Waste Heat Boilers

The paper "COGAS - A New Look for Naval Propulsion" published in the October 1974 American Society of Naval Engineers (ASNE) Journal provides some information on the design of waste heat boilers for use in a Combined Gas Turbine And Steam (CoGAS) propulsion plant. Data and trendline equations derived from this report are also included in the thesis "Ship Synthesis Model for Naval Surface Ships".

In the ASNE paper it is estimated that 54,000 lb/hr of 625 psi steam could be developed by waste heat boilers operating in combination with early 1970's era LM-2500 gas turbines. In the plant outlined in this paper each waste heat boiler attached to a 23,000 HP gas turbine is said to be able to produce an additional 7,000 HP. It should be noted here this plant also generates superheated steam and a plant that produces only saturated steam may vary a bit from this, but this information appears to be the best data that I currently have available.

Also in the ASNE paper a single waste heat boiler is estimated to output 54,000lb/hr of 625 psi steam based on the gas turbine generating 463,000lb/hr of 977^oF exhaust (with a temperature drop of the exhaust through the waste heat boiler of 527^oF, and an exhaust temperature at the boiler outlet of 450^oF).

Here is some similar information on power and exhaust output for some other typical marine gas turbines :

Type	Full Power Output	Full Power Exhaust Temp	Full Power Exhaust Flow Rate
Type	(HP)	(^oF)	(lb/hr)
LM500	6,000	1,049	129,240
LM1600	20,000	1,950	374,400
early LM2500 (as in the ASNE paper)	23,000	977	463,000
RR Marine Spey	26,150	914	530,960
modern LM2500	33,600	1,051	558,000
LM2500+	40,500	965	680,400
LM2500+G4	47,370	1,020	736,920
RR MT30	48,280	871	896,830
LM6000	56,900	853	982,800

Doing some quick back of the envelope calculations, assuming the same exhaust temperature at the outlet of a waste heat boiler of 450^oF (in order to try and prevent substances such as Sulfuric Acid from condensing) and the same temperature of the feedwater into the system of 289 ^oF, and assuming that we are trying to produce 600psi saturated steam (as opposed to the 625 psi superheated steam in the system described in the ASNE paper) I come up with the following (where the Required Steam Flow is the estimated Steam Flow that would be needed to absorb all the heat available for transfer assuming the exhaust temperature after the boiler is 450^oF);

Type	delta Temp	delta enthalpy of exhaust	m(dot) * delta h	Max Avail Power	delta enthalpy of the feedwater/steam *	Required Steam Flow
	(^oF)	BTU/lbm	BTU/hr	HP	BTU/lb	lbm/hr
LM500	599	152.8	19,750,000	7,755	946.2	20,875
LM1600	1,500	200.0	149,750,000	58,800	946.2	158,284
early LM2500	527	133.9	61,980,000	24,340	946.2	65,505
RR Marine Spey	464	117.4	62,330,000	24,470	946.2	65,879
modern LM2500	601	153.4	85,570,000	33,600	946.2	90,443
LM2500+	515	130.7	88,940,000	34,920	946.2	94,002
LM2500+G4	570	145.2	106,980,000	42,010	946.2	113,070
RR MT30	421	106.2	95,270,000	38,410	946.2	100,689
LM6000	403	101.6	99,830,000	39,200	946.2	105,510

* Where the enthalpy of the feedwater @ 289^oF is 258.4 BTU/lbm and the enthalpy for saturated steam at 600 psi is 1,204.5 BTU/lbm.
[For comparison: if we enter the enthalpy of superheated steam @ 625psi I come up very close to the 54,000lb/hr listed in the ASNE paper]

In the "Ship Synthesis Model for Naval Surface Ships" thesis the following weight estimates are provided;

Boiler Wt = 0.000883*SHP
Condensors & Air Ejectors = 0.000533*SHP
Main Steam Piping = 0.0000833*SHP
Feed Water & Condensate Systems = 0.0001833*SHP
Circulating & Cooling Water Systems = 0.000300*SHP

However, in looking further at the thesis and ASNE article it appears that the SHP used in the equations above are for the total "output" SHP of the plant (including both the Gas Turbines and Steam system).

Looking at the data in the ASNE paper two issue come to mind;

The rated steam flow of the waste heat boilers described in the ASNE paper is 54,000 lb/hr.
The Steam Turbines of the CoGAS plant described in the paper only account for 23.3% of the total power output of the plant (14,000 of the 60,000 total SHP of the plant).
The power output from each Steam Turbines (7,000 HP) is only about 28.8% of the total power output of the boilers (24,340 HP).

As such, adjusting the above equations to reference the "input" power provided by the exhaust gas flow at maximum output, I come up with;

Item	f(HP@blr)
Boiler Wt	0.0010880*HP@blr
Condensors & Air Ejectors	0.0006569*HP@blr
Main Steam Piping	0.0001027*HP@blr
Feed Water & Condensate Systems	0.0002259*HP@blr
Circulating & Cooling Water Systems	0.0003698*HP@blr

Where HP@blr is enthalpy of the steam as it exits the bloiler times the steam mass flow (in lb/hr), converted to horsepower. In general these values may be a bit off in that the original ASNE paper describes a plant capable of generating superheated steam, while non-superheated steam would likely be desired to support steam catapult operations, but these equations are currently the best information that I have for estimating the weights of waste heat boilers and associated equipment.

In addition, a general rule of thumb for auxiliary systems would add an additional 0.556% of the sum of the above weights for Spares & Tools plus another 1.77% of the above weights for Operating Fluids.

With regards to the use of waste heat boilers a couple issues may arise. Specifically;

a) it appears they are best suited to use with propulsion plants which produce a high vloume of waste heat, such as gas turbine plants, and
b) depending on the main propulsion plant configuration and the number and type of catapults fitted on a ship there may be situations wherethe waste heat boilers on their own may not be able to generate all the steam required for large scale air operations

As such, in some instances it may be possible that a secondary system, such as an additional stand alone boiler, may also be required to supplement a waste heat boiler system.

From the data above it appears that for a single C13-2 catapult a it appears that a waste heat boiler on an early generation LM2500 or RR Marine Spey may not provide enough power at the boiler outlet to meet the required 28,750 HP requirement estimated for the catapult, but that a waste heat boiler on a more modern LM 2500, LM 2500+, LM 2500+G4, RR MT30 or LM 6000 should be able to meet the steam requirements.

Assuming a Gas Turbine with enough exhaust gas flow and temperature to generate 28,750 HP in a waste heat boiler as a base case, the following weight estimates are calculated;

Item	Weight (LT)
Boiler	31.3
Condenser & Air Ejectors	18.9
Main Steam Piping	3.0
Feedwater & Condensate Systems	6.5
Circulating & Cooling Water Systems	10.6
Spares & Tools	0.2
Operating Fluids	1.5
Total	72.0