumps to allow simultaneous heating and cooling.

This system has a “hot” buffer tank and a “cold” buffer tank. The system controls monitor the temperature of each tank and call for a heat pump to turn on when the temperature of either tank deviates slightly from its target temperature or outside of a set temperature range. The water temperature in the hot buffer tank would be regulated based on outdoor reset control. The water for the cold buffer tank would be maintained between upper and lower temperature setpoints, such 45ºF and 60ºF whenever a cooling load is present.

When a heat pump is called to operate, the zone valve pairs associated with its mode of operation open. The status of the heat pump’s reversing valve is also set. A variable-speed pressure-regulated circulator operates to create flow through the appropriate buffer tank. The speed of the circulator is based on proportional differential pressure control. The speed automatically increases or decreases depending on how many heat pumps are operating.

The valving at each heat pump is also arranged so that the zone valves or heat pump can be isolated from the balance of the system if necessary for service.

Each buffer tank provides hydraulic separation between the heat pump circulators and the load circulators.

The heating zones are supplied by low temperature radiant panels. Flow to each manifold station is controlled by a zone valve. Each manifold station piping assembly is also equipped with a balancing valve and purging valve. These valves are arranged so that each manifold station and its associated zone valve can be completely isolated from the balance of the system if necessary for service.

The cooling zones are supplied by fan-coils. Each fan-coil piping assembly is equipped with a balancing valve and purging valve. These valves are arranged so that each fan-coil and its associated zone valve could be completely isolated from the balance of the system if necessary for service.

Because this system can simultaneously supply heating and cooling, one mode of operation must take priority when staging the heat pumps. Several possibilities exist. For example, during heating season the ability to maintain adequate water temperature in the “hot” buffer would likely be the priority. Once that temperature is established, at least one of the heat pumps would be allowed to operate in cooling mode if a cooling load is present. During the cooling season, it’s likely that all heat pumps would be prioritized to satisfy the cooling load.

The same piping concepts discussed for multiple air-to-water heat pumps can also be applied to multiple water-to-water heat pumps. In some applications, the heat pumps would be dissipating heat to earth loops or “open” groundwater sources. In other applications, the heat of rejection from the heat pumps may be used to heat domestic water, or to preheat process water that would eventually be used for washing or sterilization in food processing facilities. Figure 5-4 shows an array of two water-to-water heat pumps that could provide either staged heating capacity or staged cooling capacity, but not both at the same time.

Flow to both sides of each heat pump is provided by a variable-speed pressure-regulated circulator. A pair of motorized ball valves, one on the condenser side of the heat pump and the other on the evaporator side, open when their associated heat pump operates. These valves have relatively high Cv ratings, and thus minimize head loss in their associated branch piping paths. An end switch within the actuator of each motorized ball valve closes when the valve reaches 80% of its fully open status. This switch can be used as part of a “safety switching” circuit to verify that both flow pathways (e.g., evaporator and condenser) through the heat pump are open before allowing the heat pump’s compressor to operate.

The two SEP4™ separators allow the possibility of independent variable-speed control of all four circulators. They also provide high-efficiency air, dirt and magnetic particle separation in all portions of the system. The flow rate through the earth loop could be varied depending on how many heat pumps are operating.

This approach to flow control (e.g., use of variable-speed high-efficiency circulators and valve-based branch flow control) significantly reduces the electrical power required to operate the hydraulic portions of the system. This is especially relevant in cooling mode operation, since all electrical power supplied to the circulators adds to the overall cooling load or heat dissipation requirements of the system.

Figure 5-5 shows an array of two 5-ton rated water-to-water heat pumps that are used in a maintenance facility. These heat pumps provide staged heat input to a radiant floor slab. They also provide cooling through multiple chilled-water air handlers.

Each water-to-water heat pump is equipped with a desuperheater for domestic water heating (uninsulated piping connected to upper two ports on each heat pump). The two pipes that lead to the earth loop and the two pipes the lead to the heating and cooling distribution system are connected to the heat pump using reinforced flexible hose connectors to minimize vibration transfer. The earth loop piping and distribution piping are insulated with an elastomeric foam to prevent surface condensation and conserve energy. Each heat pump is mounted so that front and side service panels can be accessed.

