Thursday, 7 August 2014

Boat Fridge Installation

Although it had been previously successfully repaired (see earlier blog, 24 June 2013),  the 20 year old Electrolux gas fridge was always temperamental and finally gave up the ghost during our trip from Oundle to Welford. This time its decline looked terminal so we investigated our options.  Here it is when we bought Patience in 2009.  Note the shelf space below the fridge, which was not much used.

The latest boat safety regs make it almost impossible to fit a new gas fridge (see, for example, Waterways World) and, as electric fridges using the new Danfoss DC compressors are very efficient, we took the opportunity to go to electric power.  We carefully removed the old fridge and capped off and leak tested the gas supply pipe.  We discovered some thermal insulation panels, which looked as if they might contain asbestos, see photo below.
These were sprayed with a fine water mist and very carefully unscrewed, wearing an approved disposable face mask and disposable overalls.  The panels were immediately double bagged and sealed, prior to disposal at an approved local authority asbestos disposal site. The cavity was carefully wiped down with water to remove any residual dust.

The next task was to choose a new fridge.  Three manufacturers were investigated: Isotherm (made in the US), Waeco (made in Germany) and Shoreline (made in the UK).  All use the same Danfoss BD35F compressor, so there isn't much to choose between them on the basis of power consumption.  We did realise that by removing the bottom shelf in the cavity, we could fit a larger fridge than the old 60 litre Electrolux fridge that we were replacing.  In fact our friends at the Oundle marina chandlery had a Shoreline RR102W in stock that would just fit the space and give us an increased capacity of 98 litres.  It was also about £100 cheaper than the smaller Isotherm and Waeco 65 litre fridges.

Before finally committing to this, we did some sums to check that we had adequate domestic battery capacity to run the fridge while moored.  The Shoreline fridge is claimed to have an average energy consumption of 0.95 Ah/h. We rarely moor up for more than an overnight stay, so if we assume a period of 18 hours, the fridge would consume a nominal 17 Ah of battery charge.  We have 2 x 110 Ah domestic batteries and no other large current consuming appliances, so this seems well within their capacity, even allowing for only accessing 50% of the nominal energy from the batteries.

The next task was to check the routing of the cable back to the main switch board, where fortunately we had a spare 15 A switch and circuit breaker. It was not easy to find a route for the cable and we discovered that we would need a 10 m run each way.

All the fridge manufacturers recommend cables whose cross sectional area must be increased in proportion to their length.  Shoreline recommend 10 sq mm cable if the fridge is 10 m from the supply and Isotherm and Waeco specify even larger cables than this. 10 sq mm cables are not only very expensive, but also more difficult to route through the boat.  At this point we went back to the theory.

It seemed that the objective is to reduce the voltage drop along the cables to acceptable limits rather than keep the resistance of the cables constant, which is implied by the manufacturers' recommendations that their cross sectional area should increase in proportion to their length.  There is a very useful formula for the voltage drop along copper cables on the SmartGauge website, which gives the required copper cable size in sq mm as: (18 x length of cable in m x current in amps)/(voltage drop x 1000).  Our installation has 20 m (allowing for both live and neutral).  The current draw of the Danfoss compressor was measured at 4.2 A. In fact their data sheet gives a range of 4 A (at 0 deg C evaporator temperature) to 4.45 A (at 5 deg C evaporator temperature) when running at 2000 rpm.  The current is likely to be lower as the freezer compartment (which determines the evaporator temperature rather than the chiller compartment) will be below zero once the fridge has reached its normal operating condition. If we take a cable length of 20 m, a current of 4.25 A and a voltage drop of 0.34 V, the formula gives us a cable cross section of 4.5 sq mm, which is a commercially available option.  Quite independently of this, Nick Billingham's book on Narrow Boat Care and Maintenance (1995, The Crowood Press) suggests a 4 sq mm cable for a 4 A current draw with up to 10 m between the device and the battery, which fits in with the theory of limiting the voltage drop to about a third of a volt.  The 10 sq mm cable recommended by Shoreline would only reduce the voltage drop from 0.34 V to 0.15 V, a reduction of just 0.19 V.  In the context of the significant variation in supply voltage from a battery depending on its state of charge and the fact that the fridge cut-out is set as low as 10.4 V, it seemed a reasonable risk to go with 4.5 sq mm cables and monitor the situation. Of course, we may be proved wrong and find that the fridge cuts out when moored up for a few hours.  If this proves to be the case, we will simply double up the supply cables.

After installation we measured a voltage drop at the fridge of about 0.7V going from no load (12.5V) to when the compressor cut in (11.8V). However, in addition to the voltage drop in the cables between the switchboard to the fridge, estimated at about 0.34V, the balance is presumably due to the voltage drop between the battery and the switchboard, as well as the voltage drop in the battery itself when current is drawn from it.

A second requirement when sizing cables is to ensure that they are capable of carrying the current without overheating, although in low voltage DC systems, this is not likely to be a problem if they are correctly sized to manage the voltage drop, as described above.  The total heat generated (in Watts) in 20 m of cable is the current (4.25 A) x the voltage drop (0.34 V), which is about 1.5 W along the whole length, or 0.075 W/m of cable, which is negligible. For example, a smaller cable (2.5 sq mm) than we are using here is typically rated as being safe for up to 20 A, see for example the SmartGauge website.

Anyway, after all this theory and practice, the new fridge looks good and, at the time of writing, seems to be working fine, see the photo below.  The final task was to reverse the door hinge so that it opens more conveniently towards the galley area. This was achieved by removing the top panel of the fridge and swapping the upper hinge from the right to the left hand side and similarly switching the lower hinge plate on the base of the fridge.

We will of course have to be even more careful that we remember to switch over to the domestic batteries when we moor up to protect the charge in the starter motor battery.  

Finally, don't attempt any of these tasks unless you are competent to do so!

2 comments:

  1. John's calculations (above) proved correct and we are happy with the installation and the fridge itself. We are, however, careful about how long we leave the fridge on while the engine is off. 24 hours running directly off the domestic batteries does take a bit out of them. So we now usually switch the fridge off at night. The insulation is perfectly adequate and the contents are unharmed, while the battery is left in better shape.

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