Title: Open heat exchanger for improved energy efficiency in the heating of hot spas

Abstract

Hot spas and Jacuzzis are popular in Iceland due to the abundance of reasonably prized geothermal
heat available. However the water from the district heating system is too warm to be admitted
directly into the spa. For safety reasons the water is mixed with cold water, from 75 down to 50ºC,
which leads to wasting a large quantity of heat. Therefore a design was suggested that enables the
feeding of geothermal water directly into the pot, omitting the step of mixing it with cold water.
The idea is to employ an open heat exchanger that transfers much heat from the geothermal water to
the bulk water in the spa, before letting it mix with the spa water. A case study was done for one
particular spa. Heat load was calculated and measured when the spa was in use, and when it was
unused. A design is suggested employing a circular double-plate which is to be placed at bottom of
pot. This unit will function as an open heat exchanger feeding district heating water into the pot.
Free convection takes place at the up side of the upper plate and forced convection below the upper
plate. Heat transfer coefficient for both was calculated. Temperature field in the pool before and
after implementation of the open heat exchanger was measured at different points using
thermocouples. The measured temperatures were compared to thermal and fluid-dynamic simulation of
the temperature and flow fields obtaining good accordance. Results are reasonable and promising for
a good design that may considerably reduce the energy expenses for a continuously heated geothermal
spa. More detailed measurements were made on the upper plate of the heat exchanger and detailed
simulation of the heat exchanger itself was then used to obtain a value for the heat transfer
coefficient for the upper plate to the surrounding water. This information was used to make an
improved design for the open plate heat exchanger, stating that a diameter of 63 cm and a thickness
of 1.5 cm were suggested as final design. Due to economy consideration the recovery time of the
implementing of suggested heat exchanger is
estimated to 8 months in studied case.