One variation on standard air-cooled chillers is to add an internal refrigerant-to-water heat exchanger that can transfer some or all of the heat of rejection produced during cooling mode operation to a stream of water instead of dissipating all that heat to outside air. The heated water can then be used for loads such as space heating, domestic water heating, pool heating or other process heating. Any heat not transferred to the warm water stream is dissipated to outside air by a condenser coil and fan.

Figure 5-6 shows an example of a 5-ton rated heat recovery chiller. One set of pipes connects to the chilled-water distribution system, the other set connects to the heated water distribution system.

Heat recovery chillers are ideal in situations where simultaneous heating and cooling are required or possible. Potential applications include:

•    Cooling a building while simultaneously heating a swimming pool
•    Cooling the core area of a building while simultaneously heating perimeter areas
•    Cooling a building while providing domestic water heating
•    Cooling a building while providing hot water for an industrial process such as laundry, food preparation or greenhouse soil warming

Because both the chilled water stream and hot water stream are being used, the effective COP of a heat recovery chiller can be significantly higher than the COP of a stand-alone chiller or a stand-alone heat pump.  In theory, the effective COP of a heat recovery chiller would be:

$$COP_e = {Q_t \over e_{input} \times 3413} = \bigl( 2 \times COP_h-1 \bigr)$$

Where:
$COP_e$ = Effective COP based on total useful energy output, which is the sum of heating and cooling outputs
$Q_t$ = Total useful energy output of hot and cold water streams (Btu/hr)
$e_i$ = electrical input to heat recovery chiller (kilowatts)
$COP_h$ = Measured heating COP of the unit based on heat output only and electrical input
$3413$ = Conversion factor from kilowatt to Btu/hr

Thus, a heat recovery chiller operating at a COP of 4.0 based on measured heating output would achieve an effective COP of (2x4-1) = 7 based on the total useful output of heating and cooling flow streams.

Some heat recovery chillers can also switch between outside air or a geothermal earth loop as the “source” for low temperature heat, or the media to which excess heat (e.g., heat not used by the hot stream load) is dissipated.

Figure 5-7 shows a possible application using a heat recovery chiller to supply either 2-pipe air handlers or 4-pipe air handlers. Note that each air handler can independently operate in heating or cooling mode. It would also be possible to add different types of chilled water terminal units or low temperature heat emitters such as radiant panels to the distribution systems shown.

Figure 5-8 shows how multiple heat recovery chillers can be piped to provide staged input to either the heating or cooling distribution system.

The “hot side” and “cold side” of each chiller are connected to low head loss header piping that leads to two hydraulic separators. Depending on the chiller’s flow requirements, a pair of high Cv zone valves or a pair of motorized ball valves open and close to allow or prevent flow through the chiller. Two variable-speed pressure-regulated circulators automatically adjust speed to maintain a constant differential pressure across the headers. The distribution system is shown with variable-speed pressure-regulated circulators. The hydraulic separators prevent interaction between the chiller circulators and the load circulators. They also provide high-efficiency air, dirt and magnetic particle separation. Each chiller can be fully isolated for service if necessary.

Because the chillers are located outside, the entire system operates with an antifreeze solution. A fluid feeder is used to maintain system pressure as air is vented from either side of the system.

It’s also possible to produce chilled water by combining one or more outdoor condensers with an external refrigerant-to-water heat exchanger. The heat exchanger serves as the evaporator for the refrigeration system. This heat exchanger must be properly sized based on the refrigerant used, the desired evaporator temperature, the superheat setting of the thermal expansion valve in the condenser unit, and the required chilled-water temperature and flow rate.

In systems that require 10 or more tons of cooling, it’s possible to use two more 5-ton single phase condensers and operate them in stages based on the demand for chilled water. Figure 5-9 shows how each condenser unit is piped to a separate heat exchanger. A flow switch should be installed on each branch, as shown in Figure 5-9a.

Each flow switch verifies that a suitable flow rate exists through the water side of the refrigerant-to-water evaporator before allowing the condenser unit to operate. Insufficient water flow through the heat exchanger could result in freezing. Notice that all piping, as well as the lower body of the flow switch, are insulated to prevent condensation formation on components carrying chilled water.

