General
Process hot water often represents the single largest Btu/hr energy
requirement for a manufacturer. Development of highly efficient heat
exchange concepts for this purpose has resulted in the 'direct contact
water heating' scheme. Fundamentally, by raining water down a 'packed'
column, which also is the stack for combustion products (natural gas),
near ideal heat transfer is achieved.
Exhaust leaves the system cooled to less than
10F above the cool water inlet, and the water is able to reclaim well
above 90% of the exhaust energy.
Process Uses
Abundance, availability, safety and experience make hot water a first
choice for manufacturing processes requiring:
1. Washing/flushing
- Equipment "clean-down" and sanitizing in
food industries (meat, dairy, sugar refining, etc.), and pharmaceutical
and "bio" processes
- Continuous washing operations in raw food preparation
(cane/beet sugars, meat, etc.), textiles, wood/paper pulp, removing
oils and other excess matter (paint, dust etc.) in metals fabrication
and molded plastics industries (auto parts, sheet metal, cans, food/beverage
containers, etc.), and in synthetic rubber and fiber manufacturing
- Flushing process piping and batch equipment (paint
blenders, fermentation vessels, etc.), particularly for operations
using the same process lines/equipment to produce slightly varying
products (paints, candy slurries, pharmaceuticals, etc.).
2. Solvents for raw material preparation, leaching, separations/extractions,
and emission control operations. Water is typically chosen when these
systems handle general solid inorganics, acids, generally polar fluids,
and crystalline salts.
3. Crystallization/fermentation/reaction media for industries
including wine/malt-beverage, dairy, pharmaceutical, and inorganic
chemicals.
4. Heating jackets for vessels/operations below ~230F including
chocolate tempering, crystallizers, and storage vessels/mixers containing
viscous materials.
Hot water generated from direct contact with natural gas derived combustion
exhaust has been approved for food manufacturers including dairy,
meat plants, and beverages.
Integrating
for Cogeneration
Addressing heat transfer, either more packing media,
or extending the height of the column (or both) may be necessary to
maintain normal operation (with retrofit systems). Pressure drop and
thus back-pressure imposed on the generating system will be a key
design element. Special consideration to ensure that no process water
enters back into the DG unit's exhaust system is also crucial for
practical implementation.
Many industrial facilities may not have a constant hot
water demand, however two profiles may describe the demand well (e.g.,
normal production operation, and "clean-down" or full capacity
day shifts with part capacity night shifts). In these cases, a bypass-recuperator
option on a turbine-based cogeneration scheme can be integrated with
a variable flow water tower to switch between profiles.
Assuming precise hot-water energy requirements are known,
a recuperator with bypass can be designed to maintain total system
efficiency by diverting some or all of the exhaust past the (turbine)
recuperator to boost the hot water delivery to the desired level.
Currently Available Systems
There are currently two off-the-shelf small industrial
cogeneration systems available in the marketplace to generate hot
water. The first system is a microturbine-based solution that works
like the indirect liquid heating system. An air-to-water heat exchanger
is used with the turbine exhaust gases to heat water.
The second system is a standard reciprocating engine cogeneration
system. These system use liquid-to-water heat exchangers on the water
jacket cooling fluid, the lubricating oil system, and sometimes on
the aftercoolers. Some of these systems also use an air-to-water heat
exchanges on the engine exhaust.
For more information on each application:
