HPAC Magazine

Making the case for modular chillers

March 20, 2019 | By Dave Demma

Modular, in terms of manufacturing, is not a new concept. There is a certain logic to it: manufacturing cataloged production models rather than custom models is certainly more cost effective, and building and/or assembling in a manufacturing plant versus onsite is more cost effective and provides more consistent quality. It is not uncommon to see an 18-wheeler with a flatbed trailer transporting a portion of a residential home down the freeway on its way to a building site. Nor is it uncommon to see other modular structures, such as fences or walls. So, why not modular chillers? Indeed…why not.

The individual chiller modules are not particularly different than a standard chiller. You would have the same four basic components: compressor, condenser, expansion device, and evaporator (brazed plate heat exchanger, or conventional shell in tube heat exchanger).

It is not uncommon for the modules to be equipped with two separate circuits, two compressors, a single two-circuit condenser, two expansion devices, and a two-circuit evaporator (heat exchanger). In addition, one (or both) circuits could be equipped with hot gas bypass valve. This would allow for a wide range of capacity control. For example, a 30-ton module would be comprised of two 15-ton circuits, which would cycle based on leaving water temperature. With the utilization of hot gas bypass, this would allow the module to provide anywhere from 7.5 tons of cooling capacity to the rated 30 tons of capacity.

Now, what sets the modular approach apart from a standard chiller is this: two or more these relatively smaller capacity chillers modules can be paralleled together, to provide a very wide range of cooling capacity.

The modules are controlled in a typical master/slave control relationship, with the master module containing the main microprocessor controller, which would be linked via communication cable to the slave controller located in each slave module.

Water system installation sketch.

As stated above, each module is equipped with its own evaporator/heat exchanger. The chilled water supply and chilled water return from each module’s heat exchanger are piped respectively to the main chilled water supply header and main chilled water return header. This allows each module to provide cooling capacity to the chilled water return as needed, and sequenced by the main microprocessor.

Air cooled models would typically employ their own air cooled condenser, whereas water cooled models could employ their own water cooled condenser. In water cooled models, the condensing water inlet and condensing water outlet would respectively be connected to the cooling tower water supply header and cooling tower water return header.

In addition to providing cooling capacity, modular chillers are available in heat pump configurations, and heat recovery configurations (where one or more modules would provide cooling capacity, and one or more modules would provide heating capacity).

Benefits of Modular Chillers:

Size: When retrofitting equipment in existing buildings it can sometimes be challenging to get the new equipment into the allocated machine room “after” the building has already been built. This is one area where a modular design really offers a benefit, as the modules are substantially smaller than a single piece of equipment.

Capacity Control: While most large chillers offer some measure of capacity reduction (unloading), there is always an efficiency loss in operating a large compressor in an unloaded state. For example, a single 12 cylinder reciprocating compressor driven by a 120 HP motor, when operating 50 per cent unloaded, will realize an amp draw greater than 50 per cent of the full load amp draw. Now, imagine a modular chiller, employing three modules equipped with dual two cylinder compressors. Operating at a 50 per cent unloaded condition is achieved by de-energizing three of the compressors. This provides a 50 per cent reduction cooling capacity with a corresponding 50 per cent reduction in compressor motor amp draw, as 50 per cent of the compressors are no longer in operation.

Redundancy: This means there will be no significant downtime in a modular chiller application. Each module would have its own dedicated power. This allows for each unit to undergo maintenance without interrupting the ability of the remaining modules to provide cooling capacity.
In addition, using the above single compressor chiller application, consider what would happen when any one of the following conditions occur:

• Drive motor failure;
• Compressor failure;
• Refrigerant loss;
• Tripped main breaker;
• Blown control fuses;
• Condenser fan motor failure;
• Tripped breaker;
• TEV failure;
• Solenoid valve failure;
and the list goes on.

With the single compressor chiller, experiencing any of the issues listed above would result in the entire system being offline and no cooling capacity.

With a modular chiller, experiencing one of the issues above to a single compressor circuit in one of the modules would result in losing 16.67 per cent (1⁄6) of the system cooling capacity. Even in the middle of summer, this is much more palatable outcome than losing the entire system cooling capacity.

Mr Spock… what are the odds that all six chiller modules would experience simultaneous problems? “Captain, the odds are precisely, 2028.7 to 1 against that happening.”

Flexibility and Efficiency: As opposed to providing cooling only, modular chillers can come in heat pump configurations or heat recovery configurations (simultaneous cooling and heating). In addition, some modules can be configured for water source use, or to provide capacity to heat domestic water. Not only does this allow extreme flexibility in providing various comfort solutions, it also provides much great efficiency than separate heating/cooling equipment.

Expandability: Traditional chillers are not good candidates for capacity increases, barring major a major expense. So, when faced with a renovation or a building addition, a modular chiller offers the ability for simple and easy capacity expansion. For example, if the existing modular capacity is 120 tons, and the new building expansion requires an additional 20 tons of cooling capacity, the simple addition of a 20-ton module will satisfy the increased capacity requirement

Of course, piping into the chilled water supply header and chilled water return header (along with the cooling tower water supply header and cooling tower water return header if water cooled), providing the required electrical supply, and connecting the communication wiring to the slave controller, and you are set.

Environmental Concerns: There is no escaping the world of regulations and standards that we live in. While perhaps not a major driver in the decision as to what type of equipment to utilize, modular chillers can much easier comply with ASHRAE Standard 15 mechanical ventilation standards. This is due to the smaller refrigerant charge inherent in modular chiller systems. This potentially eliminates the need for additional and/or costly installation of ventilation, refrigerant monitoring, etc., which can lower the total cost of ownership.

How wonderful to live in an age where, for any given set of design conditions, there are multiple equipment options of varying cost and efficiency available. <>

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at ddemma@uri.com.



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