Anyone who has swam in a lake in the northern half of the United States can verify that the water temperature gets noticeably cooler just a few feet below the surface, even on a hot summer day. Measurements have shown that the water temperatures at depths of approximately 40 feet or more below the surface of lakes in climates that experience several weeks of below-freezing air temperature during winter, experience very little temperature variation on an annual basis. This is illustrated in Figure 5-10.

Water attains its maximum density at a temperature of 3.98ºC (39.2ºF). In cold climates, water at this temperature will permanently accumulate in the lowest regions of lakes having depths of at least 40 feet. Water at such a temperature is very adequate for chilled-water cooling systems, if it can be accessed from shore.

Lake source cooling has been done on several large-scale projects. The cool waters of Cayuga Lake in upstate New York provide approximately 20,000 tons of cooling capacity for Cornell University in Ithaca, NY. The city of Toronto, Ontario, also uses the cool water from about 600 feet below the surface of Lake Ontario to provide 59,000 tons of cooling for downtown high-rise office spaces. The lake water absorbs heat from a large district cooling system that is connected to several high-rise office buildings in downtown Toronto. Several large plate & frame heat exchangers, seen in Figure 5-11, provide the interface between the lake water and the water in the district cooling system.

The cost and complexity of such systems is beyond what would be practical for a single commercial building. Its feasibility depends on a cluster of larger buildings, located reasonably close to a large lake, that could connect to a district cooling system.

The feasibility study of a potential lake water cooling system should begin with assessment of any codes or regulations regarding use of lake water. Different requirements may apply to “navigable waters” versus lakes in which boating is not allowed. If these criteria allow further pursuit of the project, water temperature measurements should be taken at several depths below the lake surface to determine available water temperatures during months when cooling is needed. This helps determine the required depth of the water intake pipe.

Another method for harvesting the cooling potential of deep lakes or large ponds uses a closed plate-type heat exchanger, an example of which is shown in Figures 5-12 and 5-13.

This lake heat exchanger consists of multiple stainless steel plate assemblies. Each assembly has two stainless steel plates that are specially patterned to create flow channels and are welded together along their perimeter. Each plate assembly is connected to a supply and return header. The overall assembly is welded to a stainless steel base that supports the plates several inches above the surface they rest on. HDPE tubing is routed from the headers to the shore.

This type of heat exchanger is designed to be used with water source heat pumps. However, when properly sized and used in a lake where the water temperatures at the lakebed are stable and relatively low, it could provide direct heat exchange to a chilled-water cooling system. Figure 5-13 shows one concept for such a system.

This system uses a SEP4 hydraulic separator to provide air, dirt and magnetic particle separation of the system water. A variable-speed pressure-regulated circulator is used on the load side of this separator. The speed of this circulator automatically changes based on proportional differential pressure control as the zone valves on each air handler open and close. The SEP4™ also allows for different flow rates between the lake heat exchanger and load side of the system. The lake heat exchanger allows the lake’s cooling potential to be harvested using a completely closed loop system. It eliminates the need for filtering lake water.

Manufacturers of lake water heat exchangers provide design assistance software that can be used to select a specific heat exchanger based on total cooling load, lake water temperature, flow rate through the heat exchanger and temperature change across the heat exchanger.

Some chiller manufacturers specify minimum “loop volumes” in dedicated chilled-water distribution systems. The suggested value is typically 3 gallons of water in the chilled-water circuit per ton of cooling capacity the loop delivers. Specialized applications that require more precise control of chilled water temperature may require from 6 to 10 gallons of water volume per ton of cooling capacity.

If the distribution system does not provide the required volume, a “2-port” baffled buffer tank is often suggested by the chiller manufacturer to bring the circuit volume up to the required value. A typical installation for such a tank is shown in Figure 5-14.

Notice that flow through the 2-port buffer tank changes direction depending on the flow rate of the distribution system versus the flow rate through the chillers. In either case, the buffer tank provides the necessary water volume to stabilize chiller operation. Verify the recommended sizing and placement of 2-port buffer tanks with chiller manufacturers.

Figure 5-15 shows an example of an air-cooled chiller connected to a “2-pipe” buffer tank. The piping to the left of the buffer tank has a DiscalDirt® separator for eliminating air and capturing dirt in the hydronic system.